Template:2009 flu pandemic data

Influenza (flu)
Types Avian A/H5N1 subtype Canine Equine Swine A/H1N1 subtype
Vaccines 2009 pandemic Pandemrix Live attenuated Seasonal flu vaccine brands
Treatment Amantadine Baloxavir marboxil Laninamivir Oseltamivir Peramivir Rimantadine Umifenovir Zanamivir
Pandemics 1889-1890 Russian flu 1918 Spanish flu 1957-1958 Asian flu 1968 Hong Kong flu 1977 Russian flu 2009 swine flu
Outbreaks 1976 swine flu 2006 H5N1 India 2007 Australian equine 2007 Bernard Matthews H5N1 2008 West Bengal 2015 United States H5N2 outbreak 2020–2022 H5N8 outbreak
See also Flu season Influenza evolution Influenza research Influenza-like illness Vaccine reformulations

The 2009 flu pandemic is a global outbreak of a new strain of H1N1 influenza virus, often referred to as "swine flu" in the media. Although the virus, first detected in April 2009, contains a combination of genes from swine, avian (bird), and human influenza viruses, it cannot be spread by eating pork products or being around pigs.^[2][3]

The outbreak began in Veracruz, Mexico, with evidence that there had been an ongoing epidemic for months before it was officially recognized as such.^[4] The Mexican government closed most of Mexico City's public and private facilities in an attempt to contain the spread of the virus. However the virus continued to spread globally, clinics were overwhelmed by people infected, and the World Health Organization (WHO) and US Centers for Disease Control (CDC) stopped counting cases and in June declared the outbreak to be a pandemic.^[5]

While only mild symptoms are experienced by the majority of people,^[5] some have more severe symptoms. Mild symptoms may include fever, sore throat, cough, headache, muscle or joint pains, and nausea, vomiting, or diarrhea. Those at risk of a more severe infection include: asthmatics, diabetics,^[6] those with obesity, heart disease, the immunocompromised, children with neurodevelopmental conditions,^[7] and pregnant women.^[8]

In addition, even for persons previously very healthy, a small percentage of patients will develop viral pneumonia. This manifests itself as increased breathing difficulty and typically occurs 3–6 days after initial onset of flu symptoms.^[9][10]

Similar to other influenza viruses, pandemic H1N1 is typically contracted by person to person transmission through respiratory droplets.^[11] Symptoms usually last 4–6 days.^[12] To avoid spreading the infection, it is recommended that those with symptoms stay home, away from school, work, and crowded places. Those with more severe symptoms or those in an at risk group may benefit from antivirals (oseltamivir or zanamivir).^[13] As of December 4, 2009^[update], there are Template:Swine-flu-deaths confirmed deaths worldwide. This figure is a sum of confirmed deaths reported by national authorities and the WHO states that total mortality (including deaths unconfirmed or unreported) from the new H1N1 strain is "unquestionably higher" than this.^[14]

The initial outbreak was called the "H1N1 influenza". It is officially called pandemic H1N1/09 virus by the WHO,^[15] while the CDC refers to it as "novel influenza A (H1N1)" or "2009 H1N1 flu".^[16] In the Netherlands, it was originally called "Pig Flu", but is now called "New Influenza A (H1N1)" by the national health institute, although the media and general population use the name "Mexican Flu". South Korea and Israel briefly considered calling it the "Mexican virus".^[17] Later, the South Korean press used "SI", short for "swine influenza". Taiwan suggested the names "H1N1 flu" or "new flu", which most local media adopted.^[18] The World Organization for Animal Health proposed the name "North American influenza".^[19] The European Commission adopted the term "novel flu virus".^[20]

The symptoms of swine flu are similar to other influenzas, and may include a fever, cough (typically a "dry cough"), headache, muscle or joint pain, sore throat, chills, fatigue, and runny nose. Diarrhea, vomiting, and neurological problems have also been reported in some cases.^[21][22] People at higher risk of serious complications include those aged over 65, children younger than 5, children with neurodevelopmental conditions, pregnant women (especially during the third trimester),^[9][23] and those of any age with underlying medical conditions, such as asthma, diabetes, obesity, heart disease, or a weakened immune system (e.g., taking immunosuppressive medications or infected with HIV).^[8] More than 70% of hospitalizations in the US have been people with such underlying conditions, according to the CDC.^[24]

A study from Australia and New Zealand estimated that the demand for ICU beds due to viral pneumonia was much higher during the pandemic than in previous influenza seasons.^[25] A Canadian study reported that intensive care capacity in Winnipeg, Manitoba was "seriously challenged" at the peak of the outbreak, with full occupancy of all regional ICU beds.^[25]

In adults, shortness of breath, pain in the chest or abdomen, sudden dizziness, or confusion may require emergency care. In both children and adults, persistent vomiting or the return of flu-like symptoms that include a fever and cough may require medical attention.^[22] If it follows the same pattern as in children, a relapse with high fever may in fact be pneumonia.

