[Influence of distinct criteria for selecting patients for swabbing on estimation of the effectiveness of the influenza vaccine]. / Influencia del criterio de selección de pacientes para la toma de frotis en la estimación de la efectividad de la vacuna antigripal.

ABSTRACT

OBJECTIVE: To estimate the effectiveness of the influenza vaccine under different criteria for selecting patients for swabbing. METHOD: A case-control study was performed of laboratory-confirmed cases (n=909) and negative controls for influenza (n=732) in the 2010-2011 to 2012-2013 seasons in Navarre (Spain). The adjusted vaccine effectiveness was estimated by including all swabs from patients with influenza-like-illness and selecting only the first two cases per physician and week. RESULTS: The first two patients per physician and week were less frequently vaccinated against influenza (7.9% vs. 12.5%, p=0.021) and less often received confirmation of influenza (53.6% vs. 66.4%, p <0.001) than subsequent patients. These differences decreased after adjustment for covariates. The effectiveness of the influenza vaccine was 49% (95% CI 23-66%) when all swabs were included and was 55% (95% CI 27-72%) when we selected the first two swabs per week and physician. CONCLUSION: The selection of the first two patients per physician and week may bias assessment of the effectiveness of the influenza vaccine, although this bias was small in the seasons analyzed.