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Does the influenza jab reduce the risk of infection and symptom burden in COVID-19?

With the same group of patients at a higher risk of complications from both influenza and COVID-19, researchers investigated if prior influenza vaccination affected COVID-19 susceptibility and severity.

The risk for serious complications from COVID-19 include increasing age and the presence of the co-morbidities such as cardiovascular diseases, diabetes and obesity. These same factors also enhance the risk of complications from influenza, hence the need for vaccination in these patients. Because there are similarities between the two 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 have 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. They 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.

Overall, 27,201 patients received a COVID-19 test at the hospital, of whom 1218 (4.5%) tested positive and 12,997 (47.8%) of them 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).

In discussing 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.

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