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

Research shows recent flu vaccination is associated with a noticeable reduction in the risk of SARS-CoV-2 infection and the severity of COVID-19

Multiple previous studies have shown a beneficial effect of flu shots on the risk of infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as well as on the risk of symptomatic and severe coronavirus disease 2019 (COVID-19) after infection. A new preprint reports the effectiveness of flu vaccination in this light.

Study: Effectiveness of flu vaccination against SARS-CoV-2 infection among health workers in Qatar† Image Credit: BlurryMe/Shutterstock


The flu vaccine is protective against the flu virus, reducing the number of cases and deaths with this seasonal pathogen. Flu shots are a high priority for older adults and health professionals (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 flu vaccination. It was necessary to ensure that this was not because flu shots are more commonly taken by health-conscious people, who are also more compliant with protective health behaviors against COVID-19. This is called the healthy user effect and is a possible source of bias in such studies.

The current study, which appears on the medRxivpreprint server, was conducted in Qatar, including more than 30,000 health workers who were vaccinated against flu in the period between September 17, 2020 and December 31, 2020, when the annual flu shots are usually given. Significantly, this was before the rollout of COVID-19 vaccines.

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

Subjects were tested for the virus by a polymerase chain reaction (PCR) assay with a median of 1.5 months after vaccination. Of the more than 30,000 HCWs, more than 12,000 were tested, with nearly 600 testing positive and 10,000 always negative. About two-thirds were tested for COVID-19-like symptoms.

What did the investigation reveal?

The results showed that the flu shots 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 the nearly 130 people who tested positive for SARS-CoV-2 by PCR after taking the flu shot, one developed severe COVID-19 (requiring hospitalization) and none developed into a critical or fatal illness .

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

Recent flu vaccination is associated with a noticeable reduction in the risk of SARS-CoV-2 infection and the severity of COVID-19† Striking as this statement is, it should be noted that the study included small numbers of serious cases. Nevertheless, the evidence supports the reported effectiveness of the flu vaccine against both infection and COVID-19 disease due to SARS-CoV-2.

The mechanism of protection has not yet been elucidated, but could be due to a general increase in the immune response enhancing non-specific immunity or trained specific immunity. The first usually lasts no more than a few weeks, and as even specific COVID-19 vaccines are known to decline effectiveness quite quickly, in the long run it can play no role in the protection.

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

Another effect may be due to bystander immunity, a process in which infection and inflammation stimulate a prolonged response that exposes autoantigens to the host’s immune system.

More importantly, the study is not generalizable as it mainly includes young, healthy HCWs. However, this negates the healthy user bias. So all in all”The findings support flu vaccination benefits beyond protection against flu infection and serious illness

*Important announcement

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

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