Seasonal flu vaccination: short term protection, long term risk?

Influenza (flu) sickens millions and kills many thousands of Americans each year. Vaccination is intended to reduce the number cases and severity of illness. The flu virus changes its coat proteins each year, a ‘shape-shifting’ behavior that challenges the timely production of vaccines that are effective against flu variants. The US CDC evaluates vaccine effectiveness (VE) in thousands of outpatient respiratory illness patients, usually finding substantial vaccine protection with little risk Consequently, its Advisory Committee on Immunization Practices (ACIP) as well as the World Health Organization (WHO) recommend annual immunization of everyone over 6 months old unless contraindicated.
 
Here, the authors surveyed vaccination over 10 years among several thousand Japanese school children and several hundred school staff adults. Of the many forms of flu vaccine, Japan uses a quadrivalent (4 strains; trivalent prior to 2014) “split” vaccine based on influenza hemagglutinin (HA). “Split” vaccine means the virus was ‘disrupted’ by detergent (the equivalent of ‘heat killed’; viruses aren’t alive). The flu viruses used to prepare the vaccine are grown in eggs; alternative quadrivalent preparations are available for those with allergies to eggs, including Flublok and Flucelvax.

They found that morbidity was reduced in vaccinated elementary school students but elevated in middle school students (Fig 1). Most people who had been vaccinated ‘from infancy’ were also vaccinated in the 2019-2020 season (Fig. 2). Moreover, they “found that morbidity was significantly higher among elementary (P < 0.001) and middle (P < 0.05) school students who had been vaccinated since infancy than among those who had not been vaccinated since infancy” (Figure 3, shown).

Fig. 3 Relationship between morbidity and vaccination from infancy.

These data are self-reported via questionnaires, and therefore extra subjective. The authors propose no mechanism for how annual vaccination could cause increased morbidity. It seems probable that the association is not direct, not causative, but indirect through other behavioral or health status factors. This is an intriguing finding that should be analyzed and study that should be repeated.

Kajiume T, Mukai S, Toyota N, Kanazawa I, Kato A, Akimoto E, Shirakawa T. Effectiveness of seasonal influenza vaccine in elementary and middle schools: a 10-year follow-up investigation. BMC Infect Dis. 2022 Dec 6;22(1):909. doi: 10.1186/s12879-022-07898-y. PMID: 36474168; PMCID: PMC9724312.