Adults hospitalized for acute respiratory illness who received the influenza vaccine in 2017 to 2018 had higher antibody titers against the egg-adapted vaccine strain and lower titers against circulating virus, according to data published in the Journal of Infectious Diseases.

Investigators compared geometric mean titers against egg- and cell-grown influenza A/Hong Kong/4801/2014 A(H3N2) vaccine strains and representative circulating viruses, including A/Washington/16/2017. Researchers found titers against egg-grown influenza A/Hong Kong were significantly higher in vaccinated individuals (geometric mean titer, 173 vs 41; P =0.01). Also, a 2-fold increase in titers against egg-grown influenza A/Hong Kong was not significantly protective, according to unadjusted models (29% reduction; P =.09); however, a similar increase in cell-grown influenza A/Washington titer (3C.2a2) was protective (60% reduction; P =.02).

When adjusted for titers against A/Washington, higher egg-grown influenza A/Hong Kong titers were not significantly associated with infection (15% reduction; P =.61). After adjustment for egg-grown influenza A/Hong Kong, there was a 54% reduction of odds of infection observed with a 2-fold increase in A/Washington strain infection, but this result was not significant.

Without paired prevaccination specimens, investigators were unable to measure the magnitude of antibody response to vaccination, despite demonstrating differences in antibody titer by vaccination status. The conclusions of this study were further limited as a result of the inclusion of only a small number of samples from a single hospital. Issues with sample size also meant subanalyses regarding how the results might vary within specific demographic groups or by vaccine history were not possible. Investigators noted, however, that because of the early evidence of the severity of the 2017 to 2018 epidemic, they “prioritized rapid serologic testing of the specimens that were available to date during first half of the season.” And although this limited the results, the viruses found circulating locally overall during the study period were similar to those found nationwide during the same period.

The investigators concluded that this work, “adds to the evidence that egg adaptations in the 2017-2018 influenza vaccine strain elicited antibodies to epitopes not conserved in circulating strains,” but the extent to which this worsened the epidemic remains to be determined. They further believed that although some early data suggest cell-based vaccines provided somewhat improved protection, there remain several drawbacks to producing them, including “selecting viruses that are close antigenic matches to those that will circulate and are subject to the limitations of annual strain selection timelines.” Therefore, according to the investigators, there is still a need for the development of new influenza vaccines with improved, longer, and more comprehensive coverage.

Reference

Levine MZ, Martin ET, Petrie JG, et al. Antibodies against egg- and cell-grown influenza A(H3N2) viruses in adults hospitalized during the 2017-2018 season [published online February 4, 2019]. J Infect Dis. doi: 10.1093/infdis/jiz049.

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This article originally appeared on Infectious Disease Advisor