Flu Vaccinations Can Protect Indirect Contacts

While vaccination ensures the best protection for individuals against the influenza virus, investigators have discovered a second protective function provided by the flu vaccination for those who are unvaccinated.

By utilizing longitudinal data over 6 influenza seasons from the Household Influenza Vaccine Evaluation (HIVE) study, investigators categorized households. The proportion of members who received a seasonal influenza vaccination served as the basis for which households were categorized each season.

Household members were categorized into the following groups: Unvaccinated (0%), Low Coverage (1-50%), Moderate Coverage (50-99%), and Fully Vaccinated (100%).

Mixed effect Poisson regression models that were adjusted for sex and age group were used; random effects accounted for household clustering and repeated measures.

Additionally, the association between the proportion of vaccinated household members and the incidence of influenza virus infection in the entire cohort was estimated. Subsequently, the indirect effects of influenza vaccination were estimated by comparing to unvaccinated members of households with higher levels of vaccination to unvaccinated members of households with low levels of vaccination.

From the follow up including 7,286 person-seasons, 578 cases (8 per 100/season) of influenza virus infection were detected.

In completely unvaccinated households, the seasonal incidence rate was highest (10 per 100/seasons); all other levels of vaccine coverage were lower.

A 29% lower seasonal incidence rate of influenza infection was observed in Individuals from fully vaccinated households compared to those in unvaccinated households (IRR 0.72, 95% CI: 0.56 - 0.93).

In a comparison between unvaccinated individuals in completely unvaccinated households and those in households with moderate vaccine coverage, the estimated indirect VE was 40 % (95% CI -4 to 65).

From their results, study authors concluded that the incidence of influenza in unvaccinated members of a community can be reduced by unvaccinated individuals’ close contact with vaccinated individuals.

Although there is insufficient evidence to support the claim, public health authorities currently suggest that vaccination for the flu protects close contacts.

The authors also concluded that clearer evidence for global vaccine recommendations may be provided by the collected data as they indicate protective effects of the seasonal influenza vaccination in household contacts of unvaccinated individuals.

For the upcoming 2018-2019 flu season, the US Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) recommends individuals be vaccinated by the end of October.

The ACIP has also reintroduced the intranasally-administered live attenuated influenza vaccine (LAIV), which was previously not recommended due to poor performance during 2013-2014 and 2015-2016 seasons.

Other available flu vaccines for the 2018-2019 season include recombinant influenza vaccine (RIV) as quadrivalent vaccines, and inactivated influenza vaccines (IIVs) as both a high-dose IIV trivalent and adjuvanted IIV trivalent.

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