The 2009 flu pandemic in the United States was caused by a novel strain of the Influenza A/H1N1 virus, commonly referred to as "swine flu", that was first detected on April 15, 2009. While the 2009 H1N1 virus strain was commonly referred to as "swine flu", there is no evidence that it is endemic to pigs (i.e. actually a swine flu) or of transmission from pigs to people; instead, the virus spreads from person to person. On April 25, the World Health Organization declared a public health emergency, followed concurringly by the Obama administration on April 26.

The U.S. Centers for Disease Control and Prevention (CDC) reported that during the outbreak about half of all influenza viruses being reported were 2009 H1N1 viruses, with the other half being those of the regular seasonal influenza. Unique to this particular strain, about 60% of the 2009 H1N1 influenza cases were occurring among people between 5 years and 24 years of age, and 40% of the hospitalizations were occurring among children and young adults. About 80% of the deaths were in people younger than 65 years of age. The CDC noted that this differed greatly from typical seasonal influenza epidemics, during which about 70% to 90% of deaths are estimated to occur in people 65 years and older. Antibody studies showed that children had no existing cross-reactive antibody to the 2009 H1N1 influenza virus, while about one-third of adults older than 60 years of age had cross-reactive antibody.

By April 21, 2009, CDC had begun working to develop a virus that could be used to make a vaccine to protect against the new virus. Following preparation for distribution beginning in June, the first doses were administered in October 2009. On August 10, 2010, WHO declared an end to the global 2009 H1N1 influenza pandemic. However, the virus continues to circulate as a seasonal flu virus, and cause illness, hospitalization, and deaths worldwide every year. From April 12, 2009, to April 10, 2010, the CDC estimates there were 60.8 million cases (range: 43.3 - 89.3 million), 274,304 hospitalizations (range: 195,086 - 402,719), and 12,469 deaths (range: 8868 - 18,306) in the United States due to the virus.

A follow-up study done in September 2010 showed that the risk of serious illness resulting from the 2009 H1N1 flu was no higher than that of the yearly seasonal flu. For comparison, the CDC estimates the global H1N1 death toll at 284,000 and the WHO estimates that 250,000 to 500,000 people die of seasonal flu annually.

History The earliest reported cases in the US began appearing in early April 2009, in California. In late April, the Centers for Disease Control and Prevention (CDC) activated its Emergency Operations Center and declared a public health emergency. On April 25, the World Health Organization (WHO) declared a public health emergency of international concern. WHO declared H1N1 a pandemic on June 11.

By the end of May, the flu had infected people in all 50 states. As of June 16, the total number of confirmed cases was 27,717 and on June 25, the CDC said there were over one million (1,000,000) cases, most of which had not been reported or diagnosed. Deaths relating to influenza began appearing in the US in late April, and by early June, 15 states had reported fatalities related to or directly occurring from the virus.

By October 5, the first doses of an H1N1 vaccine were given in the U.S. The CDC distributed vaccines for the flu using mechanisms already in place for its Vaccines for Children (VFC) program.

On October 24, and the CDC said more than 1,000 had died from the flu. President Obama declared a national emergency. On December 10, 2009, the CDC reported an estimated 50 million Americans or 1 in 6 had been infected and 10,000 had died. On December 23, 2009, the CDC reported a reduction of the disease by 59% percent. On February 12, the CDC reported 57 million Americans had been sickened, 257,000 had been hospitalized and 11,690 people had died (including 1,180 children) due to flu from April through to mid-January.

Initial cases The Centers for Disease Control and Prevention (CDC) identified the first two A/09(H1N1) swine flu cases in California on April 17, 2009, via the Border Infectious Disease Program, for a San Diego County child, and a naval research facility studying a special diagnostic test, where influenza sample from the child from Imperial County was tested. By April 21, enhanced surveillance was established to search for additional cases in both California and Texas and the CDC determined that the virus strain was genetically similar to the previously known A(H1N1) swine flu circulating among pigs in the United States since about 1999.

It was established that the virus was a combination of human, North American swine, and Eurasian swine influenza viruses; the viruses from the initial two Californian cases were also noted to be resistant to amantadine and rimantadine, two common influenza antiviral drugs. No contact with pigs was found for any of the seven Californian nor either of the two Texas cases, suggesting human-to-human transmission of the virus.

On April 28, 2009, the director of the Centers for Disease Control and Prevention confirmed the first official US death of swine flu. Tests confirmed that a 23-month-old toddler from Mexico, who was probably infected there, died on April 27 from the flu while visiting Texas.

Outbreak across the US

Cases of H1N1 spread rapidly across the United States, with particularly severe outbreaks in Texas, New York, Utah, and California. Early cases were associated with recent travel to Mexico; many were students who had traveled to Mexico for spring break. On May 4, 2009, the CDC reported one death, 286 confirmed cases of H1N1 flu across 36 states, 35 hospitalizations, and expected H1N1 to eventually spread to all states. A large number of cases, according to medics, have happened in the days that preceded the launch of the alert and came out only in these days due to a massive backlog. By May 5, 2009, the number had risen to 403 and a second death was reported in Texas. The CDC and government officials had expressed cautious optimism about the severity and spread of H1N1.

