2009 H1N1 Influenza -- Just the Facts: Clinical Features and Epidemiology

John G. Bartlett, MD


November 23, 2009

Editor's Note: This article will be updated frequently, so check back often for new information. On October 15, updates were made to weekly US influenza data (including college data) and reports of bacterial coinfections. Influenza hospitalization data were added as well. On October 27, updates were made about the declaration of a national emergency, weekly US and international H1N1 activity, deaths in children and pregnant women, and treatment outcomes. Updates made on November 5 include information on the true prevalence of H1N1, pediatric deaths, and underlying conditions in children hospitalized with H1N1 influenza. Updates made on November 16 include new information on the global H1N1 pandemic, the H1N1 situation on college campuses, the effect of sick-call policies on the spread of influenza, and characteristics of patients hospitalized with H1N1 infection. On November 23, updates were made about emergency use authority for pandemic supplies, guidance for homeless shelters, a summary of the article "One Killer Virus, Three Key Questions," studies on reduced deaths in patients receiving statins, and the role of cytokines in mediating the symptoms of influenza. For the latest information on 2009 H1N1 influenza, please go to the H1N1 Alert Center.


Novel Influenza A (H1N1) Timeline

February 24, 2009 Patient zero is said to be a 6-month-old girl from northern Mexico, according to Celia Alpuche of the Institute of Epidemiological Diagnosis and Reference in Mexico City. (Cohen J. Swine flu outbreak, day by day. ScienceInsider. July 17, 2009. Available at: http://blogs.sciencemag.org/scienceinsider/special/swine-flu-timeline.html Accessed September 16, 2009.)
March 3, 2009 Initial recognition of case in Mexico City with multiple cases reported on March 18.
April 6, 2009 Outbreak in La Gloria, Mexico, with attack rate of 60%.
April 15, 2009 First virologically defined cases and first recognized US case: 10-year-old boy in California with positive test for influenza H1 antigen but negative for seasonal H1 and H3.
April 26, 2009 United States declares public health emergency.
April 29, 2009 Mexican Ministry of Health reports 1-month total of 2155 patients with severe pneumonia and 100 deaths.
May 9, 2009 Global epidemic recognized with caseloads that matched international air-traffic patterns from Mexico City. (Khan K, Arino J, Hu W, Raposo P, et al. Spread of a novel influenza A (H1N1) virus via global airline transportation. N Engl J Med. 2009;361:212-214. Available at: http://content.nejm.org/cgi/content/full/361/2/212 Accessed September 16, 2009.)
June 11, 2009 Dr. Margaret Chan, Director General of the World Health Organization (WHO), declares phase 6 pandemic and calls 2009 H1N1 "unstoppable"; also notes that most patients in the world with 2009 H1N1 are younger than 25 years of age and that one third of serious cases are in previously healthy young people.
July 1, 2009 US cases appear in all states; estimated total is more than 1 million infected; 87% of deaths in persons 5-59 years of age.
July 17, 2009 WHO reports 94,512 virologically confirmed cases and 429 deaths, but considers this the "tip of the iceberg." A decision is made to stop counting cases.(World Health Organization. Chan M. World now at the start of 2009 influenza pandemic. June 11, 2009. Available at: http://www.who.int/mediacentre/news/statements/2009/
h1n1_pandemic_phase6_20090611/en/index.html Accessed September 16, 2009
July 20, 2009 Chile reports 2009 H1N1 in turkeys, increasing sources of the virus and introducing the possibility of mixing with avian genes.
August 25, 2009 President's Council of Advisors anticipates that 2009 H1N1 may infect half of the US population, require 1.8 million hospitalizations, and result in 30,000-90,000 deaths. (President's Council of Advisors on Science and Technology. U.S. Preparations for 2009-H1N1 Influenza. August 7, 2009. Available at: http://www.whitehouse.gov/assets/documents/PCAST_H1N1_Report.pdf Accessed September 16, 2009.)
August 31, 2009 WHO predicts that within 2 years nearly one third of the world population will have been infected. (Chan M. Swine flu spreading at 'unbelievable' rate: WHO chief. Khaleej Times. August 29, 2009. Available at: http://www.khaleejtimes.com/displayarticle.asp?xfile=data/international/2009/August/international_August2077.xml&
section=international&col= Accessed September 25, 2009.)
Seasonal flu is usually acquired by 5%-20% per season with 200,000 are hospitalized and an average of 36,000 die. (CDC. Key facts about seasonal influenza (flu). Available at: www.cdc.gov/flu/keyfacts.htm Accessed September 25, 2009.)
October 24, 2009 President Obama declares H1N1 influenza a state of national emergency. This declaration waives certain regulatory requirements for healthcare facilities in response to emergencies. (FluView. President Obama signs emergency declaration for H1N1 flu. October 24, 2009. Available at: http://www.flu.gov/professional/federal/h1n1emergency10242009.html Accessed October 26, 2009.)

Emergency Use Authority and 2009 H1N1 Influenza

(Sherman SE, Foster J, Vaid S. Emergency use authority and 2009 H1N1 influenza. Biosecurity Bioterrorism 2009;7:245-250.)

  • Purpose: To persuade the FDA to allow products to be introduced to interstate commerce without the usual regulatory requirements for approval, clearance or licensing. This emergency use authorization (EUA) is justified by a relevant emergency and lasts during the emergency.

  • Process: Step 1 -- Determination of an emergency by Secretary of HHS, Department of Homeland Security, or Department of Defense; Step 2 -- Declaration of emergency by Secretary of HHS; Step 3 -- Issuance of EUA; Step 4 -- The FDA Commissioner then exercises this authority after consulting with directors of the NIH and CDC.

  • Requesting an EUA: Any entity may request emergency use of a product under the EUA by a pre-EUA submission to the FDA. This may apply to products that are not FDA approved, cleared, or licensed, or this may apply to new use of an approved product.

  • EUAs for 2009 H1N1: On April 26, 2009, the HHS Secretary determined that this influenza was a public health emergency and there have subsequently been 3 EAUs for diagnostics, antivirals, and masks, as summarized below (Table 1).

Table 1. EAUs Related to H1N1

Product Purpose Date
CDC rRT-PCR Flu Panel
Focus Diagnostics

Permitted CDC to distribute test
Cleared test for additional specimens
Permitted this test by qualified labs



Authorizes use in children
< 1 yr and dosing changes in children > 1 yr
Allows use with 5 symptoms > 48 hrs

Allows use with symptoms > 48 hrs


Disposable N95 respirators

Allows use of 15 types of N95 respirators from the National Stockpile by general public

  • Conclusion: The EUA permits use of new products necessary to respond efficiently to an emergency. Pandemic influenza is a good illustration of appropriate use.


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