Background A possible link between influenza immunization and susceptibility to the complications of COVID-19 infection has been previously suggested owing to a boost in the immunity against SARS-CoV-2. Objective This study aimed to investigate whether individuals with COVID-19 could have benefited from vaccination against influenza. We hypothesized that the immunity resulting from the previous influenza vaccination would boost part of the immunity against SARS-CoV-2. Methods We performed a population-based cohort study including all patients with COVID-19 with registered entries in the primary health care (PHC) electronic records during the first wave of the COVID-19 pandemic (March 1 to June 30, 2020) in Catalonia, Spain. We compared individuals who took an influenza vaccine before being infected with COVID-19, with those who had not taken one. Data were obtained from Information System for Research in Primary Care, capturing PHC information of 5.8 million people from Catalonia. The main outcomes assessed during follow-up were a diagnosis of pneumonia, hospital admission, and mortality. Results We included 309,039 individuals with COVID-19 and compared them on the basis of their influenza immunization status, with 114,181 (36.9%) having been vaccinated at least once and 194,858 (63.1%) having never been vaccinated. In total, 21,721 (19%) vaccinated individuals and 11,000 (5.7%) unvaccinated individuals had at least one of their outcomes assessed. Those vaccinated against influenza at any time (odds ratio [OR] 1.14, 95% CI 1.10-1.19), recently (OR 1.13, 95% CI 1.10-1.18), or recurrently (OR 1.10, 95% CI 1.05-1.15) before being infected with COVID-19 had a higher risk of presenting at least one of the outcomes than did unvaccinated individuals. When we excluded people living in long-term care facilities, the results were similar. Conclusions We could not establish a protective role of the immunity conferred by the influenza vaccine on the outcomes of COVID-19 infection, as the risk of COVID-19 complications was higher in vaccinated than in unvaccinated individuals. Our results correspond to the first wave of the COVID-19 pandemic, where more complications and mortalities due to COVID-19 had occurred. Despite that, our study adds more evidence for the analysis of a possible link between the quality of immunity and COVID-19 outcomes, particularly in the PHC setting.
BACKGROUND A possible link between influenza immunization and susceptibility to COVID-19 infection has been previously suggested due to a boost in the immunity against SARS-CoV-2. OBJECTIVE We aimed to assess this hypothetic association in our setting. METHODS Population-based cohort study including all patients with COVID-19 registers in the primary health care (PHC) electronic records during the first wave of the COVID-19 pandemic (1st March 2020 – 30th June 2020) in Catalonia, Spain. We compared those people ever exposed to influenza vaccine before the COVID-19 infection with those never exposed. The data source is SIDIAP, capturing PHC information of 5.8 million people from Catalonia. The main outcomes assessed during follow-up were diagnosis of pneumonia, hospital admission and mortality. RESULTS We included 309,039 COVID-19 patients and compared them according to their influenza immunization status, being 114,181 (36.9%) vaccinated at least once and 194,858 (63.1%) never vaccinated. 21,721 (19%) of the flu-vaccinated and 11,000 (5.7%) of the non-vaccinated had at least one of the outcomes assessed. Those vaccinated against flu at any time, recently or recurrently before COVID-19 had higher risk of presenting at least one of the outcomes than those non-vaccinated. When we excluded people living in long-term care facilities, the results were similar. CONCLUSIONS We were not able to find a protective role of the immunity conferred by the influenza vaccine on the outcomes of the COVID-19 infection, as the risk of COVID-19 complications was higher in the vaccinated than in the non-vaccinated. Our results are from the first wave of the pandemic, were more complications and mortality due to COVID-19 occurred. Despite that, our study adds more evidence to the analysis of the possible link between the quality of the immunity and the COVID-19 outcomes and to the analysis of PHC patients. CLINICALTRIAL AEMPS classification IDI-VAC-2020-21. Estudio Posautorización con Otros Diseños diferentes al de seguimiento prospectivo, EPA-OD. 15th September 2020.
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