Influenza Vaccine Effectiveness in the United States during the 2015-2016 Season

Abstract

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.).

Figure 1. Adjusted Estimates of Influenza Vaccine Effectiveness, Overall and Stratified According to Age, Virus Subtype or Lineage, and Vaccine Type

In each subgroup, the number of case patients is the number of participants in that subgroup who tested positive for influenza virus; the total number in each subgroup is the number of participants in the subgroup who tested positive or negative. For the analysis according to virus subtype or lineage, the total number included all patients who tested negative plus all those who tested positive for the subtype or lineage of interest, with the exception that participants from the Wisconsin site were excluded from the analysis of A(H3N2). Vaccine effectiveness was calculated as (1 – OR) × 100, in which OR is the odds ratio for testing positive for influenza virus among vaccinated versus unvaccinated participants in each subgroup. Horizontal bars indicate 95% confidence intervals. IIV denotes inactivated influenza vaccine, IIV3 trivalent IIV, IIV4 quadrivalent IIV, and LAIV4 quadrivalent live attenuated influenza vaccine.

Figure 2. Adjusted Estimates of Influenza Vaccine Effectiveness among Children 2 to 17 Years of Age, Overall and Stratified According to Virus Subtype or Lineage and Vaccine Type

In each subgroup, the number of case patients is the number of children 2 to 17 years of age who tested positive for influenza virus (or for the type or subtype of interest, in the subgroups that are based on these factors); the total number includes unvaccinated children who tested negative for influenza virus, children vaccinated with the relevant vaccine type who tested negative for influenza virus, and children who were vaccinated with the relevant vaccine and tested positive for influenza virus (or for the type or subtype of interest, in the subgroups that are based on these factors). Vaccine effectiveness was calculated as (1 – OR) × 100, in which OR is the odds ratio for testing positive for influenza virus among vaccinated versus unvaccinated participants in each subgroup. Horizontal bars indicate 95% confidence intervals.

Figure 3. Odds Ratios for Medically Attended Influenza, Overall and According to Age Group

Horizontal bars indicate 95% confidence intervals.

Figure 4. Adjusted Estimates of Influenza Vaccine Effectiveness, Stratified According to Receipt of Vaccines for the Current Season (2015–2016) and Previous Season (2014–2015)

In each subgroup, the number of case patients is the number of participants with the specified vaccination history who tested positive for the relevant influenza virus subtype or lineage; the total number in each subgroup is the number of participants with the specified vaccination history who tested negative for influenza or tested positive for the specified subtype or lineage. Vaccine effectiveness was calculated as (1 – OR) × 100, in which OR is the odds ratio for testing positive for influenza virus among participants who were vaccinated in the season or seasons of interest versus participants who received no vaccination in either season (the reference group). Horizontal bars indicate 95% confidence intervals.