Dr. Thomas R. Frieden, Director of the CDC,^[26] suggests that people with "underlying conditions" who come down with flu symptoms should consult their doctors first before visiting an "emergency room full of sick people," since it "may actually put them in more danger." This was especially true of pregnant women.^[8]

As with the seasonal flu, certain symptoms may require emergency medical attention. In children, signs of respiratory distress include blue lips and skin, dehydration, rapid breathing, excessive sleeping, seizures,^[27] and significant irritability including a lack of desire to be held. Although "too early" to tell for certain, so far the swine flu "seems to be taking a heavier toll among chronically ill children than the seasonal flu usually does."^[28] Of the children who have died so far, nearly two-thirds had pre-existing nervous system disorders, such as cerebral palsy, muscular dystrophy, or developmental delays. "Children with nerve and muscle problems may be at especially high risk for complications".^[28] "In children without chronic health problems, it is a warning sign if they seem to recover from the flu but then relapse with a high fever. The relapse may be bacterial pneumonia, which must be treated with antibiotics."^[28] Secondary bacterial infections have been found in 30% of fatal cases for all persons, both children and adults. The most common finding in severe cases has been primary viral pneumonia.^[9] Warning signs include difficulty breathing, a crying child who cannot be consoled, or a child who does not wake up, walk, or talk normally.^[10]

Confirmed diagnosis of pandemic H1N1/09 flu requires testing of a nasopharyngeal, nasal, or oropharyngeal tissue swab from the patient.^[29] Real-time RT-PCR is the recommended test as others are unable to differentiate between pandemic H1N1/09 and regular seasonal flu.^[29] However, most people with flu symptoms do not need a test for pandemic H1N1/09 flu specifically, because the test results usually do not affect the recommended course of treatment.^[30] The CDC recommends testing only for people who are hospitalized with suspected flu, pregnant women, and people with weakened immune systems.^[30] For the mere diagnosis of influenza and not pandemic H1N1/09 flu specifically, more widely available tests include rapid influenza diagnostic tests (RIDT), which yield results in about 30 minutes, and direct and indirect immunofluorescence assays (DFA and IFA), which take 2–4 hours; DFA and IFA are more sensitive to pandemic H1N1/09 virus than RIDT.^[31] Due to the high rate of RIDT false negatives, the CDC advises that patients with illnesses compatible with novel influenza A (H1N1) virus infection but with negative RIDT results should be treated empirically based on the level of clinical suspicion, underlying medical conditions, severity of illness, and risk for complications, and if a more definitive determination of infection with influenza virus is required, testing with rRT-PCR or virus isolation should be performed.^[32] Dr. Rhonda Medows of the Georgia Department of Community Health states that the rapid tests are incorrect anywhere from 30-90% of the time. She warns doctors in her state not to use rapid flu tests because they're wrong so often.^[33] The use of RIDTs has also been questioned by researcher Paul Schreckenberger of the Loyola University Health System, who suggests that rapid tests may actually pose a dangerous public health risk.^[34]

The virus is a novel strain of influenza for which extant vaccines against seasonal flu provide little protection. A study at the US Centers for Disease Control and Prevention, published in May 2009, found that children had no preexisting immunity to the new strain but that adults, particularly those over 60, had some degree of immunity. Children showed no cross-reactive antibody reaction to the new strain, adults aged 18 to 64 had 6–9%, and older adults 33%.^[35][36] It was also determined that the strain contained genes from five different flu viruses: North American swine influenza, North American avian influenza, human influenza, and two swine influenza viruses typically found in Asia and Europe. Further analysis showed that several proteins of the virus are most similar to strains that cause mild symptoms in humans, leading virologist Wendy Barclay to suggest on May 1, 2009 that the initial indications are that the virus was unlikely to cause severe symptoms for most people.^[37]

