Seasonal Influenza Vaccine Effectiveness for the Prevention of Laboratory-Confirmed Influenza in Asthma During the Influenza Seasons 2010-11 to 2015-16 in Scotland: A National Test-Negative Design Case-Control Study

Background: Influenza infection is an important trigger of asthma attacks. Influenza vaccination has the potential to reduce the incidence of influenza in people with asthma, but uptake remains persistently low, partially reflecting concerns about vaccine effectiveness (VE). Methods: We conducted a test-negative design case-control study to estimate the effectiveness of influenza vaccine in children and adults with asthma in Scotland over six influenza seasons (2010/11 to 2015/16). We used individual patient level data from 223 primary care practices which yielded 1,830,772 patient-years of data, which were linked with hospital and virological (n=5,910 swabs) data. Findings: Vaccination was associated with an overall 55.03% (95% confidence interval (CI): 45.84-62.66) reduction in the risk of a laboratory-confirmed influenza infection in people with asthma over the six seasons. There was substantial variation in VE between seasons, influenza strains and age groups. The highest VE (76.08%; 95% CI: 55.60-87.12) was found in 2010/11 season where the A(H1N1) strain dominated and there was a good antigenic vaccine match. High protection was observed against A(H1N1) (e.g. 2010/11: 70.68%; 95%CI: 32.46-87.27) and B strains (e.g. 2010/11: 83.17%; 95%CI: 44.31-94.92), but there was lower protection for the A(H3N2) strain (e.g. 2014/15: 26.40%; 95%CI: -12.00-51.64). The highest VE against all viral strains was observed in adults aged 18-54 years (56.98%; 95%CI: 42.25-67.95). Interpretation: Influenza vaccination gave meaningful protection against laboratory-confirmed influenza in people asthma across all six seasons. Strategies to boost influenza vaccine uptake has the potential to substantially reduce influenza triggered asthma attacks. Funding Statement: Chief Scientist's Office of the Scottish Government, National Institute for Health Research - Health Technology Assessment Programme (13/34/14), Asthma UK and the Medical Research Council's Farr Institute, Health Data Research UK (HDR UK). Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: The Privacy Advisory Committee of the Information Services Division, NSS, approved the linkage and the statistical analysis of the anonymised data used in this study.