Study: Effectiveness of influenza vaccination against SARS-CoV-2 infection among healthcare workers in Qatar. Image Credit: BlurryMe/Shutterstock

Study finds recent flu vaccination is associated with significant reduction in risk of SARS-CoV-2 infection and severity of COVID-19

Several previous studies have indicated a favorable effect of influenza vaccines on the risk of infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as well as the risk of symptomatic and severe coronavirus disease 2019 ( COVID-19) after infection. A new preprint reports the effectiveness of influenza vaccination in this regard.

Study: Effectiveness of influenza vaccination against SARS-CoV-2 infection among healthcare workers in Qatar. Image Credit: BlurryMe/Shutterstock


The flu vaccine protects against the flu virus, reducing the number of cases and deaths from this seasonal pathogen. Influenza vaccines are a high priority for older adults and healthcare workers (HCWs) who are at greater risk of infection and complications.

Previous research suggests a link between SARS-CoV-2 infection/adverse COVID-19 outcomes and prior influenza vaccination. There was a need to make sure it wasn’t because flu shots are more likely to be taken by health conscious people, who are also more compliant with health protective behaviors against COVID -19. This is called the healthy user effect and is a potential source of bias in such studies.

The current study, which appears on the medRxiv* preprint server, was carried out in Qatar, including more than 30,000 healthcare workers vaccinated against influenza during the period between September 17, 2020 and December 31, 2020, when annual flu vaccines are usually administered. Significantly, this was before the rollout of COVID-19 vaccines.

Vaccinated participants had a median age of 36, while a control group had a slightly lower median age of 35. All participants received the quadrivalent vaccine Influvac Tetra (Abbott). Cases and controls were in a 1:5 ratio.

Subjects were tested for the virus by a polymerase chain reaction (PCR) test at a median of 1.5 months after vaccination. Of more than 30,000 healthcare workers, more than 12,000 have been tested, with nearly 600 testing positive and 10,000 still negative. About two-thirds had tests due to COVID-19-like symptoms.

What did the study show?

The results showed that flu vaccines reduced the risk of SARS-CoV-2 by 30% over the next two weeks. Conversely, they reduced the risk of severe or fatal COVID-19 by 90%. Of nearly 130 people who tested positive for SARS-CoV-2 by PCR after being vaccinated against influenza, one developed severe COVID-19 (requiring hospitalization) and none progressed to critical or fatal illness.

In contrast, among nearly 400 unvaccinated patients who tested positive, there were 17 severe cases and 2 critical cases, although no deaths occurred.

Recent influenza vaccination is associated with an appreciable reduction in risk of SARS-CoV-2 infection and severity of COVID-19.” As striking as this claim is, it should be noted that the study included a small number of severe cases. Nonetheless, evidence supports the reported efficacy of the influenza vaccine against COVID-19 infection and disease resulting from SARS-CoV-2.

The mechanism of protection is still unexplained but could be due to a general increase in immunological reactivity enhancing non-specific immunity or trained specific immunity. The former usually doesn’t last beyond a few weeks, and given that even specific COVID-19 vaccines are known to decline in efficiency quickly enough, it may not play a long-term protective role.

Trained immunity is the response of the innate immune system, the first line of defense, by which immune memory is formed to provide long-term protection against foreign antigens.

Another effect could be due to bystander immunity, a process by which infection and inflammation stimulate a prolonged response that exposes self-antigens to the host immune system.

More importantly, the study is not generalizable because it primarily includes young, healthy healthcare workers. However, this negates the healthy user bias. Overall, therefore,Results confirm benefits of influenza vaccination that go beyond protection against influenza infection and serious illness.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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