RESUMO
Objetivo: estimar la efectividad de la vacuna antigripal según el criterio de selección en la toma de frotis. Método: estudio de casos y controles de casos confirmados (n = 909) y controles negativos para gripe (n = 732) en las temporadas 2010-2011 a 2012-2013 en Navarra. La efectividad ajustada de la vacuna se estimó incluyendo todos los frotis de pacientes con síndrome gripal y seleccionando sólo los dos primeros por médico y semana. Resultados: los dos primeros pacientes por médico y semana estaban menos vacunados (7,9% frente a 12,5%, p = 0,021) y se confirmaron menos para gripe (53,6% frente a 66,4%, p <0,001), diferencias que se redujeron al ajustar por covariables. La efectividad de la vacuna calculada con todos los frotis fue del 49% (intervalo de confianza del 95% [IC95%]: 23-66%) y del 55% (IC95%: 27-72%) al analizar los dos primeros frotis semanales. Conclusión: la selección de los primeros pacientes semanales puede sesgar la efectividad de la vacuna antigripal, aunque en las temporadas analizadas este sesgo fue pequeño
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
