The effect of pre-COVID and post-COVID vaccination on long COVID: a systematic review and meta-analysis.

Background: Long COVID affects millions of people and results in a substantial decrease in quality of life. Previous primary studies and reviews attempted to study the effect of vaccination against long COVID, but these studies varied in the cut-off time of long COVID. We adhered to the WHO's definition of long COVID and conducted a systematic review and meta-analysis on the effect of pre-COVID and post-COVID vaccination on long COVID.

Methods: We obtained data from 13 databases up to 18 February 2024, including peer reviewed and preprint studies. Our inclusion criteria were: (1) long COVID definition as 3 months or beyond, (2) comparing long COVID symptoms between vaccinated and unvaccinated groups, (3) subjects received vaccinations either before or after infected with COVID, (4) the number of doses received by participants was specified. We extracted study characteristics and data and computed the summary odds ratio (OR) with the DerSimonian and Laird random effects model. We then performed subgroup analyses based on the main vaccine brand and long COVID assessment method. ROBINS-I framework was used for assessment of risk of bias and the GRADE approach was used for evaluating the certainty of evidence.

Findings: We included data from 25 observational studies (n=14,128,260) with no randomised controlled trials. One-dose pre-COVID vaccination did not have an effect on long COVID (number of studies = 10, summary OR = 1.01, 95% CI = 0.88-1.15, p-value = 0.896). Two-dose pre-COVID vaccination was associated with a 24% reduced odds of long COVID (number of studies = 15, summary OR = 0.76, 95% CI = 0.65-0.89, p-value = 0.001) and 4 symptoms (fatigue, headache, loss of smell, muscle pain) out of 10 symptoms analysed. The OR of three-dose pre-COVID vaccination against overall long COVID was statistically insignificant but was far away from 1 (number of studies = 3, summary OR = 0.31, 95% CI = 0.05-1.84, p-value = 0.198). One-dose post-COVID vaccination was associated with a 15% reduced odds of long COVID (number of studies = 5, summary OR = 0.85, 95% CI = 0.73-0.98, p-value = 0.024). The OR of two-dose post-COVID vaccination against long COVID was statistically insignificant but was far away from 1 (number of studies = 3, summary OR = 0.63, 95% CI = 0.38-1.03, p-value = 0.066).

Interpretation: Our study suggests that 2-dose pre-COVID vaccination and 1-dose post-COVID vaccination are associated with a lower risk of long COVID. Since long COVID reduces quality of life substantially, vaccination could be a possible measure to maintain quality of life by partially protecting against long COVID.