Abstract
Long COVID affects millions worldwide and its prevention is a critical public health strategy. While prior analyses show primary vaccination prevents long COVID in subsequent infections, the effect of booster vaccination on long COVID after Omicron infections is unclear. This systematic review identifies 31 observational studies, of which 11 are suitable for pairwise meta-analyses. The pooled odds ratio (OR) of long COVID in those vaccinated (any dose) versus unvaccinated is 0.77 (95% confidence interval [CI] 0.70-0.85; p < 0.0001; 10 studies). ORs were also lower for primary course vaccination versus unvaccinated (OR 0.81; 95% CI 0.79-0.83; p < 0.0001; 3 studies), booster vaccination versus unvaccinated (OR 0.74; 95% CI 0.63-0.86; p = 0.0001; 4 studies), and booster vaccination versus primary course vaccination (OR 77; 95% CI 0.65-0.92; p = 0.0044; 3 studies). These findings indicate that booster vaccination can provide additional protection against long COVID, highlighting the importance of seasonal vaccination against new SARS-CoV-2 variants. They should, however, be interpreted cautiously, given the small number of studies and the low quality of evidence.