In July 2009, the CDC noted that most infections were mild, similar to seasonal flu, recovery tended to be fairly quick, and deaths to date had been only a fraction of the number of people who die every year from seasonal flu.^{[citation needed]} The 1918 flu epidemic began with a wave of mild cases in the spring, followed by more deadly waves in the autumn, eventually killing hundreds of thousands in the United States.^[38] Researchers from the University of Maryland mixed swine flu and seasonal flu and concluded that the swine flu was unlikely to get more lethal.^[39]

Regarding the probable or possible history of novel H1N1, a July 9, 2009, New England Journal of Medicine article states: "H1N2 and other subtypes are descendants of the triple-reassortant swine H3N2 viruses isolated in North America.^[40] They have spread in swine hosts around the globe and have been found to infect humans. The segments coding for the neuraminidase and the matrix proteins of the new human H1N1 virus are, however, distantly related to swine viruses isolated in Europe in the early 1990s."^[41]

Spread of the H1N1 virus is thought to occur in the same way that seasonal flu spreads. Flu viruses are spread mainly from person to person through coughing or sneezing by people with influenza. Sometimes people may become infected by touching something – such as a surface or object – with flu viruses on it and then touching their mouth or nose.^[2] The basic reproduction number (the average number of other individuals that each infected individual will infect, in a population that has no immunity to the disease) for the 2009 novel H1N1 is estimated to be 1.75.^[42]

The H1N1 virus has been transmitted to animals, including swine, turkeys, ferrets, household cats, and a cheetah.^[43][44][45]

The pandemic is expected to peak by mid-winter in the Northern hemisphere.^[46] The CDC recommended that initial vaccine doses should go to priority groups such as pregnant women, people who live with or care for babies under six months old, children six months to four years old and health-care workers.^[47] In the UK, the NHS recommended vaccine priority go to people over six months old who were clinically at risk for seasonal flu, pregnant women, and households of people with compromised immunity.^[48]

Although it was initially thought that two injections would be required, clinical trials showed that the new vaccine protects adults "with only one dose instead of two", and so the limited vaccine supplies would go twice as far as had been predicted.^[49][50] Costs would also be lowered by having a "more efficient vaccine".^[49] For children under the age of 10, two administrations of the vaccine, spaced 21 days apart, are recommended.^[51][52] The seasonal flu will still require a separate vaccination.^[53]

Health officials worldwide were also concerned because the virus was new and could easily mutate and become more virulent, even though most flu symptoms were mild and lasted only a few days without treatment. Officials also urged communities, businesses and individuals to make contingency plans for possible school closures, multiple employee absences for illness, surges of patients in hospitals and other effects of potentially widespread outbreaks.^[54]

To combat the virus, the WHO and the US government geared up for a massive vaccination campaign in late 2009, one not seen since Jonas Salk discovered the polio vaccine in 1955.^[55]

The Mayo Clinic suggested personal measures to avoid seasonal flu infection were applicable to the 2009 pandemic: vaccination when available, thorough and frequent hand-washing, a balanced diet with fresh fruits and vegetables, whole grains, and lean protein, sufficient sleep, regular exercise, and avoiding crowds.^[56] Smoking raises the risk of contracting influenza, as well as producing more severe disease symptoms.^[57]

Leading health agencies have stressed that eating properly cooked pork or other food products derived from pigs would not cause flu.^[58]

On April 27, 2009, the European Union health commissioner advised Europeans to postpone nonessential travel to the United States or Mexico. This followed the discovery of the first confirmed case in Spain.^[59] On May 6, 2009, the Public Health Agency of Canada announced that their National Microbiology Laboratory (NML) had mapped the genetic code of the swine flu virus, the first time that had been done.^[60] In England, the National Health Service launched a website, the National Pandemic Flu Service,^[61] allowing patients to self-assess and get an authorization number for antiviral medication. The system was expected to reduce the burden on general practitioners.^[48]

US officials observed that six years of concern about H5N1 avian flu did much to prepare for the current swine flu outbreak, noting that after H5N1 emerged in Asia, ultimately killing about 60% of the few hundred people infected by it over the years, many countries took steps to try to prevent any similar crisis from spreading further.^[62] The CDC and other American governmental agencies^[63] used the summer lull to take stock of the United States's response to swine flu and attempt to patch any gaps in the public health safety net before flu season started in early autumn.^[64] Preparations included planning a second influenza vaccination program in addition to the one for seasonal influenza, and improving coordination between federal, state, and local governments and private health providers.^[64] On October 24, 2009, U.S. President Obama declared swine flu a national emergency, giving Secretary of Health and Human Services Kathleen Sebelius authority to grant waivers to requesting hospitals from usual federal requirements.^[65]

