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Flu vaccination reduces infection risk and symptom burden of COVID-19

With a reduction of influenza cases during the COVID-19 pandemic, researchers wondered if prior influenza vaccination affected COVID-19 susceptibility and severity.

Risk factors for infection with COVID-19 are relatively well established e.g., increasing age and associated comorbidities including cardiovascular diseases, diabetes and obesity. Moreover, these same factors increase the risk of serious complications from influenza, hence vaccination is advised for those with these co-morbidities. Due to the similarities between patient groups, could an influenza vaccination reduce the risk of developing COVID-19? This was the question posed by a team from the Department of Internal Medicine, University of Michigan, US. Some data has suggested that prior influenza vaccination reduces the risk of complications in those with COVID-19. As a result, the Michigan team set out to retrospectively examine the relationship between a positive test for COVID-19 and prior influenza vaccination. The team examined over 4.5 million patient records within the Michigan Medicine healthcare system for reported vaccinations between August 2019 and July 2020 and compared this with information on whether these individuals had tested positive for COVID-19. The primary outcome of interest was COVID-19 positivity compared with prior influenza vaccination. Among patients who tested positive, the team examined the clinical outcomes such as the need for hospitalisation, intensive care, mechanical ventilation and mortality.

A total of 27,201 patients received a COVID-19 test at the hospital, of whom 1218 (4.5%) tested positive and 12,997 (47.8%) had previously received an influenza vaccination. Among those testing positive for COVID-19, 505 (41.5%) required hospitalisation and 182 (36%) needed mechanical ventilation. Patients testing positive for COVID-19 also had higher rates of co-morbidities including chronic pulmonary disease (19.8% vs 14.6%), diabetes (8.5% vs 2.9%) and hypertension (36% vs 22.5%).

In patients who had received an influenza vaccination, there was a significant reduction in the odds of testing positive for COVID-19 (odds ratio, OR = 0.82, 95% CI 0.73–0.92, p <0.01). In addition, influenza vaccinated patients were less likely to require hospitalisation (OR = 0.58), mechanical ventilation (OR = 0.45) and had a shorter hospital stay (OR = 0.76).

Commenting on their findings, the authors suggested that prior influenza vaccination had a potentially protective role against infection with COVID-19. While this was a retrospective analysis and therefore unable to demonstrate a causal link, the authors called for future studies to examine this association in more detail.

They concluded that until the COVID-19 vaccine becomes more widely available, the influenza vaccine should be promoted as a means of reducing COVID-19 disease burden.

Conlon A et al. Impact of the influenza vaccine on COVID-19 infection rates and severity. Am J Infect Control 2021.