Influenza activity - United States, 2013-14 season and composition of the 2014-15 influenza vaccines

Abstract

During the 2013-14 influenza season in the United States, influenza activity increased through November and December before peaking in late December. Influenza A (H1N1)pdm09 (pH1N1) viruses predominated overall, but influenza B viruses and, to a lesser extent, influenza A (H3N2) viruses also were reported in the United States. This influenza season was the first since the 2009 pH1N1 pandemic in which pH1N1 viruses predominated and was characterized overall by lower levels of outpatient illness and mortality than influenza A (H3N2)-predominant seasons, but higher rates of hospitalization among adults aged 50-64 years compared with recent years. This report summarizes influenza activity in the United States for the 2013-14 influenza season (September 29, 2013-May 17, 2014†) and reports recommendations for the components of the 2014-15 Northern Hemisphere influenza vaccines.

FIGURE 1

Number* and percentage of respiratory specimens testing positive for influenza, by type, subtype, surveillance week, and year — World Health Organization and National Respiratory and Enteric Virus Surveillance System collaborating laboratories, United States, 2013–14 influenza season^{† *} N = 53,470. ^† Data reported as of May 30, 2014.

FIGURE 2

Percentage of visits for influenza-like illness (ILI)* reported to CDC, by surveillance week — Outpatient Influenza-Like Illness Surveillance Network, United States, 2013–14 influenza season and selected previous seasons^† * Defined as a fever of ≥100.0°F (≥37.8°C), oral or equivalent, and cough or sore throat, in the absence of a known cause other than influenza. ^† Data as of May 30, 2014. ^§ The national baseline is the mean percentage of visits for ILI during weeks with little or no influenza virus circulation (noninfluenza periods) for the previous three seasons plus two standard deviations. A noninfluenza period is defined as ≥2 consecutive weeks in which each week accounted for <2% of the season’s total number of specimens that tested positive for influenza. Use of the national baseline for regional data is not appropriate.

FIGURE 3

Cumulative rates of hospitalization for laboratory-confirmed influenza, by age group and surveillance week and year — FluSurv-NET* surveillance system, United States, 2013–14 influenza season^{† *} FluSurv-NET conducts population-based surveillance for laboratory-confirmed influenza-associated hospitalizations in children aged <18 years (since the 2003–04 influenza season) and adults aged ≥18 years (since the 2005–06 influenza season). FluSurv-NET covers approximately 70 counties in the 10 Emerging Infections Program states (California, Colorado, Connecticut, Georgia, Maryland, Minnesota, New Mexico, New York, Oregon, and Tennessee) and additional Influenza Hospitalization Surveillance Project states (Michigan, Ohio, and Utah). ^† Data as of May 30, 2014.

FIGURE 4

Percentage of all deaths attributable to pneumonia and influenza (P&I), by surveillance week and year — 122 Cities Mortality Reporting System, United States, 2009–2014* ^* Data as of May 30, 2014. ^† The seasonal baseline proportion of P&I deaths is projected using a robust regression procedure, in which a periodic regression model is applied to the observed percentage of deaths from P&I reported by the 122 Cities Mortality Reporting System during the preceding 5 years. ^§ The epidemic threshold is set at 1.645 standard deviations above the seasonal baseline.