As of November 19, 2009^[update], over 65 million doses of vaccine had been administered in over 16 countries; the vaccine seems safe and effective, producing a strong immune response that should protect against infection.^[66] The current trivalent seasonal influenza vaccine neither increases nor decreases the risk of infection with H1N1, since the new pandemic strain is quite different from the strains used in this vaccine.^[67][68] Overall the safety profile of the new H1N1 vaccine is similar to that of the seasonal flu vaccine, and fewer than a dozen cases of Guillain-Barre syndrome have been reported post-vaccination.^[69] Only a few of these are suspected to be actually related to the H1N1 vaccination, and only temporary illness has been observed.^[69] This is in strong contrast to the 1976 swine flu outbreak, where mass vaccinations in the United States caused over 500 cases of Guillain-Barre syndrome and led to 25 deaths.^[70]

There are safety concerns for people who are allergic to eggs because the viruses for the vaccine are grown in chicken-egg-based cultures. People with egg allergies may be able to receive the vaccine, after consultation with their physician, in graded doses in a careful and controlled environment.^[71] A vaccine manufactured by Baxter is made without using eggs, but requires two doses three weeks apart to produce immunity.^[72]

As of late November in Canada, there have been 24 confirmed cases of anaphylactic shock following vaccination, including one death. The estimated rate is 1 anaphylactic reaction per 312,000 persons receiving the vaccine. However, there has been one batch of vaccine in which 6 persons suffered anaphylaxis out of 157,000 doses given. The relatively few remainder doses of this batch are being held pending investigation. Dr. David Butler-Jones, Canada’s chief public health officer, has stated that even though this is an adjuvanted vaccine, that does not appear to be the cause of this severe allergic reaction in these 6 patients.^[73][74]

On May 7, 2009 the WHO stated that containment was not feasible and that countries should focus on mitigating the effect of the virus. It did not recommend closing borders or restricting travel.^[75] On April 26, 2009, the Chinese government announced that visitors returning from flu-affected areas who experienced flu-like symptoms within two weeks would be quarantined.^[76] On May 2, 2009, China suspended flights from Tijuana to Shanghai.^[77]

The president of the Association of Flight Attendants told members of a US Congressional subcommittee that all flight attendants should be given training in how to handle a person with flu and help in communicating to passengers the importance of keeping clean hands.^[78] She also said that flight attendants need to be provided gloves and facemasks to deal with flu-stricken passengers.^[78] Lahey Clinic vice chairman of emergency medicine and Tufts University assistant professor of emergency medicine Mark Gendreau adds that airlines should also ensure that passenger cabins are always properly ventilated, including during flight delays in which passengers are kept aboard the plane. But he also adds that although the aviation industry in the US, along with the CDC, have tried to reassure passengers that air travel is safe, they have so far done too little to try to limit infection risks.^[78]

US airlines made no major changes as of the beginning of June 2009, but continued standing practices that included looking for passengers with symptoms of flu, measles, or other infections, and relying on in-flight air filters to ensure that aircraft were sanitized.^[79] Masks were not generally provided by airlines and the CDC did not recommend that airline crews wear them.^[79] Some non-US airlines, mostly Asian ones, including Singapore Airlines, China Eastern Airlines, China Southern Airlines, Cathay Pacific, and Mexicana Airlines, took measures such as stepping up cabin cleaning, installing state-of-the-art air filters, and allowing in-flight staff to wear face masks.^[79]

The swine flu outbreak has led to numerous precautionary school closures in several countries. Rather than closing schools, the CDC recommended in August that students and school workers with flu symptoms should stay home for either seven days total, or until 24 hours after symptoms subside—whichever is longer.^[80] The CDC also recommended that colleges should consider suspending fall 2009 classes if the virus begins to cause severe illness in a significantly larger share of students than last spring. They have additionally urged schools to suspend any rules, including penalizing late papers or missed classes, or requiring a doctor's note, to enforce "self-isolation" and prevent students from venturing out while ill;^[81] schools were advised to set aside a room for people developing flu-like symptoms while they wait to go home and that surgical masks be used for ill students or staff and those caring for them.^[82]

