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I've read that

"It's good to remember that when H1N1 influenza came out in 2009, estimates of case fatality were 10 per cent," said David Fisman, an epidemiologist at the University of Toronto, who was working in public health at the time. "That turned out to be incredibly wrong."

"As the denominator is growing in terms of case numbers, and case fatality goes down and down ... you start to realise it's everywhere," he said.

But it's not clear from that in what time frame precisely was the H1N1 fatality rate overestimated right after the outbreak. Was it days, weeks?

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    From what I remember, the "panic" was worldwide and was there for months. They were aware of number of "deaths" before they were aware of lot of mild "cases." Most people infected with H1N1 probably never saw a doctor for that, so most "cases" were not documented, and the current estimation of fatality ratio may still be exaggerated. – Jan Feb 10 '20 at 7:56
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If based on WHO situation reports, an estimate of 10% for the case fatality rate for pandemic influenza A(H1N1) 2009 lasted about five to six days, not weeks and not months. This does not mean that a 10% case fatality rate was erased from press reports or from public perception about the virus even after WHO reported substantially lower case fatality rates.

The novel influenza A (H1N1) virus that caused the 2009 influenza pandemic—eventually named the swine flu-origin influenza A virus (S-OIV)--was identified in late March 2009 in Mexico based on an outbreak in Mexico.

By April 24, 2009, WHO had established a surveillance system for the novel H1N1 virus. It issued its first Situation Report about the spread of the virus and deaths in cases attributed to the virus (early report) and eventually confirmed to have been infected with the virus on April 24, 2009. “Influenza-like illness in the United States and Mexico.” https://www.who.int/csr/don/2009_04_24/en/

This report included information about the number of confirmed cases of the novel influenza/H1N1 virus and deaths in the United States and the number of cases of influenza-like illness and deaths in three places in Mexico. There were seven confirmed and nine suspect cases in the United States with no deaths. In Mexico City, there were 854 cases of pneumonia with 59 deaths; in central Mexico, there were 24 cases of influenza-like illness with 3 deaths; in Mexicali, there were four cases of influenza-like illness with no deaths.

On April 24, 2009, the case fatality rate for confirmed and suspect cases of H1N1 in the United States was 0%. The “fatality rate” in Mexico City for pneumonia cases of 6.9%. The “fatality rate” for cases of influenza-like-illness was 12.5% in central Mexico and 0% in Mexicali. The overall “fatality rate” in Mexico was 7.0%.


Two days later, on April 26, WHO updated information about swine flu illness. “Swine flu illness in the United States and Mexico - update 2”. https://www.who.int/csr/don/2009_04_26/en/

There were 20 laboratory confirmed cases in the United States, with no deaths. There 18 laboratory confirmed cases in Mexico. Deaths were not reported for Mexico.

Because fatalities in Mexico were not reported, it is not possible to estimate the case fatality rate on April 26, 2009.

26 April 2009 - As of 26 April 2009, the United States Government has reported 20 laboratory confirmed human cases of swine influenza A/H1N1 (8 in New York, 7 in California, 2 in Texas, 2 in Kansas and 1 in Ohio). All 20 cases have had mild Influenza-Like Illness with only one requiring brief hospitalization. No deaths have been reported. All 20 viruses have the same genetic pattern based on preliminary testing. The virus is being described as a new subtype of A/H1N1 not previously detected in swine or humans.

Also as of 26 April, the Government of Mexico has reported 18 laboratory confirmed cases of swine influenza A/H1N1. Investigation is continuing to clarify the spread and severity of the disease in Mexico. Suspect clinical cases have been reported in 19 of the country's 32 states.


On April 27, 2009, WHO issued “Swine Influenza Update 3.”
https://www.who.int/csr/don/2009_04_27/en/

The United States reported 40 laboratory confirmed cases with no deaths. Mexico reported 26 confirmed cases with seven deaths. Canada reported 6 cases with no deaths. Spain reported 1 case with no deaths.

On April 27, 2009, the case fatality rates among confirmed cases were 0% in the United States, 26.9% in Mexico, 0% in Canada, and 0% in Spain.

On April 27, 2009, the overall case fatality rate in confirmed cases was 9.6% (7/73) with all deaths in Mexico. This is the first WHO estimate of the case fatality rate of the novel swine influenza virus based on confirmed cases.

