2009 H1N1 Pandemic Timeline

In 2009, a new H1N1 influenza virus emerged, causing the first global flu pandemic in 40 years. Below is a timeline of major events that took place during the 2009 H1N1 pandemic.

  • image of h1n1 virus
    April 15
    • First human infection with new influenza A H1N1 virus detected in California.
  • April 17
    • Second human infection with the new influenza A H1N1 virus detected in California about 130 miles away from first infection, with no known connection to previous patient.
  • April 18
    • First novel 2009 H1N1 flu infections were reported by CDC to the World Health Organization (WHO) through the U.S. International Health Regulations Program.
  • April 21
    • CDC publicly reported the first two U.S. infections with the new H1N1 virus.
    • CDC began working to develop a candidate vaccine virus.
  • room with people on computers
    April 22
    • CDC activated it’s Emergency Operations Center (EOC).
  • April 23
    • Two additional human infections with 2009 H1N1 were detected in Texas, transforming the investigation into a multistate outbreak and response.
  • person in lab using equipment
    April 24
    • CDC uploaded complete gene sequences of new H1N1 2009 virus to a publically-accessible international influenza database.
  • April 25
    • The World Health Organization (WHO) declared a public health emergency of international concern.
  • person watching television
    April 26
    • The United States Government declared 2009 H1N1 a Public Health Emergency of International Concern and CDC began releasing 25% of antiviral drugs needed to treat this new influenza virus from the federal stockpile.
  • April 27
    • WHO Director-General raised the level of influenza pandemic alert from phase 3 to phase 4, based on data showing person-to-person spread and the ability of the virus to cause community-level outbreaks.
  • April 28
    • The Food and Drug Administration (FDA) approved a new CDC test to detect 2009 H1N1 infections
    • CDC issued the first CDC Interim Guidance on Closing Schools and Childcare Facilities, recommending a 7-day dismissal in affected schools and childcare facilities with laboratory-confirmed cases of influenza A H1N1 virus.
  • April 29
    • WHO raised the level of influenza pandemic alert from phase 4 to phase 5, signaling that a pandemic was imminent, and requested all countries to immediately activate their pandemic preparedness plans and be on high alert for unusual outbreaks of influenza-like illness and severe pneumonia.
  • May
    • 2009 H1N1 influenza summer activity peaked in the United States during May and June.
  • H1N1 lab test
    May 1
    • Domestic and global shipments of new CDC test to detect 2009 H1N1 began.
    • CDC updated the CDC Interim Guidance on Closing Schools and Childcare Facilities, recommending affected communities with lab-confirmed cases of influenza A H1N1 consider adopting school dismissal and childcare closing measures, including closing for up to 14 days depending on the extent and severity of influenza illness.
  • May 4
    • CDC shifted from reporting confirmed cases of 2009 H1N1 to reporting both confirmed and probable cases of 2009 H1N1.
  • May 5
    • Peak school dismissal day in the spring phase of the pandemic. 980 schools were dismissed, affecting 607,778 students.
  • May 6
    • CDC distributed updated recommendations for the use of influenza antiviral medicines to provide guidance for clinicians in prescribing antiviral medicines for treatment and prevention (chemoprophylaxis) of 2009 H1N1 influenza.
  • May 8
    • CDC issued an MMWR updating the 2009 H1N1 influenza situations in Mexico, the United States, and worldwide.
  • May 12
    • CDC reported early data on 2009 H1N1 illness among pregnant women in an MMWR.
  • press conference
    June 11
    • The World Health Organization (WHO) declared a pandemic and raised the worldwide pandemic alert level to phase 6, which means the virus was spreading to other parts of the world.
    • CDC held its first press conference with former CDC Director Thomas Frieden, MD, MPH. The press conference had 2,355 participants.
  • FluView Map
    June 19
    • All 50 states, the District of Colombia, Puerto Rico and the U.S. Virgin Islands had reported cases of 2009 H1N1 infection.
