Influenza vaccine effectiveness in preventing inpatient and outpatient cases in a season dominated by vaccine-matched influenza B virus

Abstract

Studies that have evaluated the influenza vaccine effectiveness (VE) to prevent laboratory-confirmed influenza B cases are uncommon, and few have analyzed the effect in preventing hospitalized cases. We have evaluated the influenza VE in preventing outpatient and hospitalized cases with laboratory-confirmed influenza in the 2012–2013 season, which was dominated by a vaccine-matched influenza B virus. In the population covered by the Navarra Health Service, all hospitalized patients with influenza-like illness (ILI) and all ILI patients attended by a sentinel network of general practitioners were swabbed for influenza testing, and all were included in a test-negative case-control analysis. VE was calculated as (1-odds ratio)×100. Among 744 patients tested, 382 (51%) were positive for influenza virus: 70% for influenza B, 24% for A(H1N1)pdm09, and 5% for A(H3N2). The overall estimate of VE in preventing laboratory-confirmed influenza was 63% (95% confidence interval (CI): 34 to 79), 55% (1 to 80) in outpatients and 74% (33 to 90) in hospitalized patients. The VE was 70% (41 to 85) against influenza B and 43% (−45 to 78) against influenza A. The VE against virus B was 87% (52 to 96) in hospitalized patients and 56% in outpatients (−5 to 81). Adjusted comparison of vaccination status between inpatient and outpatient cases with influenza B did not show statistically significant differences (odds ratio: 1.13; p = 0.878). These results suggest a high protective effect of the vaccine in the 2012–2013 season, with no differences found for the effect between outpatient and hospitalized cases.

Keywords:

• case-control study
• effectiveness
• influenza
• influenza B virus
• influenza vaccine
• sentinel surveillance

Abbreviations:

• CI, confidence interval
• ILI, influenza-like illness
• OR, odds ratio
• RT-PCR, reverse transcription polymerase chain reaction
• VE, vaccine effectiveness

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by the Carlos III Institute of Health (PS12/00087), the Spanish Ministry of Health (EC11–302), and by the I-MOVE program of the European Center for Disease Prevention and Control (ECDC).

Note

Members of the Primary Health Care Sentinel Network of Navarra: I Abad, P Aldaz, E Álvarez, JJ Arana, I Arceiz, E Arina, I Arribas, MD Artajo, A Arza, B Azagra, FC Bartolome, C Bernués, C Bolea, A Brugos, B Cano, MV Castresana, JC Cenoz, F Cia, B Compains, JR Corpas, F Cortés, B Churío, PC Cuevas, EM Da Costa, J Díez Espino, M Doiz, FJ Escribano, MJ Esparza, V Etayo, C Fernández Alfaro, B Flamarique, J Gamboa, ML Garcés, L García Blanco, AB German, A Giner, M Gómara, N Goñi, MJ Guillorme, JO Guiu, JC Gurbindo, MJ Guruchaga, JA Heras, MC Hijos, MS Indurain, B Iñigo, SE Juan Belloc, JJ Jurio, MP León, JJ Longás, JJ Miner, M Moreno, MA Moros, U Navarro, FJ Orozco, M Orte, P Palacio, J Palau, C Pérez Lecumberri, P Pérez Pascual, B Pérez Sanz, A Prado Virto, M Prado Santamaria, A Puig Arrastia, E Ridruejo, M Ramos, BE Rípodas, M Rodríguez, MA Roncal, I Ruiz Puertas, C Sánchez Vázquez, P Sarrasqueta, MA Senosiain, J Sola, M Sota, ME Ursua, IA Urtasun, MJ Vigata, MT Virto, JM Vizcay.

Members of the Network for Influenza Surveillance in Hospitals of Navarra: P Artajo, X Beristain, E Bernaola, J Chamorro, M Esquiroz, P Fanlo, F Gil, M Gabari, J Hueto, C Martín, L Peña, C Pérez, M Ruiz (Complejo Hospitalario de Navarra), M Fernández-Alonso, J Núñez (Clínica Universidad de Navarra), JJ García Irure, M Torres, MT Ortega (Hospital Reina Sofía, Tudela), F Lameiro, L Barrado (Hospital García Orcoyen, Estella), N Alvarez (Servico Navarro de Salud), M Guevara, F Irisarri, M Arriazu, A Zabala, A Barricarte, J Castilla (Instituto de Salud Pública de Navarra).