Estimating the influenza vaccine effectiveness in elderly on a yearly basis using the Spanish influenza surveillance network—Pilot case–control studies using different control groups, 2008–2009 season, Spain
Introduction
Annual vaccination is the main intervention to prevent seasonal influenza related outcomes in elderly [1], [2]. All European Union (EU) and European Economic Area (EEA) member states recommend annual influenza vaccination in high-risk groups [3], [4]. Most estimates of the influenza vaccine effectiveness (IVE) in these countries are based on data provided by the national influenza surveillance systems [5]. It is the case of Spain, whose influenza surveillance system (Spanish Influenza Sentinel Surveillance System – SISSS) was established along with other European networks in 1996 [11], [12]. The objectives of the SISSS are to provide timely clinical and virological information on influenza activity and to identify circulating strains. In the season 2008–2009, SISSS included over 700 general practitioners (GPs) and paediatricians from 16 out of 18 Spanish regions, covering 2.08% of the Spanish population. GPs report cases of influenza like illness (ILI) on a weekly basis and take swabs for the virological surveillance. Since 2002–2003, IVE was estimated in the frame of SISSS using the screening method: the proportion of ILI cases vaccinated was compared to the vaccination coverage of the Spanish population. The source of vaccination coverage in the ≥65 years was the Ministry of Health estimates, available shortly after the end of the influenza vaccination campaign.
Published studies using different designs and outcomes provided a wide range of IVE estimates in elderly [5], [6], [7]. There are only few published studies that used laboratory-confirmed influenza as outcome in Europe [8], [9], [10]. Since most of the IVE studies are observational, they are prone to bias. Confounding factors such as underline illness severity or functional status can distort the estimates and different methods to adjust for these confounding factors have been proposed [13], [14].
To develop a system to monitor seasonal and pandemic IVE in the EU/EEA, the European Centre for Diseases Prevention and Control (ECDC) funded a project (I-MOVE) that piloted different study designs during the 2008–2009 season [5]. In the framework of I-MOVE, five case–control [15] and two cohort studies were conducted in six EU countries. Spain participated in I-MOVE project with a case–control study using two different control groups. We also estimated the IVE using the screening method. Both designs (case–control and screening method) were carried out in the frame of SISSS. We aimed at measuring IVE against laboratory-confirmed influenza in elderly ≥65 years and identifying a study design feasible and sustainable to monitor IVE in Spain.
Section snippets
Methods
We conducted a case–control study among elderly ≥65 years old. Seven out 16 Spanish regional networks included in the SISSS participated in the study. We invited to participate in the study all 219 sentinel GPs of these seven networks. The population covered by the participating GPs represented 1.74% of the elderly population in the seven Spanish regions.
Participating GPs swabbed all patients in the age group ≥65 years who were attended for influenza like illness from week 40/2008 to week
Results
In Spain, the influenza activity was moderate in the season 2008–2009. The predominant influenza circulating virus was A(H3N2). Influenza activity baseline levels were overpassed in week 50/2008 reaching the peak in week 01/2009 (218 ILI cases/100,000 inhabitants). The epidemic period lasted for nine weeks, from week 50/2008 to week 07/2009. The highest incidence rate was registered in under 15 years old, so that the cumulative incidence was 3365/100,000 in the age group under 5 years old and
Discussion
In the 2008–2009 season we estimated the IVE in Spain in the frame of the SISSS, using case–control and screening designs. The estimated crude IVEs in the case–control studies with the two control groups were similar. These estimates suggest a good protective effect of the 2008–2009 influenza vaccine against influenza confirmed medically attended ILI in elderly. The point IVE estimate using the screening method was lower than the case–control study estimates even though the confidence intervals
Acknowledgments
We are thankful to all participating sentinel GPs in the cycEVA Study. We also thank Esther Kissling (EpiConcept), Bruno Ciancio (ECDC), Johan Giesecke (ECDC) for their support during the study. We are especially grateful to Alain Moren (EpiConcept) for his guidance and support during the study. This study was partly funded by European Centre for Disease Prevention and Control.
References (28)
- et al.
Efficacy and effectiveness of influenza vaccines in elderly people: a systematic review
Lancet
(2005) - et al.
Field investigation of influenza vaccine effectiveness on morbidity
Vaccine
(1998) - et al.
New strategies are needed to improve the accuracy of influenza vaccine effectiveness estimates among seniors
J Clin Epidemiol
(2009) - et al.
Estimating vaccine effectiveness against laboratory-confirmed influenza using a sentinel physician network: results from the 2005–2006 season of dual A and B vaccine mismatch in Canada
Vaccine
(2007) - et al.
Real-time monitoring of the influenza vaccine field effectiveness
Vaccine
(2006) Influenza vaccines
Wkly Epidemiol Rec
(2002)- et al.
The scientific basis for offering seasonal influenza immunisation to risk groups in Europe
Euro Surveill
(2008) National seasonal influenza vaccination survey in Europe. Final report
(2007)- Mereckiene J, Cotter S, Nicoll A, Lévy-Bruhl D, Ferro A, Tridente G, et al., on behalf of the VENICE Project...
- et al.
First steps in the design of a system to monitor vaccine effectiveness during seasonal and pandemic influenza in EU/EEA Member States
Euro Surveill
(2008)
Vaccines for preventing influenza in the elderly
Cochrane Database Syst Rev
Using sentinel surveillance to monitor effectiveness of influenza vaccine is feasible: a pilot study in Denmark
Euro Surveill
Estimation of influenza vaccine effectiveness using routine surveillance data
Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz
Cited by (37)
Interim influenza vaccine effectiveness: A good proxy for final estimates in Spain in the seasons 2010-2014
2015, VaccineCitation Excerpt :Influenza VE estimates were obtained using the observational case-control cycEVA study (Spanish component of the European I-MOVE network [4]) for the seasons 2010/11 to 2013/14. These studies and their VE results have been described in detail elsewhere [5–7,10–14]. Briefly, influenza cases were influenza like illness (ILI) patients who tested positive for influenza virus and test-negative controls were those testing negative for any type of influenza virus.
Effectiveness of seasonal influenza vaccine in community-dwelling elderly people: A meta-analysis of test-negative design case-control studies
2014, The Lancet Infectious Diseases
- 1
cycEVA Study Team: Silvia Jiménez, Isabel Salmeán, Francisco Pozo, Inmaculada Casas, Pilar Pérez, Antonia Galmés, Juana M. Vanrell, Marisol Gutierrez, Tomas Vega, Ana Martinez, Nuria Torner, Julián M. Ramos, Maria C. Serrano, Manuel García, Jesús Castilla, Jone M. Altzibar, Jose M. Arteagoitia, Carmen Quiñones, Maria E. Lezaún, Milagros Perucha.