Vaccinated Individuals 58% Less Likely to Develop Post-COVID Symptoms

In a recent study published by the BMJ, researchers at the Karolinska Institute in Sweden have identified a strong association between receiving at least one dose of a COVID-19 vaccine before the first infection and a reduced risk of developing post-COVID-19 conditions, commonly known as long COVID.

The study, based on data from over half a million Swedish adults, revealed that unvaccinated individuals were nearly four times more likely to be diagnosed with long COVID compared to those who received the vaccine before their initial infection. While the researchers caution that direct causality cannot be inferred from observational evidence, they emphasize that the results underscore the crucial role of primary vaccination in mitigating the burden of post-COVID-19 conditions in the population. 

Existing knowledge has established the effectiveness of COVID-19 vaccines against SARS-CoV-2 infection and severe complications of acute COVID-19. However, their efficacy against long COVID has remained less clear due to previous studies relying on self-reported symptoms. 

To address this gap, the researchers explored the effectiveness of primary COVID-19 vaccination, including the first two doses and the initial booster within the recommended schedule, against post-COVID-19 conditions. The study drew on data from the SCIFI-PEARL project, a register-based study of the COVID-19 pandemic in Sweden. 

The study included 589,722 adults aged 18 and over from the two largest regions of Sweden, with a first COVID-19 infection recorded between December 27, 2020, and February 9, 2022. Participants were tracked from their first COVID-19 infection until a diagnosis of post-COVID-19 condition, vaccination, reinfection, death, emigration, or the end of follow-up (November 30, 2022), whichever occurred first.

The average follow-up was 129 days in the total study population, with 197 days for vaccinated individuals and 112 days for those not vaccinated. Individuals who had received at least one COVID-19 vaccine dose before infection were considered vaccinated. The analysis accounted for various factors, including age, sex, existing conditions, healthcare contacts, education level, employment status, and the dominant virus variant at the time of infection. 

Among the vaccinated individuals with COVID-19, 0.4% (1,201) were diagnosed with post-COVID-19 conditions during follow-up, compared to 1.4% (4,118) of unvaccinated individuals. Those who received one or more COVID-19 vaccines before the first infection were 58% less likely to receive a post-COVID-19 condition diagnosis than their unvaccinated counterparts. 

Moreover, the study revealed a dose-response effect, with vaccine effectiveness increasing with each successive dose before infection. The first dose reduced the risk by 21%, two doses by 59%, and three or more doses by 73%. 

While the study is observational and cannot establish causality definitively, the researchers highlight its robustness, relying on large, well-designed datasets with a low risk of self-reporting bias. The authors conclude that the results emphasize the importance of comprehensive primary vaccination coverage against COVID-19 to reduce not only the risk of severe acute infections but also the burden of post-COVID-19 conditions in the population. 

In a linked editorial, researchers underscore the connection between the immune system and the development of post-viral conditions, emphasizing the need for continued investigation into the long-term residual symptoms of COVID-19 and other viral illnesses. They call for improved accuracy in recording recovery and ongoing illness after infection, emphasizing the importance of quantifying key outcomes to support future research and investment in clinical services for post-viral conditions. 

Journal Reference  

COVID-19 vaccine effectiveness against post-COVID-19 condition among 589,722 individuals in Sweden: population based cohort study, The BMJ (2023). DOI: 10.1136/bmj-2023-076990.