DiscussionIn patients with long covid, we found that the first covid-19 vaccine injection was associated with a reduction in the severity of the disease and on the effect on patients' social, professional, and family lives at 120 days after baseline. In particular, our results showed that the remission rate of long covid symptoms was 16.6% in the vaccinated group compared with 7.5% in the control group .
Our study had some limitations. Firstly, despite the use of robust methods and statistical techniques to make causal inferences from observational data, the intervention was not randomly assigned, and potential unmeasured confounders could have biased our results. For example, patients' motivation to receive a covid-19 vaccine was not taken into account, although it might be related to their perception of their long covid symptoms.
Thirdly, all patients in our study were infected before 1 May 2021, and so were not infected with the recent delta and omicron variants of the SARS-CoV-2 virus. The effectiveness of vaccination on persistent symptoms occurring after infection with these variants is unknown. Also, patients in this study had not been vaccinated before their infection and long covid, and whether our results are applicable to breakthrough infections in vaccinated individuals is unclear.
Fifthly, adverse events after vaccination were collected with an online questionnaire that did not ask patients about the precise time of these events.