In California, school districts and universities are on alert and working with health officials to launch education campaigns. Many planned to stockpile medical supplies and discuss worst-case scenarios, including plans to provide lessons and meals for low-income children in case elementary and secondary schools close.^[83] University of California campuses were stockpiling supplies, from paper masks and hand sanitizer to food and water.^[83] To help prepare for contingencies, University of Maryland School of Medicine professor of pediatrics James C. King Jr. suggests that every county should create an "influenza action team" to be run by the local health department, parents, and school administrators.^[84] As of 28 October 2009^[update], about 600 Schools in the United States have been temporarily closed, affecting over 126,000 students in 19 states.^[85]

US government officials are especially concerned about schools because the swine flu virus appears to disproportionately affect young and school-age people, between ages 6 months to 24 years of age, making them one of the top priority groups when it comes to the new vaccine. They state that the most important actions are to encourage and facilitate good hand washing and covering coughs and sneezes, flu vaccinations for people at risk; and to separate sick people from healthy people as soon as possible.^[86]

The US Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC), with input from the US Department of Homeland Security (DHS), have developed updated guidance^[87] and a video for employers to use as they develop or review and update plans to respond to swine flu now and during the upcoming fall and winter influenza season. The guidance states that employers should consider and communicate their objectives, which may include reducing transmission among staff, protecting people who are at increased risk of influenza related complications from getting infected with influenza, maintaining business operations, and minimizing adverse effects on other entities in their supply chains.^[87]

The CDC estimates that as much as 40% of the workforce, in a worst-case scenario, might be unable to work at the peak of the pandemic due to the need for many healthy adults to stay home and care for an ill family member,^[88] and advising that individuals should have steps in place should a workplace close down or a situation arise that requires working from home.^[89] The CDC further advises that persons in the workplace should stay home sick for seven days after getting the flu, or 24 hours after symptoms end, whichever is longer.^[80] In the UK, the Health and Safety Executive (HSE) has also issued general guidance for employers.^[90]

The CDC does not recommend use of face masks or respirators in non-health care settings, such as schools, workplaces, or public places, with a few exceptions: people who are ill with the virus should consider wearing one when around other people, and people who are at risk for severe illness while caring for someone with the flu.^[91] There is some disagreement about the value of wearing either facial masks, some experts fearing that masks may give people a false sense of security and should not replace other standard precautions.^[92] Masks may benefit people in close contact with infected persons but it was unknown whether they prevent swine flu infection.^[92] Yukihiro Nishiyama, professor of virology at Nagoya University's School of Medicine, commented that the masks are "better than nothing, but it's hard to completely block out an airborne virus since it can easily slip through the gaps".^[93] According to mask manufacturer 3M, masks will filter out particles in industrial settings, but "there are no established exposure limits for biological agents such as swine flu virus."^[92] However, despite the lack of evidence of effectiveness, the use of such masks is common in Asia.^[93][94] Masks are particularly popular in Japan, where cleanliness and hygiene are highly valued and where etiquette obligates those who are sick to wear masks to avoid their spreading disease.^[93] In May 2009, due to high demand, some drug stores in Japan ran out of face masks, even though the country has 42 different manufacturers of them,^[93] prompting some to fashion improvised face masks out of other materials.^[95][96] In response, Shanghai, Tianjin, and Guangdong Province donated masks to Osaka, Kobe, and Hyōgo Prefecture, respectively.^[97] Taiwan also donated masks to Osaka and Hyogo.^[98]

Countries have initiated quarantines or have threatened to quarantine foreign visitors suspected of having or being in contact with others who may have been infected. In May, the Chinese government confined 21 US students and three teachers to their hotel rooms.^[99] As a result, the US State Department issued a travel alert about China's anti-flu measures and warned travelers about traveling to China if ill.^[100] In Hong Kong, an entire hotel was quarantined with 240 guests;^[101] Australia ordered a cruise ship with 2,000 passengers to stay at sea because of a swine flu threat.^[102] Egyptian Muslims who went on the annual pilgrimage to Mecca risked being quarantined upon their return.^[103] Russia and Taiwan said they would quarantine visitors with fevers who come from areas where the flu is present.^[104] Japan quarantined 47 airline passengers in a hotel for a week in mid-May,^[105] then in mid-June India suggested pre-screening "outbound" passengers from countries thought to have a high rate of infection.^[106]

