Influenza vaccine coverage from 1998/99 to 2010/11
Between 1998/99 and 2010/11, IV coverage in Portugal varied between 14.2% and 17.5%. There was no consistent increase in percent IV uptake in the general population (p for trend 0.097).
During this time period (Figure 1, Table 3), there were four seasons that had notably higher IV coverage estimates: 2003/04, 2005/06, 2008/09, and 2009/10. However, except for 2003/04, these seasons did not have the highest coverage proportions in the elderly segment of the population (≥65 years), for whom vaccination has been long recommended. The seasonal IV coverage increase in the general population was primarily due to an increase in coverage in the 45–64-year age group in the 2005/06 and 2008/09 seasons and to an increase in the <15-year age group in the 2009/10 season (Table 4).
During the previous 12 years, healthy children and adolescents were not recommended risk groups for IV uptake in Portugal, except for during the 2009/10 influenza pandemic. Therefore, seasonal IV coverage levels among individuals <15 years old remained low. For most of the seasons, it was <10%. There were four seasons with slightly higher coverage levels in this age group: the 1998/99, 1999/00, 2005/06, and 2009/10 seasons, with coverage point estimates of 12.1%, 14.6%, 11.3%, and 12.9%, respectively.
Children are not considered to be a risk group for seasonal influenza. However, the general increased awareness related to the A (H1N1)2009 pandemic, and specific recommendations from Portuguese health authorities to vaccinate children may have affected seasonal vaccine uptake. More investigation is needed to address this hypothesis. However, the potential effect was not particularly strong considering that elevated vaccine uptake in season 2009/10 was not significantly higher than the other three seasons.
Individuals >65 years old had the highest coverage in all seasons. Values ranged from 31.3% in 1998/99 to 53.3% in the 2008/09 season (point estimates).
Regarding the 12-year pattern, a statistically significant increasing trend (p < 0.001) was observed for IV coverage in Portugal since 1998 in the elderly. However, after 50% coverage was achieved in 2006/07, a plateau was reached and no further improvement was observed in the last 4 years of the study period. The 2009 pandemic did not affect seasonal vaccination coverage in the elderly population in Portugal. Coverage was around 50%, which was similar to the three previous periods. This result is consistent with the fact that seasonal vaccine recommendations did not change throughout this period. Unchanged seasonal coverage rates were also observed in an elderly population in France [24].
In Europe, estimates for IV coverage in the elderly indicate that some countries failed to meet the target of 50% coverage in 2006. Portugal was not one of these countries [25]. However, other countries achieved the 75% coverage target for 2010. These countries were England, Scotland, Wales (79% in the 2005/06 season) [25], and The Netherlands (74% CI 95%: 71%–77% in 2001/02) [26]. A recent study of 11 European countries (2006/07 season) found that Spain, with 71% coverage, was close to meeting the goal [27]. This study also estimated that there was 53% IV coverage in Portugal for individuals ≥65 years, which is consistent with the estimate obtained in our study (50.4%, CI 95%: 44.8%–55, 9%). Compared with the other 10 countries included in the study (United Kingdom, Germany, Italy, France, Spain, Austria, Czech Republic, Ireland, Finland, and Poland), IV coverage in the elderly (≥ 65 years) in Portugal was sixth, but was similar to the overall estimate for all countries (53.2%).
In another European study that included 22 European countries (2006/07 season), Portugal ranked thirteenth for IV coverage in the elderly, and represented the median value for coverage in Europe [28]. This finding was consistent with the findings of the previous study.
Given that the elderly had been targeted for vaccination for many years and are the largest risk group, these results indicate that progress has been made toward meeting the WHO coverage goals. They reflect positively on Portuguese vaccination policies. Nevertheless, the 2010 75% coverage goal for 2010 remained far from being achieved. The 50% coverage plateau since 2006/07 indicates that additional effort is needed to further improve vaccination levels. Receiving advice from the family doctor/nurse has been identified as the main motivation to get vaccinated among the Portuguese population [29], followed (at a considerable distance) by old age as a reason. This result suggests that efforts directed at family doctors could have the greatest effect on IV uptake in the elderly. Public information campaigns that are directed at risk groups may also have an effect.
