Flu Vaccine Could Reduce Severity of COVID-19

Gunther Fink, PhD

Influenza vaccination is recommended to help avoid catching the flu and to reduce the burden of the "twindemic" of flu and coronavirus 2019 (COVID-19) on the healthcare system, and a new study finds that it could also reduce the severity of COVID-19.

The study comparing outcomes of patients hospitalized for COVID-19 who did or did not receive influenza vaccination prior to hospitalization was conducted by Günther Fink, PhD, University of Basel and the Swiss Tropical and Public Health Institute, and colleagues.

"This is the first study assessing the associations between influenza vaccination and COVID-19 severity as well as COVID-19 mortality using clinical patient records," the investigators declare.

Fink and colleagues identified over 53,000 patients hospitalized for COVID-19 in Brazil between January 1 and June 23, 2020 from that country's national electronic respiratory infection data system, distinguishing patients who had received inactivated trivalent influenza vaccination. The primary outcome measure was age-specific mortality rates between vaccinated and unvaccinated patients, and secondary measures included level of required care.

The investigators reported that mortality was consistently lower across all age groups of patients receiving vaccination in March 2020 or later, with absolute mortality differences ranging from a risk difference of 12% in patients 10-19 years of age to 3% in the very elderly patients ≥90 years old. The putative-protective association was not found in those vaccinated earlier, or in 2019.

Overall, influenza vaccination received in 2020 and prior to onset of COVID-19 pandemic was associated with 17% lower odds of mortality, and with 36% lower odd of mortality ifadministered after onset of clinical COVID-19 symptoms.The investigators characterize this finding as "surprising," but note the two estimates were not statistically significantly different given the numbers of patients in those groups.

Influenza vaccination was also associated with a 7% reduction in the odds of receiving intensive care and a 17% reduction in odds of receiving respiratory support.

Fink and colleagues considered factors that could confound the apparent protective effect of vaccination, including the possibility that persons who vaccinate follow other healthy behaviors and are less likely to have the underlying conditions and comorbidities associated with severe COVID-19.

"The hospital records we work with have relatively detailed information on comorbidities like diabetes or obesity, and we can rule out confounding through these factors. What we cannot rule out are other behavioral differences, (such as) subjects who got the vaccine must lead healthier lives in ways that are not reflected in comorbidities." Fink told HCPLive.

"The most obvious such factor is likely smoking, which is generally more common among people not getting vaccines, and which may change disease outcomes even when the observed pre-existing comorbidities are the same," Fink said.

The investigators posit that a protective effect from influenza vaccine could emerge from the resultant "trained innate immunity." They pointed to the differential protective effects observed in the older (>60 years) versus younger COVID-19 patient groups; and note that influenza vaccines are generally less effective in older than in younger individuals, with development of trained innate immunity less likely among older patients.

"The 2020/21 winter season will of course also give us interesting data to work with because flu vaccination rates vary a lot across age groups and countries, and we will be able to at least explore some further correlations by the time we reach Summer 2021," Fink commented.

The study, “Inactivated Trivalent Influenza Vaccination is Associated with Lower Mortality Among Patients with COVID-19 in Brazil,” was published online in BMJ Evidence-Based Medicine.