The pandemic virus is a type of swine influenza, derived originally from a strain that lived in pigs and this origin gave rise to the common name of "swine flu". This term is widely used by mass media. The virus has been found in American^[107] and Canadian^[108] hogs, as well as in hogs in Northern Ireland, Argentina, and Norway.^[109] However, despite its origin in pigs, this strain is transmitted between people and not from swine to people.^[4] The United States Secretary of Agriculture made clear that despite its common name being "swine flu", there is no risk of contracting flu from eating cooked pork products.^[110] Nevertheless, on April 27, Azerbaijan imposed a ban on the importation of animal husbandry products from America.^[111] The Indonesian government also halted the importation of pigs and initiated the examination of 9 million pigs in Indonesia.^[112] The Egyptian government ordered the slaughter of all pigs in Egypt on April 29, 2009.^[113]

A number of methods have been recommended to help ease symptoms, including adequate liquid intake and rest.^[114] Over-the-counters pain medications such as acetaminophen and ibuprofen may also be useful to reduce symptoms. These measure however do not kill the virus.^[115] Most people recover without medical attention, although those with pre-existing or underlying medical conditions are more prone to complications and may benefit from further treatments.^[116]

People in at-risk groups should be treated with antivirals (oseltamivir or zanamivir) as soon as possible when they first experience flu symptoms. The at risk groups includes pregnant women, children under 2 years old, and people with ‘underlying conditions' such as respiratory problems.^[13] People who are not from the at-risk group who have persistent or rapidly worsening symptoms should also be treated with antivirals. These symptoms include difficulty breathing and a high fever that lasts beyond 3 days. People who have developed pneumonia should be given both antivirals and antibiotics, as in many severe cases of H1N1-caused illness, bacterial infection develops.^[117] Antivirals are most useful if given within 48 hours of the start of symptoms and may improve outcomes in hospitalized patients.^[118] In those beyond 48 hours who are moderately or severely ill antiviral may still be beneficial.^[11] If oseltamivir (Tamiflu) is unavailable or cannot be used zanamivir (Relenza) is recommended as a substitute.^[13][119] Peramivir is an experimental antiviral drug approved for hospitalized patients in cases where the other available methods of treatment are ineffective or unavailable.^[120]

To help avoid shortages of these drugs, the CDC recommended oseltamivir treatment primarily for people hospitalized with pandemic flu; people at risk of serious flu complications due to underlying medical conditions; and patients at risk of serious flu complications. The CDC warned that the indiscriminate use of antiviral medications to prevent and treat influenza could ease the way for drug-resistant strains to emerge which would make the fight against the pandemic that much harder. In addition, a British report found that people often failed to complete a full course of the drug or took the medication when not needed.^[121]

Both medications have known side effects, including lightheadedness, nausea, vomiting, loss of appetite, and trouble breathing. Children were reported to be at increased risk of self-injury and confusion after taking oseltamivir.^[114] The WHO warns against buying antiviral medications from online sources, and estimates that half the drugs sold by online pharmacies without a physical address are counterfeit.^[122]

As of December 2009^[update], the World Health Organization (WHO) reported 96 out of over 10,000 samples of the prevalent 2009 pandemic H1N1 (swine) flu tested worldwide have shown resistance to oseltamivir (Tamiflu).^[123] This is not totally unexpected as 99.6% of the seasonal H1N1 flu strains tested have developed resistance to oseltamivir.^[124] No circulating flu has yet shown any resistance to zanamivir (Relenza), the other available anti-viral.^[11]

Template:2009 flu pandemic data

While it is not known precisely where or when the virus originated,^[125][126] analyses in scientific journals have suggested that the H1N1 strain responsible for the current outbreak first evolved in September 2008 and circulated amongst humans for several months before being formally recognized and identified as a novel strain of influenza.^[125][127]

The virus was first reported in two US children in March 2009, but health officials have reported that it apparently infected people as early as January 2009 in Mexico.^[128] The outbreak was first detected in Mexico City on March 18, 2009;^[129] immediately after the outbreak was officially announced, Mexico requested material support from the US, and within days of the outbreak Mexico City was "effectively shut down".^[130] Some countries canceled flights to Mexico while others halted trade. Calls to close the border to contain the spread were rejected.^[130] Mexico already had hundreds of cases before the outbreak was officially recognized, and was therefore in the midst of a "silent epidemic". As a result, Mexico was reporting only the most serious cases, possibly leading to a skewed initial estimate of the case fatality rate.^[129]

