This year's influenza vaccine prevented just more than half of medical visits for the flu but was significantly less effective against influenza A among the elderly, according to new data from the Centers for Disease Control and Prevention (CDC).
The findings were posted in the February 22 issue of the CDC's Morbidity and Mortality Weekly Report.
Some older adults may not mount an effective immune response to the influenza A (H3N2) component of this season's vaccine, the CDC says.
"Nonetheless, this finding should not discourage future vaccination by persons aged ≥65 years, who are at greater risk for more severe cases and complications from influenza. Influenza vaccines remain the best preventive tool available, and [vaccine effectiveness] is known to vary by virus type/subtype, age group, season, host immunity, and the outcome measured," according to the CDC.
The new analysis was based on data from 2697 children and adults enrolled in the US Influenza Vaccine Effectiveness Network from December 3, 2012, through January 19, 2013. Of the 41% (1115) of the participants who tested positive for influenza, 32% had received the 2012-2013 influenza vaccine. Of the 1582 who did not have influenza, 50% had received the vaccine.
After adjustment for age, race/ethnicity, site, self-rated health, and days from illness to enrollment, overall vaccine effectiveness against medically attended acute respiratory illness was 56%. This follows an earlier interim estimate of 62%.
Estimated vaccine effectiveness was 47% against influenza A (H3N2) and 67% against influenza B.
For influenza A, based on a total 546 confirmed infections, the estimated adjusted effectiveness was 58% for children aged 6 months to 17 years, 46% for adults aged 18 to 49 years, and 50% for those aged 50 to 64 years. However, for adults aged 65 years and older, efficacy against influenza A was only 9%.
Based on 366 confirmed cases of influenza B, the adjusted vaccine effectiveness estimate was 67% overall, ranging from 64% to 75% across age groups. Here, the estimate was 67% among those aged 65 years and older, but the numbers were too small for a precise estimate.
"Variability among studies and across seasons and age groups is to be expected and should not change recommendations for annual vaccination," the CDC adds, noting that these data are limited to the prevention of outpatient visits and do not include hospitalization or death.
New Influenza Vaccines
At a meeting on February 21, the CDC's Advisory Committee on Immunization Practices (ACIP) voted to include 4 recently approved influenza vaccines into its yearly recommendations: quadrivalent live attenuated inactivated vaccine (Flumist, MedImmune), quadrivalent inactivated influenza vaccine (Fluarix, GlaxoSmithKline), cell-culture based trivalent inactivated influenza vaccine (Flucelvax, Novartis), and recombinant hemagglutinin influenza vaccine (FluBlok, Protein Sciences).
Of those options, the 2 quadrivalents are expected to provide broader protection against circulating influenza B viruses. The recombinant hemagglutinin influenza vaccine is egg-free but is currently only licensed for ages 18 to 49 years.
Information on any relevant financial disclosures was not provided.
Morb Mortal Wkly Rep. 2013;62;119-123. Full text
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