The 2017-2018 influenza vaccine reduces the odds of getting the disease by about 33%, but it is 25% effective against H3N2, the most prevalent virus this season, according to an update from the Centers for Disease Control and Prevention (CDC) and published in Morbidity and Mortality Weekly Report.

The interim estimate of 25% vaccine effectiveness (VE) against influenza A(H3N2) viruses this season indicates that vaccination provided some protection, in contrast to recently reported, nonsignificant interim estimates of 17% from Canada and 10% from Australia and is similar to final (32%) VE estimates in the United States against A(H3N2) viruses during 2016 to 2017 season. However, among children aged 6 months through 8 years, the interim estimates against any influenza and A(H3N2) virus infection were higher; the risk for A(H3N2) associated medically attended influenza illness was reduced by more than half (59%) among vaccinated children.

This year does not appear to be the worst recent year for overall vaccine effectiveness. For the 2014-2015 season, the CDC reported a 19% flu prevention rate that was due to a mismatch between the strains in the vaccine and the ones causing illness.

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Immune responses to vaccination differ by age and previous infection or vaccination history and can affect vaccine protection. Higher VE against A(H3N2) viruses among young children suggests that vaccination may provide better protection against circulating A(H3N2) viruses in this age group, according to the CDC. In addition, genetic changes in the vaccine virus hemagglutinin protein that arise during passage in eggs may result in a vaccine immune response that is less effective against circulating viruses. Additional studies are needed to assess whether VE against circulating A(H3N2) viruses varies by vaccine type, including comparisons between egg-based and non–egg-based vaccines.

The CDC will continue to monitor VE through the remainder of the season and is investigating these factors. In addition, many efforts are under way to improve selection and development of candidate vaccine viruses that are optimal for vaccine production and provide protection against a majority of circulating viruses.

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“Annual monitoring of VE supports ongoing efforts to improve influenza vaccines,” stated the research team led by Brendan Flannery, PhD. “Although more effective vaccines are needed, vaccination prevents a substantial burden of influenza-related illness annually.”


  1. Painter K. Flu vaccines just 25% effective against worst strain this year, CDC says. USA Today. February 15 2018. Accessible at:
  2. Flannery B, Chung JR, Belongia EA, et al. Interim estimates of 2017–18 seasonal influenza vaccine effectiveness — United States, February 2018. MMWR Morb Mortal Wkly Rep. 2018;67. DOI: