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

Abstract

BackgroundIn 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.MethodsMedically 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.ResultsThe 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.ConclusionsThe 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.

Estudio cycEVA: casos y controles para la estimación de la efectividad de la vacuna antigripal en España, 2008-2013

“…Durante la fase piloto 2008-09 16 se demostró la viabilidad del estudio siendo una etapa crucial para afrontar el reto que supuso la estimación de la efectividad de la vacuna estacional 2009-10 y la monovalente pandémica durante la pandemia de 2009 18 . Posteriormente el estudio cycEVA se consolidó a lo largo de las tres ediciones post-pandémicas [19][20][21][22] .…”

“…A partir de la edición 2009-10 se recogió información sobre los dos nuevos factores de riesgo de complicaciones por gripe descritos en la pandemia de 2009, embarazo y obesidad mórbida (IMC≥40 kg/m 2 ). Gracias a la incorporación de la variable "pertenencia a los grupos recomendados de vacunación", desde la edición 2010-11 se obtuvieron estimaciones de EV antigripal en España para los grupos diana de vacunación [20][21] . La inclusión de la variable "número de consultas al MP en los últimos 12 meses" permite ajustar las estimaciones de EV por la conducta de frecuentación médica (health seeking behavior) de los pacientes incluidos en el estudio 28 .…”

“…Una mejor participación de los MP, la recogida de información de más calidad y la consolidación del diseño de estudio posibilitan la consecución de estimaciones más precisas y menos sesgadas en los distintos subgrupos analizados [20][21] . Una fortaleza de cycEVA es la mejora con el tiempo de su contribución al estudio europeo I-MOVE, convirtiéndose en un componente básico para obtener las estimaciones europeas.…”

Influenza vaccine effectiveness: Maintained protection throughout the duration of influenza seasons 2010–2011 through 2013–2014

“…Yearly revaccination is necessary because circulating strains and/or vaccine composition change every influenza season. Additionally, previous studies have found that influenza vaccine effectiveness (VE) declines over time since vaccination in some populations does not provide significant protection in most cases after 90-120 days [3][4][5][6][7]. Many factors impact VE estimates and speed of decline -including age, comorbidities, herd immunity, use of adjuvants, type of vaccine administered (live attenuated or inactivated), prior natural influenza exposure, prior influenza vaccination, antigenic drift, and study design [6,[8][9][10][11][12][13].…”

“…Previous studies have assessed influenza VE declines in relatively small sample sizes or from 1 influenza season [3][4][5][6]14]. Additionally, many of these studies have been conducted outside the United States, in regions that have different vaccine composition (adjuvant vs. non-adjuvanted) [6,15], and vaccine recommendations [2,16], and may also have differences in circulating strains and in the proportion of LAIV vs. IIV and quadrivalent versus trivalent vaccine which is administered.…”