Background: Clinical guidelines recommend seasonal influenza vaccination for individuals with asthma in pediatric and adult populations but the effectiveness in preventing asthma episodes is debated.
Objectives: To examine if influenza vaccination among patients with persistent asthma reduces the frequency of episodes requiring either an emergency visit or hospitalization.
Methods: A retrospective cohort of children and adults who met the HEDIS definition of persistent asthma was drawn from the MarketScan® database (2005-07). The influenza season was defined August 2006 through March 2007 and individuals with and without the seasonal influenza vaccine during this flu season were identified. Asthma-related emergency department visits or hospitalizations between August 2006 and March 2007 were the outcomes of interest. Data from January 2005 through July 2006 for this cohort was used to construct Charlson Comorbidity Score and to identify asthma-related ED/hospitalization as a proxy for asthma severity. Instrumental Variable and Propensity Score Matching (PSM) methods were used to examine if vaccinated persistent asthma patients had a reduced risk of acute asthma episodes, controlling for comorbidity and severity.
Results: Both methods resulted in similar and statistically significant protective odds ratio for the vaccinated group in the 2006-2007 flu season. The Instrumental Variables method indicated that the odds ratio of having acute asthma episodes for the vaccinated group was 0.77 [0.73, 0.81]. Across the different Propensity Score Matching models that were estimated (nearest neighbor, kernel and local linear regression), the vaccinated group consistently had an odds ratio of 0.7 for experiencing acute asthma episodes.
Conclusions: Influenza vaccination was associated with fewer acute asthma episodes. Although the possibility of residual confounding inherent in observational data remains, these results support the policy recommendation to routinely deliver influenza vaccine to patients with persistent asthma.