Background: There is good evidence of vaccine effectiveness in healthy individuals but less robust evidence for vaccine effectiveness in the populations targeted for influenza vaccination. The live attenuated influenza vaccine (LAIV) has recently been recommended for children in the UK. The trivalent influenza vaccine (TIV) is recommended for all people aged ≥ 65 years and for those aged < 65 years who are at an increased risk of complications from influenza infection (e.g. people with asthma).
Objective: To examine the vaccine effectiveness of LAIV and TIV.
Design: Cohort study and test-negative designs to estimate vaccine effectiveness. A self-case series study to ascertain adverse events associated with vaccination.
Setting: A national linkage of patient-level general practice (GP) data from 230 Scottish GPs to the Scottish Immunisation & Recall Service, Health Protection Scotland virology database, admissions to Scottish hospitals and the Scottish death register.
Participants: A total of 1,250,000 people.
Interventions: LAIV for 2- to 11-year-olds and TIV for older people (aged ≥ 65 years) and those aged < 65 years who are at risk of diseases, from 2010/11 to 2015/16.
Main outcome measures: The main outcome measures include vaccine effectiveness against laboratory-confirmed influenza using real-time reverse-transcription polymerase chain reaction (RT-PCR), influenza-related morbidity and mortality, and adverse events associated with vaccination.
Results: Two-fifths (40%) of preschool-aged children and three-fifths (60%) of primary school-aged children registered in study practices were vaccinated. Uptake varied among groups [e.g. most affluent vs. most deprived in 2- to 4-year-olds, odds ratio 1.76, 95% confidence interval (CI) 1.70 to 1.82]. LAIV-adjusted vaccine effectiveness among children (aged 2-11 years) for preventing RT-PCR laboratory-confirmed influenza was 21% (95% CI -19% to 47%) in 2014/15 and 58% (95% CI 39% to 71%) in 2015/16. No significant adverse events were associated with LAIV. Among at-risk 18- to 64-year-olds, significant trivalent influenza vaccine effectiveness was found for four of the six seasons, with the highest vaccine effectiveness in 2010/11 (53%, 95% CI 21% to 72%). The seasons with non-significant vaccine effectiveness had low levels of circulating influenza virus (2011/12, 5%; 2013/14, 9%). Among those people aged ≥ 65 years, TIV effectiveness was positive in all six seasons, but in only one of the six seasons (2013/14) was significance achieved (57%, 95% CI 20% to 76%).
Conclusions: The study found that LAIV was safe and effective in decreasing RT-PCR-confirmed influenza in children. TIV was safe and significantly effective in most seasons for 18- to 64-year-olds, with positive vaccine effectiveness in most seasons for those people aged ≥ 65 years (although this was significant in only one season).
Future work: The UK Joint Committee on Vaccination and Immunisation has recommended the use of adjuvanted injectable vaccine for those people aged ≥ 65 years from season 2018/19 onwards. A future study will be required to evaluate this vaccine.
Trial registration: Current Controlled Trials ISRCTN88072400.
Funding: This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 24, No. 67. See the NIHR Journals Library website for further project information.
Keywords: ADVERSE EFFECTS; ASTHMA; HOSPITALISATION; LIVE ATTENUATED INFLUENZA VACCINE; TRIVALENT INFLUENZA VACCINE.
In Scotland, a new type of influenza vaccine (live attenuated influenza vaccine), administered via the nose, was introduced in 2014/15 for all children aged between 2 and 11 years. It can be difficult to evaluate any changes in health as a result of new immunisation programmes, given that randomised controlled trials of vaccines are impractical and can also be seen as unethical. These changes are therefore typically not evaluated, making it difficult to inform future policy in this field. Observational studies can be used to assess the effects of health-care interventions without influencing the care that is provided or affecting the people who receive it. An evaluation (effectiveness and safety) of this change in the immunisation programme was conducted. The vaccine programme, an inactivated vaccine administered as an injection, for other groups for whom the evidence available is limited was also evaluated [i.e. for people aged ≥ 65 years and people aged < 65 years who have a medical condition (e.g. asthma) that puts them at risk of severe illness from influenza]. The findings support the view that the intranasal vaccine is effective and safe in preventing influenza in children. The injectable vaccine in people aged < 65 years who are more at risk of complications from flu was safe and effective. Lower effectiveness was found in people aged ≥ 65 years. Both the injectable vaccine and the intranasal vaccine have high levels of uptake in the population offered vaccination. When considering these results, the important limitation of bias in observational study designs should be noted [for instance, residual confounding, whereby it is not possible to measure a characteristic of those people receiving the vaccine (e.g. being healthier)], and this is accounted for in this analysis.