27 April 2009 -- The current situation regarding the outbreak of swine influenza A(H1N1) is evolving rapidly. As of 27 April 2009, the United States Government has reported 40 laboratory confirmed human cases of swine influenza A(H1N1), with no deaths. Mexico has reported 26 confirmed human cases of infection with the same virus, including seven deaths. Canada has reported six cases, with no deaths, while Spain has reported one case, with no deaths.


The next day, April 28, 2009, WHO issued “Swine Influenza update 4.” https://www.who.int/csr/don/2009_04_28/en/

There were reports from seven countries with confirmed cases and deaths—the United States (64 cases with no deaths), Mexico (26 cases with 7 deaths), Canada (6 cases with no deaths), New Zealand (3 cases with no deaths), the United Kingdom (2 cases with no deaths), Israel (2 cases with no deaths) and Spain (2 cases with no deaths).

On April 28, 2009, the case fatality rate was 6.7% (7/105), with all deaths in Mexico.

28 April 2009--The situation continues to evolve rapidly. As of 19:15 GMT, 28 April 2009, seven countries have officially reported cases of swine influenza A/H1N1 infection. The United States Government has reported 64 laboratory confirmed human cases, with no deaths. Mexico has reported 26 confirmed human cases of infection including seven deaths.

The following countries have reported laboratory confirmed cases with no deaths - Canada (6), New Zealand (3), the United Kingdom (2), Israel (2) and Spain (2).


The next day, April 29, WHO issued its first report referring specifically to Influenza A(H1N1). This was “Influenza A(H1N1) - update 5.” https://www.who.int/csr/don/2009_04_29/en/

Data were reported from nine countries. There were a total of 148 cases of swine influenza A/H1N1. One death was reported in the United States and there were seven from Mexico. There were no deaths in the other seven countries.

On April 29, 2009, the case fatality rate for confirmed influenza A(H1N1) was 5.4%.

29 April 2009 - The situation continues to evolve rapidly. As of 18:00 GMT, 29 April 2009, nine countries have officially reported 148 cases of swine influenza A/H1N1 infection. The United States Government has reported 91 laboratory confirmed human cases, with one death. Mexico has reported 26 confirmed human cases of infection including seven deaths.

The following countries have reported laboratory confirmed cases with no deaths - Austria (1), Canada (13), Germany (3), Israel (2), New Zealand (3), Spain (4) and the United Kingdom (5).


On May 2, 2009 WHO issued ”Influenza A(H1N1) - update 9.” This update came just 5 days after the first WHO estimate of the case fatality rate for influenza A(H1N1) based on confirmed cases. https://www.who.int/csr/don/2009_05_02/en/

15 countries reported 615 cases of influenza A(H1N1) with 17 deaths.

On May 2, 2009, the case-fatality rate was 2.8%.

2 May 2009 - The situation continues to evolve. As of 06:00 GMT, 2 May 2009, 15 countries have officially reported 615 cases of influenza A(H1N1) infection.

Mexico has reported 397 confirmed human cases of infection, including 16 deaths. The 241 rise in cases from Mexico compared to 23:30GMT of 1 May reflects ongoing testing of previously collected specimens. The United States Government has reported 141 laboratory confirmed human cases, including one death.

The following countries have reported laboratory confirmed cases with no deaths - Austria (1), Canada (34), China, Hong Kong Special Administrative Region (1), Denmark (1), France (1), Germany (4), Israel (2), Netherlands (1), New Zealand (4), Republic of Korea (1), Spain (13), Switzerland (1) and the United Kingdom (13).


WHO began to label its influenza A(H1N1) situation reports as pandemics reports on July 1, 2009 when update 56 was issued. “Pandemic (H1N1) 2009 - update 56.” https://www.who.int/csr/don/2009_07_01a/en/

On that date, worldwide, there were 77,201 cumulative total laboratory-confirmed cases of pandemic (H1N1) 2009 with 332 deaths.

On July 1, 2009, the case fatality rate was 0.4%.

1 July 2009. Laboratory-confirmed cases of pandemic (H1N1) 2009 as officially reported to WHO by States Parties to the International Health Regulations (2005)

Table Cumulative Total Cases 77201 with 332 deaths

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