The 2009 pandemic influenza A virus H1N1, the cause of substantial disease worldwide, emerged in the United States in April 2009. During August 30, 2009-January 9, 2010, the World Health Organization (WHO; Geneva, Switzerland) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 310,151 respiratory specimens for influenza viruses. Of those 81,179 (26.2%) were positive, and 80,951 (99.7%) of these were positive for influenza A, and 228 (0.3%) were positive for influenza B. Of the 61,726 influenza A viruses for which subtyping was performed, 61,332 (99.4%) were 2009 H1N1 viruses. Only 29 viruses (<0.1%) were seasonal influenza A (H1), 52 (<0.1%) were influenza A (H3) viruses, and 313 (0.5%) were influenza A, but were not subtyped because of specimen quantity or quality.

The emergence and spread of the 2009 pandemic influenza A virus H1N1 (2009 H1N1) resulted in much higher than usual influenza activity in the United States throughout the summer and fall months of 2009. During this period, influenza activity reached its highest level in the week ending October 24, 2009, with 49 of 50 states reporting geographically widespread disease. By January 9, 2010, overall influenza activity declined substantially. Since April 2009, the dominant circulating influenza virus in the United States was Influenza A virus H1N1 2009.

During the surveillance period declines were observed in 2009 H1N1 influenza activity; however, rates of influenza-related hospitalizations and deaths among persons aged < 65 years during this season were substantially higher than in previous influenza seasons.

The Council of State and Territorial Epidemiologists (CSTE) instituted reporting of 2009 H1N1-confirmed hospitalizations and deaths to the Center for Disease Control and Prevention (CDC; Atlanta, GA, USA). On August 30, 2009, CDC and CSTE instituted modified case definitions for aggregate reporting of influenza-associated hospitalizations and deaths.

CDC antigenically characterized 944 viruses that were 2009 H1N1, one seasonal influenza A virus, seven influenza A (H3N2), and six influenza B viruses collected since September 1, 2009. A total of 942 (99.8%) 2009 H1N1 viruses tested were related to the A/California/7/2009 (H1N1) reference virus selected by WHO as the 2009 H1N1 vaccine virus; only two viruses (0.2%) showed reduced titers with antisera produced against A/California/7/2009.

Surveillance for resistance of circulating influenza viruses to both classes of influenza antiviral medications: adamantanes (amantadine and rimantadine) and neuraminidase inhibitors (zanamivir and oseltamivir) were conducted by CDC.

The largest number of states to date reporting widespread activity occurred during the week ending October 24, 2009, when 49 jurisdictions reported widespread activity. During the week ending January 9, 2010, no jurisdiction reported widespread activity. The early widespread state-specific activity contrasts with the previous three influenza seasons (October to May), when state-specific influenza activity did not reach comparable levels until mid-February or early March.

In the week ending October 24, 2009, the weekly percentage of outpatient visits for influenza-like illness (ILI) reported by the U.S. Outpatient ILI Surveillance Network (ILINet) reached 7.7%, the highest level to date this influenza season. By January 9, 2010, ILI activity had decreased to 1.9%. During the previous three influenza seasons, peak ILI activity occurred later in the season and ranged from 3.5% during the week ending February 17 of the 2006-07 seasons to 6.0% during the week ending February 17 of the 2007-08 season.

During August 30, 2009-January 9, 2010, 1,779 deaths associated with laboratory-confirmed influenza virus infections were reported to CDC. The 1,779 laboratory-confirmed deaths were in addition to the 593 laboratory-confirmed deaths from 2009 H1N1 that were reported to CDC from April through August 30, 2009.

Epidemiologic data in this report support expanded recommendations by CDC that the influenza A (H1N1) 2009 monovalent vaccine be offered to all persons aged 6 months or older, depending on local availability. The report appeared online in CDC's Morbidity and Mortality Weekly Report (MMWR) on January 22, 2010.

Related Links:
World Health Organization
Center for Disease Control and Prevention
National Respiratory and Enteric Virus Surveillance System
Council of State and Territorial Epidemiologists
Outpatient ILI Surveillance Network