The influenza vaccine introduced in 2009 showed reduced effectiveness during the 2015-2016 influenza season, but only in adults born between 1958 and 1979, according to an analysis published online in the Journal of Infectious Diseases.

Using the Influenza Vaccine Effectiveness Network, researchers analyzed data from 2,115 patients with medically attended acute respiratory illness who tested positive for A(H1N1)pdm09 influenza virus, and 14,696 patients who tested negative for the influenza virus, from 2010-2011 to 2015-2016 (excluding the 2014-2015 influenza season).

Overall, 48% of the influenza virus–negative patients and 28% of the virus-positive patients had received at least one dose of the seasonal inactivated influenza vaccine more than 2 weeks before they fell ill.

However, the vaccine, which was based on the A/California/07/2009 strain of the A(H1N1)pdm09 virus, was only 47% effective during the 2015-2016 season, compared with 61% effectiveness during the 2010-2011 season through to the 2013-2014 season.

When researchers looked at vaccine effectiveness by birth cohort, they found that one particular cohort – individuals born between 1958 and 1979 – showed a significantly reduced vaccine effectiveness (22%) during the 2015-2016 season. By comparison, vaccine effectiveness in this cohort was 61% during the 2010-2013 seasons, and 56% during the 2013-2014 season.

When this birth cohort was excluded from analysis of the 2015-2016 season, the overall vaccine effectiveness for that season was 61%.

While the vaccine was based on an early reference strain of A(H1N1)pdm09, the virus itself later acquired mutations in the hemagglutinin gene, leading to the emergence of new genetic clades, including 6B, which dominated in the 2013-2014 influenza season, and 6B.1, which dominated in 2015-2016.

“Limited serologic data suggest that some adults born during 1958-1979 (age range in 2015-2016, 36-57 years) have decreased antibody titers against A(H1N1)pdm09 group 6B and 6B.1 viruses,” wrote Brendan Flannery, PhD, from the Centers for Disease Control and Prevention, and his coauthors.

They suggested that individuals in this cohort may have been immunologically primed with A/USSR/90/1977-like viruses, which were the first group of A(H1N1) viruses that this cohort would have been exposed to. A(H1N1) strains didn’t circulate between 1958 and 1977. Vaccination with A(H1N1)pdm09 viruses may have induced antibodies against shared antigenic components found on early versions of A(H1N1)pdm09.

If these shared antigenic epitopes were then altered in the later 6B and 6B.1 viruses, that might account for decreased antibody titers in this age group.

“Replacement of the A/California/07/2009(H1N1)pdm09 vaccine reference strain with A/Michigan/45/2015 (group 6B.1) should lead to improved [vaccine effectiveness] against circulating A(H1N1)pdm09 viruses,” the investigators noted.

The study was supported by the Centers for Disease Control and Prevention, the National Institutes of Health, and the National Center for Advancing Translational Sciences. Eight authors declared funding, grants, and consultancies with the pharmaceutical industry, with five also declaring funding from the CDC.

SOURCE: Flannery B et al. J Infect Dis. 2018 Jan 18. doi: 10.1093/infdis/jix634 .


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