Changes in surveillance of cases of influenza-like illness, including new guidelines for identifying cases to test, increased laboratory testing, and new test kits able to distinguish this novel strain, resulted in a spike in the percent of cases tested positive for influenza. Of the positive cases, about a third were due to the novel strain. Also found were a substantial number of cases where the strain could not be subtyped.

The proportion of US deaths due to pneumonia and influenza climbed above the epidemic threshold in the 2007–2008 winter flu season but not in the 2008–2009 season. Although the 2009 H1N1 outbreak reached epidemic levels of infection early in 2009, it did not contribute to epidemic levels of pneumonia and influenza related deaths until October 2009.

2009 US Swine Flu Summary Number of Confirmed Cases/Deaths See Table Above Number of States/Territorieswith Reported Cases 56 Number of States/Territorieswith Confirmed Deaths 55 Earliest Confirmed Infection in US March 28, 2009 First Death Inside the US April 27, 2009 First Death of US Citizen May 5, 2009Number of People Hospitalized 9,079 (as of September 3)Fatalities 593 (as of September 3)

The second wave In early October 2009, the Centers for Disease Control and Prevention announced that swine flu was widespread across the country. It also said there was significant flu activity in virtually all states, which was considered to be quite unusual for this time of year. There was particular worry about pregnant women. As of late August, 100 had been hospitalized in intensive care units and 28 had died since the beginning of the outbreak in April. On October 1, it was acknowledged that a recruit in basic training in Fort Jackson, South Carolina, was the Army's first swine flu death. The recruit fell ill on September 1 and died of pneumonia on September 10.

Dell Children's Medical Center in Austin, Texas, erected two tents in its parking lot to handle emergency room visits, and hospitals around Colorado Springs recorded a 30 percent spike in flu visits. As pediatric cases were increasing, the Dept. of Health and Human Services released 300,000 courses of children's liquid Tamiflu from the national pandemic stockpile in late September, with the first batches sent to Texas and Colorado.

In late September, the disease centers reported that 936 had died of flu symptoms or of flu-associated pneumonia since August 30, when it began a new count of deaths, including some without laboratory-confirmed swine flu.

The Agriculture Department reported on October 16 that three pigs at the Minnesota State Fair, in St. Paul, were tested positive in late August for H1N1 virus, which were the first cases in the country, although infected pigs had been found in eight other countries. There were 103 pigs tested at the Fair, including the three infected, though all appeared healthy. Scientists said the virus was already spreading widely among people, and, in fact, was more common in humans than in pigs, so humans were more likely to catch it from others than from pigs.

In mid-October, it was reported that flu caused by the H1N1 virus was widespread in 41 states, and flu-like illnesses accounted for 6.1 percent of all doctor visits, which was considered high, particularly for October. Forty-three children had died from H1N1 since August 30, which is approximately the number that usually dies in an entire flu season. Nineteen of the forty-three were teenagers while sixteen were between ages five to eleven. The rest were under five. It is reported that the severity of the disease was not increasing. About fifteen to twenty percent of the patients hospitalized for the flu were placed in the intensive care unit, a level similar to that for seasonal flu.

Projections of the supply of H1N1 vaccine had decreased significantly from a level of 120 million doses ready in October, estimated during the summer, to an estimate of 28 to 30 million doses by the end of the month. On October 14, 11.4 million doses of the H1N1 vaccine were said to be available. As of November 20, 2009, the CDC reported sharp declines in H1N1 activity throughout the United States, with influenza-like illness (which may also include meningitis, pneumonia, strep pharyngitis, gastroenteritis, and the common cold) accounting for 5.5% of doctors visits, down sharply from 8% in late October, the peak of the second wave. However, taking the vaccine is still urged by the CDC, as a third wave of the disease may sweep across the US, possibly in January/February 2010. As of December 24, the second wave of H1N1 has clearly peaked, with pneumonia and influenza deaths falling below the epidemic threshold for the first time in 11 weeks, and the proportion of doctors visits due to influenza-like illness falling to baseline (2.3%), down from 5.5% 1 month before, on November 20. However, it was reported that influenza activity was beginning to increase in West Virginia, with 5.2% of patients treated by West Virginia health care providers having influenza-like illness, a major increase from 2% of patients treated by West Virginia health care providers having influenza-like illness in November.

On August 10, 2010, WHO declared an end to the global 2009 H1N1 influenza pandemic. However, the virus continues to circulate as a seasonal flu virus, and cause illness, hospitalization, and deaths worldwide every year.

H1N1 characteristics and severity