Previous research has shown that influenza vaccination reduces the likelihood of infection with the SARS-CoV-2 coronavirus and the risk of developing symptomatic and severe coronavirus disease 2019 (COVID-19) after infection. Given this, a recent preprint investigates the efficacy of influenza vaccination.

The influenza vaccine protects against the influenza virus, reducing the number of seasonal illnesses and deaths caused by this infection. Vaccinating the elderly and healthcare workers (HCWs), who are more vulnerable to sickness and its consequences, is a major priority.

Preceding research has linked SARS-CoV-2 infection to unfavorable COVID-19 outcomes, as well as prior influenza vaccination. There was a need to confirm that this was not because health-conscious people are more likely to get flu shots and follow COVID-19 preventative health practices. The phenomenon known as the healthy user effect may bring bias into such investigations.

The most recent study, published on the preprint service medRxiv*, was conducted in Qatar and included roughly 30,000 healthcare personnel who received annual flu vaccines between September 17, 2020, and December 31, 2020. This occurred before the availability of COVID-19 vaccines.

The average age of the vaccinated participants was 36, whereas the unvaccinated participants were 35. Everyone received the quadrivalent Influvac Tetra vaccine (Abbott). The case-to-control ratio was 1 to 5. The subjects were tested for the virus using polymerase chain reaction 5 months after vaccination (PCR). Around 600 of the nearly 30,000 HCWs tested were positive, whereas the remaining 10,000 were consistently negative. Approximately two-thirds of the people tested positive for COVID-19-like symptoms.

The flu vaccines, according to the findings, lowered the risk of developing SARS-CoV-2 by 30% during the next two weeks. In contrast, they lowered the chance of severe or fatal COVID-19 by 90%. After getting the flu vaccine, one person had severe COVID-19 (requiring hospitalization), but no one developed a critical or fatal disease. In comparison, there were 17 severe and 2 critical cases among the nearly 400 unvaccinated persons with positive test results, but no fatalities.

Recent influenza vaccination is associated with a significant reduction in the risk of SARS-CoV-2 infection and COVID-19 severity. This finding is significant, but keep in mind that the study only looked at a small number of severe instances. Despite this, the findings support the influenza vaccination’s reported efficacy against SARS-CoV-2 infection and COVID-19 illness.

The protective mechanism is unknown, however, it may be linked to an increase in immunologic responsiveness, which increases nonspecific immunity or trained specific immunity. Because even specific COVID-19 vaccines are known to lose efficacy quickly, the former may not give long-term protection.

Trained immunity is the innate immune system’s first line of defense, where immunological memory is formed to provide long-term protection against foreign antigens. Bystander immunity can occur when infection or inflammation causes a prolonged immune response that exposes autoantigens to the host’s immune system.

Furthermore, because the study only involved young, healthy HCWs, the findings cannot be generalized. Nonetheless, this eliminates the positive user bias. Overall, “the findings show that the influenza vaccine’s effects extend beyond protection against influenza infection and severe illness.”

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