The most relevant finding of this study is that influenza vaccination coverage among Spanish asthmatics aged ≥16 years (35.2 %) is below desirable levels. Several studies have investigated the prevalence and predictors of influenza vaccination among asthmatics [13, 16, 19–23].
Our results are similar to those reported in Canada (36.3 %) [16] and the USA [19], where 39.9 % of adults aged 18–64 years had been vaccinated, with coverage of 33.9 % for people aged 18–49 years and 54.7 % for those aged 50–64 years.
Few European studies have described influenza vaccine coverage among asthmatics [7, 20–23]. Keenan et al. [20] reported coverage of 40.2 % among asthmatic patients for the year 2003. Ten years previously, based on data from elderly individuals with asthma in a cohort from the UK General Practice Research Database [7], coverage of 43 % (1991–1992), 39 % (1992–1993) and 41 % (1993–1994) was reported.
In the 2003–2004 campaign in the Netherlands, the results from two medical information sources [the Great Influenza Survey (an Internet-based survey of influenza-like illness) and the official data obtained by Dutch GPs participating in the Dutch Sentinel Practice Network] revealed coverage rates of 73 and 68 %, respectively [21]. A study from Portugal based on a telephone survey reported 34.3 % coverage among persons with self-reported asthma [22].
Another relevant finding of our investigation was that no change was observed in vaccination coverage among Spanish asthmatics between 2006 and 2010. A similar observation was made in the USA for the period 2000–2007 [13, 19].
Our multivariate analysis showed that older age (especially age >65 years) was a positive predictor for vaccination among asthmatics. The fact that vaccination coverage is higher as age increases in populations with asthma or other chronic lung diseases has already been described in Spain and elsewhere [12–14, 16, 19, 23].
We found vaccination coverage to be lower among smokers, as did Jiménez-García et al. [23]. This finding is important, as smokers who also have chronic respiratory diseases are at a higher risk of suffering from respiratory complications [24]. Tobacco usage has also been found to be a negative predictor for vaccination in previous studies examining people who suffer from chronic lung diseases [14, 16].
Our results showed that asthmatics who perceived their health to be bad or very bad were 1.86 times more likely to be vaccinated than those who perceived their health to be good, a finding that was similar to Ford et al. [13]. One possible explanation is that asthmatics visit health care professionals more frequently and, therefore, are more likely to be vaccinated.
The reasons for low vaccination coverage among asthmatics are mixed and involve both patients and health care professionals. Vaccination may be refused by people who are already sick, because they think that they will develop influenza or because they do not perceive any benefit from vaccination [25]. Younger people may not be aware of the need for vaccination to prevent complications of asthma or they may feel that being vaccinated could cause them to have adverse reactions because of their existing disease [13].
Health care professionals play a significant role in encouraging patients to be vaccinated. By providing adequate information about immunization, they could improve coverage among asthmatics [13]. In addition, urgent action must be taken to inform asthmatics of the potential dangers of influenza and of the effectiveness and safety of the vaccine [26].
Our study has several limitations. First, the validity of the questions used by the EHS to classify vaccination status has not been evaluated. As data from the EHS are de-identified, it is not feasible to cross the self-reported information with medical records. In Spain, the only study to assess the validity of the self-reporting of influenza vaccination [27] was conducted among health care professionals over four influenza campaigns and found that sensitivity ranged from 73 to 91 % and specificity ranged from 73 to 94 %. Our literature review revealed eight studies comparing self-reporting with influenza vaccination records in high-risk populations. In all cases, the sensitivity was very high, ranging from 92 to 100 % and the specificity ranged from 38 to 96 %. In addition, the overall agreement determined using Cohen’s kappa ranged from 0.36 to 0.88 [28–35]. Therefore, we believe that self-reporting is a valid source for the assessment of influenza vaccination coverage. Furthermore, many other authors from different countries have published papers on vaccination coverages using population surveys with self-report data and without validating their answers with medical records [12–14, 16, 19, 23, 36–39]. Second, the timing of the survey with respect to the availability of the vaccination may affect recall bias, and the information obtained may be affected by the subjects’ desire to provide socially acceptable answers. Third, information on relevant variables, such as pharmacological treatment, use of other recommended vaccinations (pneumococcal vaccine), and the duration and severity of asthma are not included in the EHS and may act as confounders. And, lastly, the response rate to the NHS was 64 %, with the result that a non-response bias may exist [18].
In conclusion, influenza vaccination coverage among Spanish asthmatics is lower than desired and has not improved in recent years. Urgent strategies for increasing vaccination coverage among asthma sufferers are necessary and must target young adults, smokers and those who perceive their health to be good, as these are the groups who least follow vaccination recommendations.