In a Swedish cohort, the risk of long COVID was much lower for vaccinated than unvaccinated participants in the year after infection, even when restricting the analyses to subgroups based on variant, age, sex, and previous infection status, estimates a study published last week in the Journal of Infectious Diseases.

Karolinska Institutet researchers in Stockholm analyzed data from five registries to compare rates of persistent COVID-19 symptoms, or post-COVID condition [PCC], in adults infected from January 2021 to February 2022 by vaccination status in the 14 days before infection. Follow-up was 365 to 660 days.

“We have previously shown that a PCC diagnosis correlates with excess healthcare usage in SARS-CoV-2 infected individuals, indicating a good accuracy of the diagnosis for severe PCC, and a Swedish cohort study found an association between COVID-19 vaccination and a reduced risk of being diagnosed with PCC,” the study authors wrote.

Prevention of severe infection

Of all 331,042 participants, 2,546 (0.7%) had long COVID at any point, and 852 of those with long COVID (33%) had the condition for a year or more. The incidence of long COVID was 0.4% for unvaccinated adults, 0.3% for one vaccine dose, and 0.1% for two or three. The adjusted risk ratio for long COVID in vaccinated versus unvaccinated participants was 0.81 for one dose, 0.42 for two doses, and 0.37 for three. More than two-thirds of the doses were of the Pfizer/BioNTech vaccine.

Collectively, we demonstrate that COVID-19 vaccinations are strongly associated with a reduced risk of persistent PCC, reinforcing the importance of obtaining an adequate protection against COVID-19 and its long-term effects.

“Reduced risks for vaccinated individuals were also observed when restricting the analyses to pre-Omicron and Omicron, as well as all subgroups including sex, age, and previous infection,” the authors wrote.

Part of the protective effect likely stems from the vaccine’s efficacy against severe COVID-19, which has been tied to an increased risk of long COVID. “Collectively, we demonstrate that COVID-19 vaccinations are strongly associated with a reduced risk of persistent PCC, reinforcing the importance of obtaining an adequate protection against COVID-19 and its long-term effects,” they wrote.

Because of its broad clinical case definition and range of presentations, long COVID may be either under- or over-diagnosed in healthcare settings, the researchers noted. “National survey data and electronic healthcare record data from the UK have demonstrated that underestimation is highly plausible,” they wrote. “However, we find it unlikely that a major differential outcome misclassification between vaccinated and unvaccinated individuals would yield biased estimates.”

Editor’s note: This story was corrected on March 18 to accurately distinguish between the numbers and percentages of participants with long COVID at any point versus those with the condition at a year or more after infection.