This article covers the chronology of the 2009 novel influenza A (H1N1) pandemic. Flag icons denote the first announcements of confirmed cases by the respective nation-states, their first deaths (and other major events such as their first intergenerational cases, cases of zoonosis, and the start of national vaccination campaigns), and relevant sessions and announcements of the World Health Organization (WHO), the European Union (and its agency the European Centre for Disease Prevention and Control), and the U.S. Centers for Disease Control (CDC).

Unless otherwise noted, references to terms like S-OIV, H1N1 and such, all refer to this new A(H1N1) strain and not to sundry other strains of H1N1 which are endemic in humans, birds and pigs.

Timeline Take note that the date of the first confirmations of the disease or any event in a country may be before or after the date of the events in local time because of the International Dateline.

March 2009

Mexico In La Gloria, Veracruz 60% of the town's population is sickened by a respiratory illness of unknown provenance. The government of Mexico believes it to be caused by H3N2 influenza, though at least one patient in La Gloria tested positive for A/H1N1. Two babies died in the outbreak but both were buried without testing.

March 7 United States In the ninth week of its routine influenza surveillance, the CDC reports on FluView that thirty-five states have reported widespread influenza activity, and 14 states have reported regional activity, but that although the rate of activity was high, that the proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 09, 2008-2009 Season . Cdc.gov

March 14 United States The CDC reports on the 10th week of FluView that thirty states reported widespread influenza activity and 18 states reported regional activity.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 10, 2008-2009 Season . Cdc.gov

March 17 Mexico Earliest known onset of a case that is later to be confirmed as Swine-Origin Influenza A (H1N1) Virus Infection.

March 21 United States CDC FluView, Week 11: Widespread influenza activity in twenty-four states; regional activity in 19. Influenza activity continues to decrease.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 11, 2008-2009 Season . Cdc.gov

March 28

United States Earliest known onset of a USA case later confirmed as swine flu, that of a nine-year-old girl residing in Imperial County, California. Thirteen states reported widespread influenza activity and 19 reported regional activity on the CDC's FluView, Week 12.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 12, 2008-2009 Season . Cdc.gov

March 30

United States A sample is collected from a nine-year-old female patient which is later confirmed to contain the novel virus strain (genetically sequenced as A/California/05/2009(H1N1)).

United States Onset of illness for a ten-year-old boy residing in San Diego County, California; his case is eventually the first to be confirmed as swine flu in the US.

April 2009

April 1

United States A nasopharyngeal swab is collected from a ten-year-old male patient in San Diego County, later confirmed as containing the novel virus and the first organism of that strain to be completely sequenced (A/California/04/2009(H1N1)).

April 2

Mexico In La Gloria, Veracruz, a four-year-old boy falls ill at the end of the outbreak. Only his sample, which was eventually sent abroad, tested positive for A(H1N1). Veracruz officials state that there were no plans to exhume the bodies of two infants who died in the outbreak.

April 4 United States CDC FluView, Week 13: Widespread influenza activity in four states, regional activity in 18.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 12, 2008–2009 Season . Cdc.gov

April 5 European Union The media monitoring website MedISys reports on a Mexican article about the epidemiological alert.. Archived 2009-09-10.

April 6

Mexico Public health authorities begin investigating unusual cases of pneumonia. 400 people had reportedly sought treatment for pneumonia/influenza-like illness (ILI) in La Gloria the preceding week.

United States Biosurveillance firm Veratect reports the unusual respiratory illness in Mexico. Veratect publishes the alert "La Gloria: 'Strange' Respiratory Affects 60% of Local Population; Three Pediatric Deaths May be Associated with the Outbreak."

April 11 United States CDC FluView, Week 14: Widespread influenza activity in one state; regional activity in 14.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 12, 2008-2009 Season . Cdc.gov

April 12 Mexico The General Directorate of Epidemiology (DGE) reports the outbreak of an ILI in a small community in Veracruz to the Pan American Health Organization (PAHO), which is the Regional Office of the World Health Organization (WHO). Furthermore, a 39-year-old woman dies of severe viral pneumonia in the city of San Luis Potosí; this is later believed to be the earliest known fatality related to the outbreak.

April 13

Mexico First death in Oaxaca due to what would later be identified as swine flu.

United States The U.S. Centers for Disease Control (CDC) is advised of a ten-year-old boy with a respiratory illness in San Diego County, California. Test results revealed an Influenza A virus but were negative for standard human strains. The San Diego County Health Department is notified.

April 14 United States The CDC receives its first sample from California (from the ten-year-old boy in San Diego County), and identifies the virus as a strain of swine influenza A(H1N1).

April 16

Mexico Authorities notify the PAHO of the atypical pneumonia.

United States Veratect publishes the alert "Atypical Pneumonia Cases Reported at Hospital" regarding the Oaxaca cases.

April 17

Mexico A case of atypical pneumonia in Oaxaca prompts enhanced national surveillance. A field investigation is started. Mexico contacts Canada to request more specialized testing.

