Influenza vaccines have a surprising health benefit: they might also prevent COVID-19, particularly in its most severe forms1.

A study of more than 30,000 health-care workers in Qatar found that those who got a flu jab were nearly 90% less likely to develop severe COVID-19 over the next few months, compared with those who hadn’t been recently vaccinated against flu.

The study, which was conducted in late 2020, before the roll-out of COVID-19 vaccines, is in line with previous work suggesting that ramping up the immune system using influenza vaccines and other jabs could help the body to fend off the coronavirus SARS-CoV-2.

Collateral benefit

In the early months of the pandemic — while COVID-19 vaccines were still in development — researchers were intensely interested in the possibility that existing vaccines might provide some protection against SARS-CoV-2. But collecting strong evidence for such an effect is difficult, because people who seek vaccination for diseases other than COVID-19 might also make other choices that reduce their risk of being infected with SARS-CoV-2.

To minimize the impact of this ‘healthy-user effect’, a team led by Laith Jamal Abu-Raddad, an infectious-disease epidemiologist at Weill Cornell Medicine–Qatar in Doha, analysed the health records of 30,774 medical workers in the country. There is probably less variation in health-related behaviour among such workers than in the general population, reducing — but probably not eliminating — bias, Abu-Raddad says.

The researchers tracked 518 workers who tested positive for SARS-CoV-2 and matched them to more than 2,000 study participants who had tested negative for the virus. Those who had received an influenza vaccine that season were 30% less likely to test positive for SARS-CoV-2, and 89% less likely to develop severe COVID-19, compared with workers who had not (although the number of severe cases was small in both groups). The study was posted on the medRxiv preprint server on 10 May.