Influenza Vaccination Failed to Reduce COVID-19 Risk
In a new study from the University of Copenhagen, vaccination against influenza was found to not change the risk of hospitalization or symptoms due to COVID-19.
And influenza vaccination was not associated with the risk of contracting SARS-CoV-2 infection as assessed by the presence of antibodies against SARS-CoV-2.
This study focused on the working population and subgroup analyses of outcomes related to the severity of disease included a sparse number of HCW aged ≥65 who had contracted SARS-CoV-2.
‘To our knowledge, this is the first large-scale study to examine the effect of influenza vaccination on the severity of disease due to COVID-19 in 42,112 health care works (HCW) in Denmark,’ wrote these researchers.
The relative risk of hospitalization due to SARS-CoV-2 for influenza vaccinated compared with unvaccinated participants was 1.00 for the seasonal vaccination in 2019/2020 (CI 0.56-1.78, p=1.00).
The Journal of Infectious Diseases published these findings on January 5, 2022, indicating that influenza vaccination does not affect the risk of SARS-CoV-2 coronavirus infection or the disease it causes COVID-19.
The current study is in line with the previous null-effect studies on infection with SARS-CoV-2 as measured by pharyngeal swabs or serology.
Previous findings on non-specific, protective effects of influenza vaccination have been divergent.
One prior, cross-sectional registry study (n≈56million) by Huang et al. examined the severity of COVID-19 as defined by the need for mechanical ventilation in patients aged ≥65 years.
When comparing influenza vaccinated individuals (n≈13million) to unvaccinated (n≈43million), Huang et al. found a 28% reduction in the odds (CI 0.68–0.76) of needing mechanical ventilation due to COVID-19 and a 24% reduction in the odds (CI 0.75–0.77) of SARS-CoV-2 infection.
All baseline comorbidities except asthma (diabetes, hemoglobin disorder, immunocompromised, severe obesity, and chronic kidney, lung, heart, and liver disease) were more frequent in the unvaccinated individuals and confounding is likely as no adjustment for comorbidity was performed.
Furthermore, as Huang et al. examined influenza vaccination in patients aged ≥65 years, the population is not directly comparable to the current study.
Another study, Noale et al. found influenza vaccination to reduce the probability of SARS-CoV-2 infection as measured by nasopharyngeal swab testing in a cross-sectional questionnaire study of participants aged <65 years (n=6,061, OR 0.85, CI 0.74–0.98), while no significant effect was found for participants aged ≥65 years (n=619, OR 0.87, CI 0.59–1.28).
Several other studies did not find an effect of influenza vaccination on infection with SARS-CoV-2.
The current study has several limitations.
As the information on the severity of COVID-19 is based on a survey filled in by participants, contemporary hospitalizations may be underreported. Participants were, however, able to fill in the survey from home or at a later screening round.
As for all observational studies confounding cannot be excluded. When it comes to influenza vaccination this issue may be of even higher susceptibility and importance as vaccination might be more frequent among individuals with better health care insurance or higher awareness of personal health.
Even though influenza vaccination in Denmark is offered to HCW free of charge and administered during work hours a higher proportion of educated HCW and HCW with chronic diseases received the influenza vaccine.
In conclusion, these researchers wrote ‘present findings indicate that influenza vaccination does not affect the risk of SARS-CoV-2 infection or COVID-19 disease.’
This work was supported by an unrestricted grant by the Lundbeck Foundation[R349-2020-731], and several researchers disclosed potential conflicts of interest.
As of December 8, 2021, the U.S. CDC says ‘Flu vaccines do not protect against COVID-19. However, flu vaccination reduces the risk of flu illness, hospitalization, and death from influenza-related viruses.
Likewise, getting a COVID-19 vaccine is the best protection against COVID-19. But those vaccines do not protect against flu.
Visit the CDC’s Frequently Asked Questions page for information about COVID-19 vaccinations and flu shots.
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