Trends in influenza vaccine coverage in the Portuguese population: Are there contributing factors?

Abstract

Introduction In Portugal as in most European countries, annual influenza vaccination is recommended for certain groups at-risk of post-infection complication, namely the elderly. The annual monitoring of the influenza vaccine (IV) coverage allows understanding the success of the vaccination program on a given season. On the other hand, evaluating IV coverage over a long period of time permits understanding factors that contribute to the trend and evolution of the vaccine coverage in specific groups of the population. In public health perspective, and considering that IV coverage is below the desirable target in all groups for which is recommended, it is important to understand what factors contribute to the IV coverage. This study aims to analyse trends in influenza vaccine coverage in the Portuguese population, overall and in specific age groups, and identify factors that contribute to the IV coverage evolution. Methods We used data from a population-based survey conducted annually to estimate IV coverage (ECOS panel) from 1998 to 2015. ECOS is composed by a sample of approximately 1000 households stratified by region (homogeneous allocation) selected from a dual sample frame - random digit dialing mobile and landline phones (ECOS sample). The relevant information is collected by Computer Assisted Telephone interview (CATI) using with the same questionnaire every year and one respondent by household (proxy for the rest of the household members). IV coverage was estimated taking in consideration the design effect. Meta-regression was used to test a linear trend in IV coverage. Models were fitted with the logit of the IV as dependent variable, weighting each survey estimate by the inverse of variance in the logit scale and adjusted for influenza related events [SARS, A(H5N1) and A(H1N1)2009 pandemic]. For the 65 and plus age group, the introduction of free vaccination (in the 2012/13 season) was also modelled. These events where tested given their direct effect on IV recommendations. Results Overall IV coverage estimates varied between 14.2% (95% CI: 11.6; 16.8) in the 1998/99 season and 19.5% (95% CI: 17.6% a 21.6%) in the 2009/10 season. Meta-regression results indicate a non-statistical significant increasing trend of IV coverage along the 18 years in analyse (OR = 1.01; 95% CI: 0.99–1.02). All influenza related events, had an increasing effect on increasing the IV coverage [SARS 2003–2004 - OR = 1.23; 95% CI = 1.02 to 1.48; A(H1N5) 2005–2006 - OR = 1.26; 95% CI = 1.05 to 1.51 and A(H1pdm09) 2009–2010 - OR = 1.25; 95% CI = 1.03 to 1.50]. Age stratified analysis revealed a decreasing IV coverage trend in children with less than 14 years (OR = 0.94; 95% CI = 0.89 to 0.99), an increasing trend in the elderly population (OR = 1.05; 95% CI= 1.02 to 1.09) and no trend in the remain age groups (15–44 years: OR = 0.99; 95% CI: 0.97 to 1.02 and 45-64 years: OR = 1.00; 95% CI: 0.9 to 1.02). None of the influenza related events influenced IV coverage in children. Equivalent results were observed in the elderly, where none of the tested influenza related events or the introduction of free vaccines in this age group revealed an impact on IV coverage. Conclusion Overall results indicate that periods with threats of circulating virus with severe consequences for health have an impact on IV coverages, towards increasing the vaccine coverage. This result may be related to a higher awareness of health risks during such periods and the adoption of influenza vaccine uptake. However, after age stratification, all these events and including the free vaccine for elderly had no impact on respective IV coverages. Further analysis should be performed in order to identify factors that are influencing the trend in the age stratified IV coverage.