Open Access Highly Accessed Research article

Effectiveness of influenza vaccine against laboratory-confirmed influenza, in the late 2011–2012 season in Spain, among population targeted for vaccination

Silvia Jiménez-Jorge12*, Salvador de Mateo12, Concha Delgado-Sanz1, Francisco Pozo3, Inmaculada Casas3, Manuel Garcia-Cenoz24, Jesús Castilla24, Esteban Pérez5, Virtudes Gallardo5, Carolina Rodriguez6, Tomás Vega6, Carmen Quiñones7, Eva Martínez7, Juana María Vanrell8, Jaume Giménez8, Daniel Castrillejo9, María del Carmen Serrano10, Julián Mauro Ramos10, Amparo Larrauri12 and on behalf of the Spanish Influenza Sentinel Surveillance System

Author Affiliations

1 National Centre of Epidemiology, Institute of Health Carlos III, c/Monforte de Lemos no. 5, Madrid 28029, Spain

2 Ciber Epidemiología y Salud Pública (CIBERESP), Ministry of Science and Innovation, Institute of Health Carlos III, Madrid, Spain

3 National Centre for Microbiology, National Influenza Reference Laboratory, WHO-National Influenza Centre, Institute of Health Carlos III, Majadahonda, Madrid 28220, Spain

4 Instituto de Salud Pública de Navarra, Navarra, Spain

5 Servicio de Epidemiología y Salud Laboral. Secretaría General de Salud Pública y Participación. Consejería de Salud de Andalucía, Consejería, Spain

6 Dirección General de Salud Pública, Consejería de Sanidad de Castilla y León, Spain

7 Servicio de Epidemiología, Subdirección de Salud Pública de La Rioja, La Rioja, Spain

8 Servicio de Epidemiología, Dirección General de Salut Pública, Baleares, Spain

9 Servicio de Epidemiología. Dirección General de Sanidad y Consumo, Consejería de Bienestar Social y Sanidad, Ciudad Autónoma de Melilla, Spain

10 Dirección General de Salud Pública, Servicio Extremeño de Salud, Junta de Extremadura, Spain

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BMC Infectious Diseases 2013, 13:441  doi:10.1186/1471-2334-13-441

Published: 22 September 2013

Abstract

Background

In Spain, the influenza vaccine effectiveness (VE) was estimated in the last three seasons using the observational study cycEVA conducted in the frame of the existing Spanish Influenza Sentinel Surveillance System. The objective of the study was to estimate influenza vaccine effectiveness (VE) against medically attended, laboratory-confirmed influenza-like illness (ILI) among the target groups for vaccination in Spain in the 2011–2012 season. We also studied influenza VE in the early (weeks 52/2011-7/2012) and late (weeks 8-14/2012) phases of the epidemic and according to time since vaccination.

Methods

Medically attended patients with ILI were systematically swabbed to collect information on exposure, laboratory outcome and confounding factors. Patients belonging to target groups for vaccination and who were swabbed <8 days after symptom onset were included. Cases tested positive for influenza and controls tested negative for any influenza virus. To examine the effect of a late season, analyses were performed according to the phase of the season and according to the time between vaccination and symptoms onset.

Results

The overall adjusted influenza VE against A(H3N2) was 45% (95% CI, 0–69). The estimated influenza VE was 52% (95% CI, -3 to 78), 40% (95% CI, -40 to 74) and 22% (95% CI, -135 to 74) at 3.5 months, 3.5-4 months, and >4 months, respectively, since vaccination. A decrease in VE with time since vaccination was only observed in individuals aged ≥ 65 years. Regarding the phase of the season, decreasing point estimates were only observed in the early phase, whereas very low or null estimates were obtained in the late phase for the shortest time interval.

Conclusions

The 2011–2012 influenza vaccine showed a low-to-moderate protective effect against medically attended, laboratory-confirmed influenza in the target groups for vaccination, in a late season and with a limited match between the vaccine and circulating strains. The suggested decrease in influenza VE with time since vaccination was mostly observed in the elderly population. The decreasing protective effect of the vaccine in the late part of the season could be related to waning vaccine protection because no viral changes were identified throughout the season.

Keywords:
Influenza; Vaccine effectiveness; Case–control studies; Sentinel networks; Discordant strain; Waning immunity