Vaccine Cuts By Half Medical Visits for Influenza-Associated Illnesses

ATLANTA – Early estimates indicate that this year’s influenza vaccine effectiveness reduced the risk for associated medical visits by about half.

In an interim report in the Morbidity & Mortality Weekly Report, national Centers for Disease Control and Prevention (CDC) researchers emphasized that, because flu activity is likely to continue for several more weeks in the United States, vaccination efforts should continue as well.1

Thus far in the flu season, overall vaccine effectiveness (VE) against influenza A and influenza B virus infection associated with medically attended ARI was 48%, with most influenza infections caused by A (H3N2) viruses, the CDC reported. VE was estimated to be 43% against illness caused by influenza A (H3N2) virus and 73% against influenza B virus. The report used data from 3,144 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network from Nov. 28, 2016–Feb. 4, 2017.

Because, during 2014–15, influenza A (H3N2) 3C.2a viruses were antigenically different from the recommended A (H3N2) vaccine component, low vaccine effectiveness of 1% resulted, contributing to high rates of influenza-associated hospitalizations that season, especially among older adults.

This flu season, however, rates of influenza-associated hospitalizations observed so far have been substantially lower:  Virologic surveillance indicates that the majority of influenza A (H3N2) viruses collected by U.S. laboratories during the 2016–17 season remain antigenically similar to the A/Hong Kong/4801/2014–like cell propagated reference virus belonging to genetic group 3C.2a, which is the recommended influenza A (H3N2) component of the 2016–17 Northern Hemisphere vaccine.

“Early antiviral treatment can reduce severity and complications of influenza-associated illness. Early antiviral treatment is recommended for persons with suspected influenza with severe or progressive illness (e.g., hospitalized persons) and persons at high risk for complications from influenza, such as children aged <2 years, adults aged ≥65 years and persons with underlying health conditions, even if illness is less severe,” CDC said. “Antiviral medications should be used as recommended for treatment in patients with suspected influenza, regardless of vaccination status.”

It further emphasized that the decision to initiate antiviral treatment should not be delayed while waiting for laboratory confirmation of influenza and should not be dependent on insensitive assays, such as rapid influenza diagnostic tests.

  1. Flannery B, Chung JR, Thaker SN, et al. Interim Estimates of 2016–17 Seasonal Influenza Vaccine Effectiveness — United States, February 2017. MMWR Morb Mortal Wkly Rep 2017;66:167–171. DOI:

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