Impact of the influenza vaccine on COVID-19 infection rates and severity

Abstract

Background: With a unique influenza season occurring in the midst of a pandemic, there is interest in assessing the role of the influenza vaccine in COVID-19 susceptibility and severity.

Methods: In this retrospective cohort study, patients receiving a laboratory test for COVID-19 were identified. The primary outcome was comparison of positive COVID-19 testing in those who received the influenza vaccine versus those who did not. Secondary end points in patients testing positive for COVID-19 included mortality, need for hospitalization, length of stay, need for intensive care, and mechanical ventilation.

Results: A total of 27,201 patients received laboratory testing for COVID-19. The odds of testing positive for COVID-19 was reduced in patients who received an influenza vaccine compared to those who did not (odds ratio 0.76, 95% CI 0.68-0.86; P < .001). Vaccinated patients testing positive for COVID-19 were less likely to require hospitalization (odds ratio, 0.58, 95% CI 0.46-0.73; P < .001), or mechanical ventilation (odds ratio, 0.45, 95% CI 0.27-0.78; P = .004) and had a shorter hospital length of stay (risk ratio, 0.76, 95% CI 0.65-0.89; P < .001).

Conclusion: Influenza vaccination is associated with decreased positive COVID-19 testing and improved clinical outcomes and should be promoted to reduce the burden of COVID-19.

Keywords: COVID-19; Influenza vaccination.

Fig 1

Study Flow Diagram of Search Results for Included Patients. A total of 4,549,598 unique patient charts were searched within the Michigan Medicine health care system using DataDirect. The search was limited to include only patients who received a laboratory test for COVID-19 within the Michigan Medicine healthcare system. Record of influenza vaccination between August 1, 2019 to July 15, 2020 was then obtained.

Fig 2

Effect of Influenza Vaccination Status on COVID-19 Clinical Outcomes. Total cohort includes all patients who tested positive for COVID-19 (n = 1,218) and is stratified by those who did (n = 525) or did not (n = 693) receive the influenza vaccine. Mechanical ventilation, intensive care and length of stay were assessed only for those who were hospitalized (n = 505). The proportion of patients requiring hospitalization, mechanical ventilation, intensive care as well as mortality rates are represented by values on the primary Y-axis. Average length of stay (days) is represented on the secondary Y-axis. All clinical outcomes reported are adjusted for baseline covariates including ethnicity, race, gender, age BMI, Elixhauser score, smoking status and the combined metric for chronic pulmonary, congestive heart failure, diabetes, and hypertension. *P < .05.