The influenza vaccine is protective against the influenza virus, reducing the number of cases and deaths that occur with this seasonal pathogen. Flu shots are a high priority for older adults and healthcare workers (HCWs) at a greater risk of infection and complications.
Earlier research suggests a link between SARS-CoV-2 infection/adverse COVID-19 outcomes and prior influenza vaccination. There was a need to make sure that this was not because flu shots are more likely to be taken by the health-conscious, who are also more compliant with protective health 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 conducted among health care workers at Hamad Medical Corporation, the principal provider of public healthcare services in Qatar and the nationally designated entity for COVID-19-related healthcare needs.
It included over 30,000 healthcare workers vaccinated against influenza during 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. All participants received the quadrivalent Influvac Tetra vaccine (Abbott). Cases and controls were in a ratio of 1:5.
The results showed that the flu shots reduced the risk of SARS-CoV-2 over the next two weeks by 30%. Conversely, they reduced the risk of severe or fatal COVID-19 by 90%. Of nearly 130 individuals who tested positive for SARS-CoV-2 by PCR after taking the flu shot, one developed severe COVID-19 (requiring hospitalization), and none progressed to critical or fatal disease.
In contrast, among nearly 400 unvaccinated patients who tested positive, there were 17 severe and 2 critical cases, though no deaths occurred.
It must be noted however, that the study included small numbers of severe cases. Nonetheless, the evidence supports the reported effectiveness of the influenza vaccine against both infection with and COVID-19 disease as a result of SARS-CoV-2.
The protection mechanism is as yet unexplained. More importantly, the study is not generalizable because it includes mostly young, healthy HCWs. However, this negates the healthy user bias. Overall, therefore, “The findings support benefits for influenza vaccination that extend beyond protection against influenza infection and severe disease.”