    • By late-June, more than 30 summer camps in the U.S. had reported outbreaks of 2009 H1N1 influenza illness. CDC released guidance for day and residential camps to reduce spread of influenza.
  • June 25
    • CDC estimated at least 1 million cases of 2009 H1N1 influenza had occurred in the United States.
  • Early July
    • Reported cases of 2009 H1N1 nearly doubled since mid-June 2009.
    • Three 2009 H1N1 influenza viruses that were resistant to the antiviral drug, oseltamivir, were detected in three countries.
  • July 10
    • CDC reported findings in an MMWR that indicated a large prevalence of obesity in intensive care patients with confirmed 2009 H1N1 influenza infection.
    • After mid-July, 2009 H1N1 influenza activity declined in most countries.
  • person filling out paperwork
    July 22
    • Clinical trials testing the 2009 H1N1 flu vaccine began.
  • August
    • Additional oseltamivir-resistant 2009 H1N1 viruses were detected by CDC.
  • August 3
    • CDC School Dismissal Monitoring System (SDMS) activated.
  • August 19
    • CDC Guidance for Businesses and accompanying toolkit posted to CDC.gov.
  • swine flu action center
    August 20
    • CDC Guidance for Institutions of Higher Education (IHE) and accompanying toolkit posted to CDC.gov. Calls were conducted with Secretary Duncan and Sebelius to explain guidance. Press briefings followed.
    • Second wave of 2009 H1N1 influenza activity began in the U.S.
  • August 30
    • New reporting season for the 2009-2010 influenza season began.
  • H7N9 reagent kits
    September 1
    • More than 1,000 test kits shipped to 120 domestic and 250 international laboratories in 140 countries since May 1, 2009.
  • September 3
    • CDC published a study that analyzed data related to H1N1 influenza pediatric deaths reported to CDC from April to August 2009 in MMWR. Data showed 477 deaths with lab-confirmed 2009 H1N1 flu in the U.S. had been reported to CDC as of August 8, 2009.
  • September 10
    • HHS secretary and CDC Director joined the National Foundation for Infectious Diseases (NFID) in a news conference to stress the importance of getting vaccinated for the upcoming influenza season.
  • September 15
    • The FDA announced its approval of four 2009 H1N1 influenza vaccines.
  • September 30
    • U.S. states placed first orders of 2009 H1N1 vaccine.
  • October
  • October 5
    • First doses of H1N1 vaccine were given in the U.S.
  • October 24
    • Influenza activity reached its highest level in the reporting week ending October 24, 2009, with 48 of 50 states reported widespread activity.
  • Late October
    • Second wave of H1N1 flu activity peaked in the U.S.
  • November 12
    • CDC released its first estimates official estimates of 2009 H1N1 cases, hospitalization and deaths.
  • November 16
    • FDA announced its approval of a fifth 2009 H1N1 vaccine.
  • November 23
    • No school closures throughout United States; first time since 8/25/2009.
  • December
    • Results of trials conducted among adults were published in December, and the data indicated that the immune response among vaccinated adults was excellent.
  • December 4
    • CDC published preliminary safety results for the 2009 H1N1 vaccines for the first months of reports received through the U.S. Vaccine Adverse Event Reporting System (VAERS).
  • December 18
    • First 100 million doses of 2009 H1N1 vaccine were available for ordering.
  • Late December
    • 2009 H1N1 vaccination had been opened up to anyone who wanted it.


  • January
    • Activity declined to levels below baseline, but persisted for several more months at lower levels.
  • January 10-16
    • President of the United States proclaimed National Influenza Vaccination Week (NIVW) and encouraged all Americans to observe the week by getting vaccinated with the 2009 H1N1 flu vaccine.
  • February
    • FDA’s VRBPAC selected 2009 H1N1 virus for inclusion in 2010-2011 seasonal flu vaccine
  • February 18
    • WHO published recommendations for the composition of influenza virus vaccines for the upcoming Northern Hemisphere influenza season. Components included a 2009 H1N1-like virus.
  • CDC telebriefing
    • Between April 2009 and April 2010, CDC held 60 related media events – 39 press briefings and 22 telebriefings – reaching more than 35,000 participants.
  • August 11
    • WHO announced the end of 2009 H1N1 influenza pandemic.