Science conflict: why repeat flu vaccines may lose their effectiveness
Two major studies question the annual vaccination strategy – and show that protection may even be reversible. Here and here.
Every fall, the same campaign begins: health authorities urge people to get vaccinated against the flu. Elderly people, caregivers and people with chronic diseases in particular are encouraged to “protect themselves every year.” But two of the most important studies in recent years paint a much more complex and worrying picture.
Canadian and Italian researchers reported in the Journal of Infectious Diseases (Skowronski et al., 2017) that the effectiveness of influenza vaccines decreases significantly with repeated use – and may even be negative in some years. A meta-analysis by Ramsay et al. (2019) confirmed that repeated vaccinations show inconsistent results, which vary greatly depending on the season, virus variant and individual vaccination history.
When protection becomes weakness
A Canadian-Italian study examined data from five flu seasons between 2010 and 2015. Researchers found that people who were vaccinated multiple years in a row were more likely to get the flu than those who were vaccinated in only one year.
Scientists explained this phenomenon with the so-called antigenic distance hypothesis: if last year’s vaccine and the current vaccine are too similar, but the virus itself has changed significantly, the immune system may respond to the old antigens and not effectively fight the new variants.
In short, repeated vaccinations can “mis-prime” the immune system.
“We found evidence of negative interference, especially during seasons when H3N2 strains had changed significantly,” Skowronski and his colleagues write.
“Individuals vaccinated in consecutive years had a significantly higher risk of developing laboratory-confirmed influenza than those vaccinated once.”
Meta-analysis: protection only under favorable conditions
A systematic review by Ramsay et al. (2019), which evaluated more than 30 studies, came to a similar conclusion: the effectiveness of repeat influenza vaccines varies greatly. In some years, it provides little protection, while in others it provides moderate protection (30–60%). However, the negative effects are particularly evident for influenza A(H3N2) – that is, a higher incidence of the disease among people who have been repeatedly vaccinated.
The authors warn that such results should not be ignored:
“Repeated influenza vaccination may reduce vaccination efficacy under certain antigenic conditions. These dynamics need to be taken into account in vaccination recommendations.”
Health policy on autopilot
Despite these findings, health authorities worldwide maintain their policy of vaccinating all at-risk groups annually. Public communication conveys the message that booster vaccination is always safe and effective. However, data show that immune responses are individual and context-dependent – and the “once a year” routine remains scientifically unfounded.
Some experts are already calling for a rethinking of the vaccination strategy: instead of general campaigns, more targeted recommendations are needed that take into account the development of the virus, the type of vaccine, and immune history.
Summary
Research shows:
- Flu vaccines can protect, but not every year and not for everyone.
- Repeated vaccinations can interfere with natural immunity and be harmful in certain years.
- The general recommendation to vaccinate every year ignores these complex interactions.
As long as public health communication ignores these scientific uncertainties, the annual flu campaign will remain more of a ritual than evidence-based medicine.
