Influenza vaccination linked to reduced risk of secondary influenza infections among household contacts: JAMA

A new study published in the Journal of American Medical Association showed that with an estimated 21% efficacy, influenza vaccination was linked to a lower risk of subsequent influenza infections among household contacts. 

In the US, anyone 6 months of age and older are advised to get an influenza vaccination in order to avoid influenza and its associated consequences. The efficacy of the influenza vaccine (VE) is frequently evaluated in relation to the avoidance of illnesses that necessitate medical care. Evaluations of the ability of VE to prevent subsequent influenza infections are few. To ascertain the estimated efficacy of influenza vaccinations in preventing secondary infections following the introduction of influenza into homes, Carlos Grijalva and his colleagues carried out this investigation.

A prospective case-ascertained household transmission cohort study was conducted in Tennessee and Wisconsin over 3 successive influenza seasons (2017–2020) to recruit primary patients (the first household members with laboratory-confirmed influenza) and their household contacts. For a maximum of 7 days, the participants collected nose swabs and daily symptom diaries.

The analysis of the data took place between September 2022 and February 2024. History of vaccinations, as self-reported and confirmed by examination of registry and medical records. Reverse transcription-polymerase chain reaction was used to evaluate the specimens for influenza infection. The efficiency of influenza vaccinations in avoiding infection among household contacts overall, as well as by virus type, subtype, and/or lineage, was estimated using longitudinal chain binomial models.

The study examined a total of 699 main patients and 1581 family contacts. The median (IQR) age of the primary cases was 13 (7-38) years, with 381 (54.5%) females, 60 (8.6%) Hispanics, 46 (6.6%) non-Hispanic Blacks, 553 (79.1%) non-Hispanic Whites, and 343 (49.1%) vaccinated.

The median age of household contacts was 31 (10-41) years, with 833 (52.7%) females, 116 (7.3%) Hispanics, 78 (4.9%) non-Hispanic Blacks, 1283 (81.2%) non-Hispanic Whites, 792 (50.1%) vaccinated, and 356 (22.5%) having laboratory-confirmed influenza during follow-up. The total secondary infection risk from influenza among home contacts was 18.8%. The risk was highest among children, at 15.9% for influenza B and 20.3% for influenza A. 

Variable by type, the total estimated VE for avoiding secondary infections across unvaccinated household contacts was 21.0%; the estimated VE for influenza A was 5.0%, whereas for influenza B, it was 56.4%. Overall, this study demonstrated that there is a substantial chance of transmission to household members once influenza virus infections are introduced in homes.

Source:

Grijalva, C. G., Nguyen, H. Q., Zhu, Y., Mellis, A. M., McGonigle, T., Meece, J. K., Biddle, J. E., Halasa, N. B., Reed, C., Fry, A. M., Yang, Y., Belongia, E. A., Talbot, H. K., & Rolfes, M. A. (2024). Estimated Effectiveness of Influenza Vaccines in Preventing Secondary Infections in Households. In JAMA Network Open (Vol. 7, Issue 11, p. e2446814). American Medical Association (AMA). https://doi.org/10.1001/jamanetworkopen.2024.46814