Background:

There are controversies regarding the effect of the influenza vaccine on the risk of suffering from COVID-19. The aim of this paper was to compare the risk of suffering from COVID-19 between vaccinated and unvaccinated patients against influenza.

Methods:

A retrospective case-control study of patients assigned to an urban CAP (coverage: 44,564 inhabitants) was carried out during 2020. Patients with diagnostic confirmation of COVID-19 (ART, PCR or serology) were defined as cases, and those who have not had said infection, selected by simple random sampling, were defined as controls. The case/control ratio was 1:1. The data sources used were the Khalix health data registry databases and the computerized medical record (E-cap). Data were analyzed with a multivariable logistic regression model.

Results:

A total of 4,320 patients were included in the study: 2,160 cases (50%) and 2,160 controls. 18% of the cases and 14.3% of the controls were vaccinated against influenza [OR=1.3 (95% CI: 1.2-1.5; p=0.001)]. Multivariate analysis showed that in patients under eighty years of age, there were no differences between vaccinated and unvaccinated patients [OR=1.02 (95% CI: 0.8-1.3; p=0.8)]; while in those over eighty years of age, there was a lower risk of presenting COVID-19 in vaccinated patients [OR=0.4 (95% CI: 0.3-0.7; p<0.001)].

Conclusions:

The adjusted model shows that at ages above than 80 years, flu-vaccinated patients have decreased risk of COVID-19 with statistical significance.

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