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Influenza Vaccine Effectiveness in the United States during the 2015-2016 Season
Michael L Jackson1, Jessie R Chung1, Lisa A Jackson1
1From the Group Health Research Institute (now Kaiser Permanente Washington Health Research Institute), Seattle (M.L.J., L.A.J., C.H.P., J.B.); Centers for Disease Control and Prevention, Atlanta (J.R.C., A.M.F., B.F.); University of Michigan, Ann Arbor (A.S.M., E.T.M.); Marshfield Clinic Research Foundation, Marshfield WI (E.A.B., H.Q.M.); Baylor Scott and White Health, Texas A&M College of Medicine, Temple (M.G., K.M.); and University of Pittsburgh, Pittsburgh (R.Z., M.P.N.).
Insights
Influenza vaccines were effective in 2015-2016, but the live attenuated vaccine showed no effectiveness in children. This led to an interim recommendation against its use for the following season.
Area of Science:
- Immunology
- Virology
- Public Health
Background:
- The A(H1N1)pdm09 strain in the live attenuated influenza vaccine (LAIV) was changed for the 2015-2016 season due to poor performance in young children during 2013-2014.
- The Influenza Vaccine Effectiveness Network (IVEN) assessed the impact of this strain change on vaccine effectiveness (VE).
Purpose of the Study:
- To evaluate the VE of influenza vaccines during the 2015-2016 season.
- To assess the effectiveness of both inactivated influenza vaccine (IIV) and LAIV, particularly in children.
Main Methods:
- A test-negative design was employed with participants aged 6 months and older presenting with acute respiratory illness.
- VE was calculated as (1-OR)×100, where OR is the odds ratio for a positive influenza test in vaccinated versus unvaccinated individuals.
- Separate VE estimates were determined for IIV and LAIV.
Main Results:
- Overall VE against any influenza illness was 48%.
- IIV demonstrated 60% effectiveness in children aged 2-17 years, while LAIV showed 5% effectiveness (not statistically significant).
- VE against A(H1N1)pdm09 was 63% for IIV in children, compared to -19% for LAIV (not statistically significant).
Conclusions:
- Influenza vaccines were effective in reducing illness during the 2015-2016 season.
- LAIV was ineffective in children during this period, despite substantial effectiveness of IIV.
- Due to the lack of LAIV effectiveness and the unchanged A(H1N1)pdm09 strain for 2016-2017, an interim recommendation advised against its use.
Background:
The A(H1N1)pdm09 virus strain used in the live attenuated influenza vaccine was changed for the 2015-2016 influenza season because of its lack of effectiveness in young children in 2013-2014. The Influenza Vaccine Effectiveness Network evaluated the effect of this change as part of its estimates of influenza vaccine effectiveness in 2015-2016.
Methods:
We enrolled patients 6 months of age or older who presented with acute respiratory illness at ambulatory care clinics in geographically diverse U.S. sites. Using a test-negative design, we estimated vaccine effectiveness as (1-OR)×100, in which OR is the odds ratio for testing positive for influenza virus among vaccinated versus unvaccinated participants. Separate estimates were calculated for the inactivated vaccines and the live attenuated vaccine.
Results:
Among 6879 eligible participants, 1309 (19%) tested positive for influenza virus, predominantly for A(H1N1)pdm09 (11%) and influenza B (7%). The effectiveness of the influenza vaccine against any influenza illness was 48% (95% confidence interval [CI], 41 to 55; P<0.001). Among children 2 to 17 years of age, the inactivated influenza vaccine was 60% effective (95% CI, 47 to 70; P<0.001), and the live attenuated vaccine was not observed to be effective (vaccine effectiveness, 5%; 95% CI, -47 to 39; P=0.80). Vaccine effectiveness against A(H1N1)pdm09 among children was 63% (95% CI, 45 to 75; P<0.001) for the inactivated vaccine, as compared with -19% (95% CI, -113 to 33; P=0.55) for the live attenuated vaccine.
Conclusions:
Influenza vaccines reduced the risk of influenza illness in 2015-2016. However, the live attenuated vaccine was found to be ineffective among children in a year with substantial inactivated vaccine effectiveness. Because the 2016-2017 A(H1N1)pdm09 strain used in the live attenuated vaccine was unchanged from 2015-2016, the Advisory Committee on Immunization Practices made an interim recommendation not to use the live attenuated influenza vaccine for the 2016-2017 influenza season. (Funded by the Centers for Disease Control and Prevention and the National Institutes of Health.).
