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The emergence in April 2009 and subsequent spread of the 2009 pandemic influenza A (H1N1) virus resulted in the first pandemic of the 21st century. This historic event was associated with unusual patterns of influenza activity in terms of the timing and persons affected in the United States throughout the summer and fall months of 2009 and the winter of 2010. The US Influenza Surveillance System identified 2 distinct waves of pandemic influenza H1N1 activity--the first peaking in June 2009, followed by a second peak in October 2009. All influenza surveillance components showed levels of influenza activity above that typically seen during late summer and early fall. During this period, influenza activity reached its highest level during the week ending 24 October 2009. This report summarizes US influenza surveillance data from 12 April 2009 through 27 March 2010.

Diagnostic tests for detecting emerging influenza virus strains with pandemic potential are critical for directing global influenza prevention and control activities. In 2008, the Centers for Disease Control and Prevention received US Food and Drug Administration approval for a highly sensitive influenza polymerase chain reaction (PCR) assay. Devices were deployed to public health laboratories in the United States and globally. Within 2 weeks of the first recognition of 2009 pandemic influenza H1N1, the Centers for Disease Control and Prevention developed and began distributing a new approved pandemic influenza H1N1 PCR assay, which used the previously deployed device platform to meet a >8-fold increase in specimen submissions. Rapid antigen tests were widely used by clinicians at the point of care; however, test sensitivity was low (40%-69%). Many clinical laboratories developed their own pandemic influenza H1N1 PCR assays to meet clinician demand. Future planning efforts should identify ways to improve availability of reliable testing to manage patient care and approaches for optimal use of molecular testing for detecting and controlling emerging influenza virus strains.

The article offers updates on the influenza endemic activity during the 2009-2010 influenza season in the U.S. It notes that the detected second wave of the H1N1 influenza A which peaked in late-October 2009 resulted in higher pediatric mortality and higher hospitalization rates in children and young adults in various U.S. states. It also accounts the highest proportion of outpatient visits to healthcare providers and clinics for influenza-like illness (ILI) in the U.S. during the 2009-2010 influenza season.

The article reports on the number of infections of the pandemic influenza A (H1N1) virus in the U.S. from April to August 2009. It states that the virus activity was highest in May and June 2009 and decreased in the following two months. The Centers for Disease Control and Prevention (CDC) uses several systems to monitor the spread of the virus. The recommendations of the Advisory Committee about immunization programs are also presented.