The new strain was first identified by the CDC in two children, neither of whom had been in contact with pigs. The first case, from San Diego County, California, was confirmed from clinical specimens (nasopharyngeal swab) examined by the CDC on April 14, 2009. A second case, from nearby Imperial County, California, was confirmed on April 17. The patient in the first confirmed case had flu symptoms including fever and cough on clinical exam on March 30, and the second on March 28.^[131]

The first confirmed swine flu death occurred at Texas Children's Hospital in Houston, Texas.^[132]

Influenza surveillance information "answers the questions of where, when, and what influenza viruses are circulating. It can be used to determine if influenza activity is increasing or decreasing, but cannot be used to ascertain how many people have become ill with influenza".^[133] For example, as of late June 2009 influenza surveillance information showed the US had nearly 28,000 laboratory-confirmed cases including 3,065 hospitalizations and 127 deaths; but mathematical modeling showed an estimated 1 million Americans currently had the 2009 pandemic flu according to Lyn Finelli, a flu surveillance official with the CDC.^[134] Estimating deaths from influenza is also a complicated process. In 2005, influenza only appeared on the death certificates of 1,812 people in the US. The average annual US death toll from flu is, however, estimated to be 36,000.^[135] The CDC explains^[136] that "...influenza is infrequently listed on death certificates of people who die from flu-related complications." and furthermore that "Only counting deaths where influenza was included on a death certificate would be a gross underestimation of influenza's true impact."

With respect to the current swine flu pandemic, influenza surveillance information is available but almost no studies have attempted to estimate the total number of deaths attributable to swine flu. One study has been performed by the CDC, however; it estimates that there were 3,900 deaths (range 2,500-6,100) attributable to swine flu from April to October the 17th. During the same period, 1,004 deaths were officially confirmed as caused by swine flu.^[137]

The initial outbreak received a week of near-constant media attention. Epidemiologists cautioned that the number of cases reported in the early days of an outbreak can be very inaccurate and deceptive due to several causes, among them selection bias, media bias, and incorrect reporting by governments. Inaccuracies could also be caused by authorities in different countries looking at differing population groups. Furthermore, countries with poor health care systems and older laboratory facilities may take longer to identify or report cases.^[138] "...[E]ven in developed countries the [numbers of flu deaths] are uncertain, because medical authorities don't usually verify who actually died of influenza and who died of a flu-like illness."^[139] Dr. Joseph S. Bresee (the CDC flu division's epidemiology chief) and Dr. Michael T. Osterholm (director of the Center for Infectious Disease Research) have pointed out that millions of people have had swine flu, usually in a mild form, so the numbers of laboratory-confirmed cases were actually meaningless, and in July 2009 the WHO stopped keeping count of individual cases and focused more on major outbreaks.^[140]

Annual influenza epidemics are estimated to affect 5–15% of the global population. Although most cases are mild, these epidemics still cause severe illness in 3–5 million people and 250,000–500,000 deaths worldwide.^[141] On average 41,400 people die each year in the United States based on data collected between 1979 and 2001.^[142] In industrialized countries, severe illness and deaths occur mainly in the high-risk populations of infants, the elderly, and chronically ill patients,^[141] although the swine flu outbreak (as well as the 1918 Spanish flu) differs in its tendency to affect younger, healthier people.^[143]

In addition to these annual epidemics, Influenza A virus strains caused three global pandemics during the 20th century: the Spanish flu in 1918, Asian flu in 1957, and Hong Kong flu in 1968–69. These virus strains had undergone major genetic changes for which the population did not possess significant immunity.^[144] Recent genetic analysis has revealed that three-quarters, or six out of the eight genetic segments of the 2009 flu pandemic strain arose from the North American swine flu strains circulating since 1998, when a new strain was first identified on a factory farm in North Carolina, and which was the first-ever reported triple-hybrid flu virus.^[145]

The great majority of deaths in the 1918 flu pandemic were the result of secondary bacterial pneumonia. The influenza virus damaged the lining of the bronchial tubes and lungs of victims, allowing common bacteria from the nose and throat to infect their lungs. Subsequent pandemics have had many fewer fatalities due to the development of antibiotic medicines that can treat pneumonia.^[146]

The influenza virus has also caused several pandemic threats over the past century, including the Pseudopandemic of 1947 (thought of as mild because although although globally distributed, caused reletively few deaths),^[157] the 1976 swine flu outbreak, and the 1977 Russian flu, all caused by the H1N1 subtype.^[144] The world has been at an increased level of alert since the SARS epidemic in Southeast Asia (caused by the SARS coronavirus).^[158] The level of preparedness was further increased and sustained with the advent of the H5N1 bird flu outbreaks because of H5N1's high fatality rate, although the strains currently prevalent have limited human-to-human transmission (anthroponotic) capability, or epidemicity.^[159]

