What were the characteristic of the 2009-2010 H1N1 influenza (swine flu) outbreak?

Updated: May 18, 2020
  • Author: Michael Stuart Bronze, MD; Chief Editor: Russell W Steele, MD  more...
  • Print

Human cases of influenza A (H1N1) were reported worldwide. In 2009, cases of influenzalike illness were first reported in Mexico on March 18; the outbreak was subsequently confirmed as H1N1 influenza A. [9] Suspected clinical cases had been reported in 19 of the country's 32 states, although only 97 of the Mexican cases had been laboratory-confirmed as Influenza A/H1N1 [10] (12 of them genetically identical to Influenza A/H1N1 viruses from California [9] ). As of May 5th, 2009, nearly 600 H1N1 influenza cases had been confirmed in Mexico, including 25 deaths. [11]

On April 17, 2009, the CDC determined that two cases of febrile respiratory illness in children who resided in adjacent counties in southern California were caused by infection with a swine influenza A (H1N1) virus. [12] By April 26, 2009, the US Department of Health and Human Services declared a national public health emergency involving H1N1 influenza A, citing its significant potential to affect national security. [13] By June 25, 2009, 27,717 lab-defined cases of H1N1 influenza had been confirmed in the United States. [10, 14, 15, 16]

Estimates in the United States for the first 6 months of the pandemic reported approximately 22 million people in the United States became ill from the H1N1 influenza, nearly 100,000 were hospitalized, and about 3900 died. Deaths include an estimated 540 children younger than 18 years, 2900 adults aged 18-64 years, and about 440 elderly individuals. These estimates are from the CDC's Emerging Infection Program, rather than using only laboratory-confirmed cases. [17]

For an updated tally and case counts in specific states, see the CDC's H1N1 Flu (Swine Flu) Web page.

On June 11, 2009, WHO raised the pandemic alert level to phase 6 (indicating a global pandemic) because of widespread infection beyond North America to Australia, the United Kingdom, Argentina, Chile, Spain, and Japan. [10] As of September 1, 2009, the World Health Organization (WHO) reported that H1N1 influenza had been confirmed in over 200,000 people in more than 100 countries and that they are aware of at least 2185 confirmed deaths. For an updated tally of affected countries and counts, see WHO's Influenza A (H1N1) Web page.

On October 24, 2009, President Obama declared the 2009 H1N1 influenza pandemic a national emergency, explaining that "...the rapid increase in illness across the Nation may overburden health care resources and that the temporary waiver of certain standard Federal requirements may be warranted in order to enable U.S. health care facilities to implement emergency operations plans, the in the United States 2009 H1N1 influenza pandemic constitutes a national emergency." This declaration made way for waiving certain requirements of the Medicare, Medicaid, and State Children's Health Insurance programs and of the Health Insurance Portability and Accountability Act Privacy Rule throughout the duration of the public health emergency. [18]

Government and public health officials are monitoring this situation worldwide to assess the threat from H1N1 influenza and to provide guidance to health care professionals and the public. Because the situation is changing rapidly, it is important to check regularly for changes in recommendations as new information becomes available. Online resources for daily guidance include the Centers for Disease Control and Prevention (CDC), World Health Organization (WHO), and Medscape's H1N1 Influenza A (Swine Flu) Alert Center.

Domínguez-Cherit et al (2009) conducted an observational study of consecutive critically ill patients in Mexican hospitals that treated most patients with confirmed, probable, or suspected H1N1 influenza during the 2009 epidemic. Critical illness occurred in 58 (6.5%) of 899 patients. Median age of critically ill patients was 44 years (range, 10-83 y). All presented with fever, and all but one with respiratory symptoms. Few patients had comorbid respiratory disorders, but 36% were obese. All patients but 2 received mechanical ventilation for severe acute respiratory distress syndrome and refractory hypoxemia. By 60 days, 24 patients had died (41.4%; 95% confidence interval, 28.9%-55%). Patients who died had greater initial severity of illness, worse hypoxemia, higher creatine kinase and creatinine levels, and ongoing organ dysfunction. Treatment with neuraminidase inhibitors was associated with improved survival (odds ratio, 8.5; 95% confidence interval, 1.2-2.8). [19]

See Medscape's H1N1 influenza algorithm adaptation for guidance in managing suspected cases.

A WHO report on the 2009 influenza pandemic indicated that nearly all countries reported cases of H1N1 virus infection, with more than 17,000 deaths worldwide. In the United States, the number of clinical illnesses was estimated at 59 million, 265,000 hospitalizations, and 12,000 deaths. [20] . H1N1 strains are still prevalent in India, whose Health Ministry has reported over 2500 deaths to date.

In the second half of 2011, a novel swine influenza virus emerged. Twelve cases from 5 states were reported by the CDC in January 2012. The new strain, dubbed A (H3N2)v, includes a gene from the human pandemic strain and affects mostly children. In 3 of the 5 states where the virus emerged (Pennsylvania, Maine, and Indiana), the virus was a result of pig-to-human transmission. [21] According to the Department of Health and Human Services (HHS), a precautionary vaccine against this variant is in development and will likely be ready for clinical trials in the spring of 2012.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!