Effect of major pandemic threats on influenza vaccine coverage from 1998/99 to 2010/11
We hypothesized that 2003/04, 2005/06, and 2009/10 were seasons during which there was special awareness about vaccination among the general Portuguese population, and/or among subgroups defined by age. We used a meta-regression model to test this hypothesis. These seasons were selected based on major public health threats worldwide: SARS in 2003/04, the increase in H5N1 virus infections in humans in Southeast Asia in 2005/06, and the declaration of an influenza pandemic in 2009/10.
Global and national health authorities, and the medical community worldwide, reinforced flu vaccination recommendations in 2003/04 [30]. In 2005/06, the pandemic threat, with associated vaccination recommendations, led to news and opinions that circulated in the medical community and in the general population. This news coverage could have led to an increased demand for the influenza vaccine. In fact, in a study on IV coverage in Germany (2001–2006), the authors found that coverage increased during the 2005/06 season. They suggested that the greater media focus on pandemic influenza was one factor that explained the increased vaccine demand [31].
In 2009/2010, the declaration of a worldwide influenza pandemic by WHO focused the attention of the media on influenza. Health ministries worldwide, including in Portugal, issued broad guidance to health services that was also reported by the media.
The results of the meta-regression model (Table 5) indicate that in the general population, influenza coverage was significantly greater in two seasons, 2003/2004 (p = 0.032) and 2005/2006 (p = 0.018). The seasonal influenza coverage in the general population during the 2009/2010 season was not significantly greater (p = 0.084). However, an examination of IV coverage differences for these three seasons, according to the age group, revealed that two coverage estimates were significant: the increase in vaccine uptake of the 45–64 segment of the population in the 2005/06 season (p = 0.044) and the increase in IV uptake in the 0–14 group in the 2009/10 season (p = 0.015).
These findings indicate that when accounting for the baseline trend, there was an increase in IV coverage during the 2003/04 and 2005/06 seasons, which suggests that awareness about vaccination could have increased in the general population and in specific age groups.
Although the 12.9% coverage in children was not the highest of all the seasons was still an important increase. Risk perception regarding flu in children may have increased during the 2009/10 season, and pandemic awareness and special recommendations for children to be vaccinated during the pandemic could have affected seasonal vaccine uptake in Portugal in the 2009/10 season in this group.
Risk perception affects vaccine uptake [32, 33]. Although results vary in degree and direction, perceptions about the risk of disease and severity of infection that follow major pandemic threats may affect uptake of seasonal influenza vaccine [33–35]. The results of our study suggest that these events may have had a positive effect on IV uptake in Portugal. More research is needed to understand the factors underlying an individual’s decision to be vaccinated against influenza.
Limitation of the study
ECOS is a sample of families from mainland Portugal, with landline and mobile telephones, who agree to complete periodic health surveys. Residents of Portugal who do not have a landline or mobile telephone were not represented in this study.
Between 1998/1999 and 2010/2011, the ECOS panel sample was renewed three times (2002, 2006, and 2010). Therefore, the estimates of IV coverage were obtained using four different samples that were selected using the same methodology. Using the same sample for more than one consecutive season could lead to biased coverage estimates. Specifically, the application of the questionnaire to the same sample in two consecutive seasons could lead to a greater proportion of individuals who choose to be vaccinated the next season. This change in behavior could artificially increase IV coverage, which would not represent coverage in the general population. There is no evidence of this bias in our study, because in each population group, IV coverage increase was not consistent or systematic for the four periods between changes in the sample population (1998 to 2001, 2002 to 2005, 2006 to 2009, and 2010 to the present).
As previously described, the representativeness of the samples studied in comparison with estimates from the 2001 census of the Portuguese mainland population found age deviations from this reference population. These deviations could be translated into an IV coverage bias in the general population. However, in risk groups for whom vaccination is recommended, particularly the elderly (≥ 65 years), and for whom IV coverage monitoring is more critical for control measures, the age bias presented by our samples was less relevant.
Recall bias may occur when individual recall of information is used to obtain data. However, individuals were vaccinated some months before each survey and this time differed from survey to survey. Additionally, only one individual (>18 years) per household answered questions about vaccination status of household members. Surveying all of the individuals in a household would make the procedure more complex and could lead to a failure of the entire process, which occurred during previous surveys using the ECOS panel [17].