Objective: To determine whether the prior usage of the flu vaccine is a risk factor for bacterial co-infection in patients with severe influenza.

Design: This was a retrospective observational cohort study of subjects admitted to the ICU. A propensity score matching, and logistic regression adjusted for potential confounders were carried out to evaluate the association between prior influenza vaccination and bacterial co-infection.

Settings: 184 ICUs in Spain due to severe influenza.

Patients: Patients included in the Spanish prospective flu registry.

Interventions: Flu vaccine prior to the hospital admission.

Results: A total of 4175 subjects were included in the study. 489 (11.7%) received the flu vaccine prior to develop influenza infection. Prior vaccinated patients were older 71 [61-78], and predominantly male 65.4%, with at least one comorbid condition 88.5%. Prior vaccination was not associated with bacterial co-infection in the logistic regression model (OR: 1.017; 95%CI 0.803-1.288; p=0.885). After matching, the average treatment effect of prior influenza vaccine on bacterial co-infection was not statistically significant when assessed by propensity score matching (p=0.87), nearest neighbor matching (p=0.59) and inverse probability weighting (p=0.99).

Conclusions: No association was identified between prior influenza vaccine and bacterial coinfection in patients admitted to the ICU due to severe influenza. Post influenza vaccination studies are necessary to continue evaluating the possible benefits.