A new study on flu vaccine effectiveness for the 2015-16 season in the United States found that the vaccine in general helped cut the risk of flu modestly, but a change in the 2009 H1N1 vaccine strain didn’t fix the problem with the inhaled vaccine’s effectiveness in children.
Scientists who are part of the Influenza Vaccine Effectiveness Network (IVEN) reported their findings today in the New England Journal of Medicine. Their preliminary findings were among the data that the Advisory Committee on Immunization Practices (ACIP) considered last summer when—amid growing evidence of a lack of effectiveness in children—it pulled its recommendation for the live attenuated influenza vaccine (LAIV), or nasal spray vaccine, for the 2016-17 flu season.
In late 2016, researchers from Medimmune, which produces FluMist, the only US-approved LAIV, reported that their initial investigation into the decreased effectiveness of the vaccine, seen in the United States but not necessarily in other countries, found that reduced fitness of the H1N1 vaccine virus strains were the likely culprit, but that they were looking into other possibilities.
For the study, the research team enrolled patients age 6 months and older who were seen during the 2015-16 flu season at study-site clinics in Michigan, Pennsylvania, Texas, Washington state, and Wisconsin. Using a test-negative design, they estimated flu vaccine effectiveness as the odds of testing positive for flu in both vaccinated and unvaccinated people.
Of 6,879 participants, 19% (1,309) tested positive for flu, mostly 2009 H1N1 and influenza B. The overall efficacy of flu vaccines was 48% (95% confidence interval [CI], 41% to 55%), which the team said was in line with other seasons in which vaccine virus strains were similar to the circulating flu viruses.
For children ages 2 to 17 years old, inactivated flu vaccines (flu shots) were 60% effective (95% CI, 47% to 70%), but the researchers saw no effectiveness for the LAIV, which officially measured a not-statistically significant 5% (95% CI, -47% to 39%). When the investigators looked at effectiveness against just the 2009 H1N1 virus in kids, inactivated vaccines were 63% effective. compared with -19% for LAIV.
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