People who contracted flu before 1957 appeared to have some immunity to swine flu. Dr. Daniel Jernigan of the CDC has stated: "Tests on blood serum from older people showed that they had antibodies that attacked the new virus [...] That does not mean that everyone over 52 is immune, since Americans and Mexicans older than that have died of the new flu."^[160]

• Cannell, JJ; Zasloff, M; Garland, CF; Scragg, R; Giovannucci, E (February 25, 2008). "On the epidemiology of influenza". Virology Journal. 5 (29): 29. doi:10.1186/1743-422X-5-29. PMC 2279112. PMID 18298852. {{cite journal}}: More than one of |author2= and |last2= specified (help); More than one of |author3= and |last3= specified (help); More than one of |author4= and |last4= specified (help); More than one of |author5= and |last5= specified (help)CS1 maint: unflagged free DOI (link)
• Smith, Gavin; Vijaykrishna, D; Bahl, J; Lycett, SJ; Worobey, M; Pybus, OG; Ma, SK; Cheung, CL; Raghwani, J (June 25, 2009). "Origins and evolutionary genomics of the 2009 swine-origin H1N1 influenza A epidemic". Nature. 459 (459): 1122–1125. doi:10.1038/nature08182. PMID 19516283. {{cite journal}}: More than one of |first1= and |first= specified (help); More than one of |last1= and |last= specified (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Soundararajan, Venkataramanan; Tharakaraman, K; Raman, R; Raguram, S; Shriver, Z; Sasisekharan, V; Sasisekharan, R (2009). "Extrapolating from sequence — the 2009 H1N1 'swine' influenza virus" (PDF). Nature Biotechnology. 27 (6): 510–513. doi:10.1038/nbt0609-510. PMID 19513050. {{cite journal}}: More than one of |first1= and |first= specified (help); More than one of |last1= and |last= specified (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
• "Introduction and Transmission of 2009 Pandemic Influenza A (H1N1) Virus --- Kenya, June--July 2009". Morbidity and Mortality Weekly Report. Centers for Disease Control and Prevention (CDC). October 23, 2009. Retrieved October 23, 2009. {{cite web}}: Text "Malaysia Vietnamese consultant Nha Bao I Care Medical Jessy Nguyen we should expect 2nd wave of H1N1 asking the general public especially the diabetics and pregnant wome were of vulnerable groups and should also be protected. However, she added that vaccine access was a big issue as Vietnam lacked the facilities to manufacture the vaccine. According to Jessy Nguyen, even countries manufacturing the vaccine such as China and Thailand could hardly produce enough vaccines to cover their own population. To lessen the risk of infection during the second wave, Dr Lam advised Malaysians to cut down on travelling." ignored (help)

• Influenza: H1N1 at Curlie
• Pandemic (H1N1) 2009 at the World Health Organization
• International Society for Infectious Diseases PROMED-mail news updates
• H1N1 Flu Resource Centre of The Lancet
• Influenza Research Database – Database of influenza genomic sequences and related information.
• CDC 2009 H1N1 Influenza Vaccine Supply Status
• The H1N1 Pandemic and Global Health Security, Dean Julio Frenk, 2009-09-17
• Graphical Image of the viral makeup of the 2009 pandemic h1n1 virus – NEJM

• Health-EU Portal EU response to influenza
• Pandemic (H1N1) 2009 at the European Centre for Disease Prevention and Control (ECDC)
• European Commission - Public Health EU coordination on Pandemic (H1N1) 2009.
• UK National Pandemic Flu Service
• Official UK government information on swine flu from Directgov
• Official swine flu advice and latest information from the UK National Health Service
• Human/Swine A/H1N1 Influenza Origins and Evolution - Analysis of genetic data for the origin and evolution of swine flu virus.

• Health Canada flu portal
• Pan-American Health Organization (PAHO) Swine Influenza portal
• H1N1 Influenza (Flu) portal at the US Centers for Disease Control (CDC)
• US Government swine, avian and pandemic flu portal
• Medical Encyclopedia Medline Plus: Swine Flu
• Swine Flu Outbreak, Influenza Virus Resource - Sequences and related resources (GenBank, NCBI)

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