Vaccinations reduce the risk of long covid

4 minute read

Being up to date with your boosters could cut the chances by three quarters or more, a major Swedish study suggests.

Three or more vaccinations reduce the risk of long covid by 73%, but even one or two doses appear to be protective.

A study of more than half a million Swedish adults with diagnosed covid found that having just one vaccine before their infection reduced the risk of long covid by 21% and having two reduced it by 59%.

The findings, based on data from 2020 to 2022, showed that long covid was fourfold higher in the unvaccinated group compared to those who had at least one jab.

“The results of this study suggest a strong association between covid-19 vaccination before infection and reduced risk of receiving a diagnosis of PCC [post-covid-19 condition],” the authors wrote in the BMJ.

“The findings highlight the importance of primary vaccination against covid-19 to reduce the population burden of PCC.”

While the benefits of vaccines against infection and severity is clear, it’s uncertain how effective they are against long covid.

The authors pointed to data showing one in 50 Swedish adults who were infected went on to be diagnosed with long covid, but other research based on self-reported symptoms suggest as many as half of all infected with covid have such issues.

This usually includes symptoms such as fatigue, dyspnoea, cognitive impairment, headaches, muscle pain, and heart abnormalities such as chest pain and palpitations.

In this study, the authors used a diagnosis of post-covid condition as the primary outcome.

Analysing the population-based database, the researchers found one vaccine dose was 19% effective against covid, two doses was 47% effective, and three or more doses was 58% effective.

Women were more likely than men to be vaccinated, as were older people compared with younger people. The chances of vaccinated people ending up in hospital were 1.5%, compared to 4% in the unvaccinated group.

Around 60% of the unvaccinated group was first infected during the Alpha wave, while 75% of the vaccinated group was infected during the Omicron wave.

“When stratifying by the median time between last vaccination and infection (126 days) to assess the potential different effects of recent versus earlier vaccination, we found that receiving the last vaccine dose more than 126 days before covid-19 was still associated with a relatively high vaccine effectiveness against PCC, and only slightly lower than in the main analysis,” the authors said.

“In addition, to ensure sufficient time between vaccination and the acute infection, in a sensitivity analysis we restricted the vaccinated population to those who received their last vaccine dose more than 14 days before covid-19, and the estimated vaccine effect did not markedly change from the main analysis.

“Furthermore, in the main analyses we only considered the first PCC diagnosis at least 28 days from infection, but in sensitivity analyses we required at least 90 days from infection, with similar results.”

Their study did find that more than one in three patients with covid who went to the ICU were diagnosed with long covid.

“Covid-19 vaccines have been shown to protect against hospital admission with covid-19, which could be one pathway for the vaccines to exert a protective effect against PCC,” they wrote.

But reduced hospital admissions seemed to only account for part of this link, they said.

“Analyses stratified on severity of acute disease as indicated by the need for hospital admission showed that vaccine effectiveness was similar in both the group admitted to hospital without ICU admission and the group with no hospital admission,” the authors wrote.

But data suggested those vaccinated after the infection had a lower risk of developing long covid compared to adults vaccinated in the three months after their infection.

“This, together with the findings in the present study, support the hypothesis of pathways beyond the protective effect against hospital admission that may contribute to the protective effect of covid-19 vaccines against PCC,” the authors concluded.

BMJ 2023, online 16 October

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