Ever since Covid-19 vaccines became available, some people with long Covid have said vaccination eases the constellation of symptoms that persist weeks and months after their original infection clears. Research exploring this anecdotal evidence has so far been intriguing but inconclusive, in part because of the small numbers of people studied.
A new study published Wednesday in BMJ solves the size problem, combing through responses from more than 28,000 adults taking part in the U.K.’s nationally representative Covid-19 Infection Survey. Vaccination after infection was associated with a lower likelihood of long Covid, the researchers report, but more data will be needed to clinch any cause-and-effect connection.
Led by Daniel Ayoubkhani of the U.K.’s Office for National Statistics, the researchers arrived at their conclusion — which they emphasize is only observational — by tracking people from February to September 2021 who received at least one Covid-19 vaccine dose after testing positive. The study ended before boosters were rolled out and before Omicron and its subvariants emerged.
Before vaccination, the odds of experiencing long Covid changed little over time, the researchers said, based on symptoms such as fatigue, shortness of breath, and brain fog. Long Covid symptoms of any severity were reported by almost a quarter of participants at least once during follow-up.
After vaccination, among all participants, the first dose was associated with an initial 13% decrease in their odds of long Covid, but the data didn’t show a sustained improvement over the following 12 weeks, when the second vaccine dose was given. That second dose was associated with another 9% decrease in the odds of long Covid and that improvement lasted, on average, for at least nine weeks.
“Our results suggest that vaccination of people previously infected may be associated with a reduction in the burden of long Covid on population health, at least in the first few months following vaccination,” study author Ayoubkhani told STAT in an email. “The sustained 9% fall in the odds of long Covid after the second dose — this may appear modest, but we need to remember that this is out of a sample of people from the population with and without long Covid, so the reduction among only people who had pre-existing symptoms will be larger.”
The results held up after factoring in sociodemographic characteristics, health-related differences, vaccine type, or the span of time between infection to vaccination, the study said. The biological reason why symptoms improve is another question, the researchers wrote — a point underlined by an editorial published with the study.
“Given the small numbers of patients reported to benefit … and the uncertainty around the true effect of vaccines relative to natural recovery, a clear explanation for how vaccines might reduce the multisystem manifestations of long Covid is still lacking,” Manoj Sivan of the University of Leeds and co-authors wrote in the editorial. “More research is needed on the link between antibody titres and symptoms over time before we can hope to predict the effects of vaccination on individuals.”
On that note, Peter English, a retired consultant in communicable disease control and former chair of the British Medical Association Public Health Medicine Committee, offered this comment on the study via the Science Media Centre: “Anything that can reduce the burden of disease from long Covid at reasonable cost is, therefore, important and valuable. The large scale of this study means that we can be fairly confident about what has been observed; but it does not mean we can be sure what it means.”
Ayoubkhani said his next targets are studies looking at longer-term outcomes of people with pre-existing long Covid, including after receiving a booster dose and in people infected with the Omicron variant.
“Studies are needed to understand the biological mechanisms underpinning any improvements in symptoms following vaccination, which may contribute to the development of therapeutics for long Covid,” he said.
Meanwhile, evidence is building for the value of vaccination before infection to lessen chances of long Covid. A February review of 15 international studies by the U.K. Health Security Agency found that people who had at least one dose of Covid vaccine were less likely than unvaccinated people to develop long Covid. “Vaccination to reduce risk of reinfection remains important for people with long Covid,” the editorial writers said.