A major Swiss study of 1,745 healthcare workers, published in Nature’s Communications Medicine, found that recent COVID-19 booster shots were linked to a significantly higher risk of illness and missed work compared to the unvaccinated.
The illness measured was Influenza-Like Illness (ILI) — sudden onset of fever (≥38°C or feeling feverish) plus at least one respiratory symptom (cough, sore throat, runny nose, loss of smell) within 7 days.
National surveillance showed that during the study period, ~21% of ILI cases were COVID-19, ~20% were influenza, and the rest were caused by other respiratory viruses — meaning boosters increase risk of infection from multiple different pathogens.
- 3 doses: aIRR 1.56 (95% CI 1.22–2.03) — 56% higher risk of ILI vs. unvaccinated.
- 4 doses: aIRR 1.70 (95% CI 1.27–2.28) — 70% higher risk.
- More recent boosters: aIRR 1.32 (95% CI 1.07–1.62) — strongest effect soon after vaccination.
More Sick Days After Boosters
- 3 doses: aIRR 1.49 (95% CI 1.08–2.01) — 49% more workdays lost.
- 4 doses: aIRR 1.50 (95% CI 1.04–2.13) — 50% more workdays lost.
Robust Even After Adjusting for Confounders
- Inverse probability weighting confirmed the association: recent boosters aIRR 1.26 (95% CI 1.12–1.43).
Hellbound and Down 