United States The CDC receives a second sample from Southern California (taken from the nine-year-old girl in Imperial County), and again identifies the virus as a strain of swine influenza A(H1N1). The California Department of Public Health is notified.

April 18 Mexico Mexico sends 14 mucus samples to the CDC and dispatches health teams hospitals to look for patients showing severe influenza- or pneumonia-like symptoms.

United States CDC FluView, Week 15: "Nine states reported regional activity; 17 states reported local influenza activity; the District of Columbia and 22 states reported sporadic influenza activity; and two states reported no influenza activity. Seven human infections with swine influenza A (H1N1) virus have been confirmed." This is the first mention of A(H1N1) in FluView.CDC - Influenza (Flu)|Weekly Report: Influenza Summary Update Week 15, 2008-2009 Season . Cdc.gov

April 20

United States Veratect advises the CDC of the Mexican events. The CDC is already investigating the California and Texas cases.

April 21

United States The CDC alerts physicians to a similar novel strain of swine influenza A(H1N1) in two cases from Southern California in a Morbidity and Mortality Weekly Report Early Release on its website.Swine Influenza A (H1N1) Infection in Two Children - Southern California, March-April 2009 . Cdc.gov Local investigations, including investigations in Texas, are already underway, and overall surveillance is enhanced. The Associated Press covers the alert, the first mention of the A(H1N1) outbreak in English-language news media.

April 22

Canada Canada receives the samples from Mexico for testing.

April 23

Mexico The Public Health Agency of Canada confirms Mexico cases of swine-origin influenza A (H1N1) virus (S-OIV) infection. Genetic sequence analysis reveals that the Mexican patients were infected with the same S-OIV strain detected in two California children. The PAHO is informed that a cluster in Mexico of severe respiratory illnesses has been laboratory-confirmed as S-OIV infection.

April 24

The WHO issues its first Disease Outbreak Notice on the matter, confirming the infection of a number of people in Mexico and the United States by "Swine Influenza A/H1N1 viruses... not... previously detected in pigs or humans".

Mexico The Minister of Health confirms the Mexican cases of human infection by swine influenza and states that it believes that some of these cases had resulted in death. Health authorities implement public health measures for all airport passengers and the vaccination of health care workers with seasonal influenza vaccine.

United States The CDC tells a press conference that seven of the 14 Mexican samples contained the same virus strain as the known in California and Texas, and that indications suggested that containment in the USA was "not very likely". The novel strain had already been reported on the CDC's Morbidity and Mortality Weekly Report website.

April 25

WHO Under the International Health Regulations (IHR), the Emergency Committee convenes for the first time since its establishment in 2007, resulting in the WHO Director-General declaring a formal "public health emergency of international concern," (PHEIC), the first ever.

The PAHO Vaccination Week In The Americas starts. The 2009 Week was planned to emphasize the vaccination of entire families, and health worker immunization.

United States First closure of an entire school district, the Schertz-Cibolo-Universal City Independent School District outside San Antonio, Texas.

April 26 United States United States declares a Public Health Emergency.

April 27

WHO The Emergency Committee meets for the second time. The WHO Director-General issues a statement that containment of the outbreak is not feasible, and elevates the pandemic alert from Phase 3 to Phase 4.

European Union (EU) Health Commissioner advises Europeans not to travel to the United States or Mexico unless the need is urgent. This follows the first confirmed case in Spain.

Canada First six cases confirmed, four in Nova Scotia and two in British Columbia.

Mexico First seven confirmed deaths

Spain First confirmed case of swine flu, in Almansa, and thus the first case in Europe; A(H1N1) has spread from the WHO Region of the Americas to the WHO European Region.Almansa (Spain) . Silobreaker

( ) United Kingdom First two confirmed cases, in Scotland.

April 28

WHO Confirmed cases are now extant in four of six WHO regions (see map). As of 19:15 GMT seven countries have officially reported cases of swine influenza A(H1N1) infection.

Canada Confirmed: two cases and another four in Alberta and Ontario, respectively.

Israel First confirmed case in Israel and thus the WHO Eastern Mediterranean Region (color-coded yellow), the third region to be affected.

New Zealand First three confirmed cases in New Zealand and thus the WHO Western Pacific Region (color-coded red), the fourth region to be affected.

Spain The second confirmed case in Spain, in Valencia.

April 29

WHO The Emergency Committee meets for the third time, and the WHO raises its pandemic alert level from Phase 4 to Phase 5, its second highest. As of 1800 GMT, nine countries have officially reported 148 cases of swine influenza A(H1N1) infection.

ASEAN ASEAN officials are looking at coordinating measures to address the potential pandemic.

EU Foreign Relations Commissioner Benita Ferrero-Waldner announces that the halt of all travel to Mexico and disinfecting all airports due to the global flu outbreak is being considered.

Austria First confirmed case.

Germany First three confirmed cases, two in Bavaria and one in Hamburg.

Spain