Effectiveness of inactivated influenza vaccines

The efficacy (prevention of illness among vaccinated individuals in controlled trials) and effectiveness (prevention of illness in vaccinated populations) of influenza vaccines is dependent on several factors. The age, immune status and health of the recipient are important as well as the match between circulating viral strains and the vaccine. Research comparing vaccinated with unvaccinated participants show outcome measures that include laboratory-confirmed infection with influenza virus provide the most robust evidence of vaccine efficacy.

Trivalent influenza vaccines contain two influenza A strains (a H1N1 and a H3N2 strain) and one influenza B strain (from either the Yamagata or Victoria line). Quadrivalent influenza vaccines contain two influenza A strains (a H1N1 and a H3N2) and two influenza B strains (one from each line). Receipt of a quadrivalent influenza vaccine broadens the immune response, which may provide additional protection if influenza B viruses from both lines are circulating or the predominant circulating influenza B virus is not from the line included in the trivalent vaccine.

When influenza vaccine strains match circulating influenza viruses, protection against influenza is primarily dependent on circulating antibodies. These peak during the first month after vaccination and decrease over 6 months (influenza A (H1N1) and B viruses) or 5 months (influenza A (H3N2) viruses) after vaccination.

Influenza vaccines are effective in children. However, less evidence is available for children aged under 2 years. In healthy adults influenza vaccines are effective in reducing cases of influenza particularly when the vaccine and circulating virus strains are well matched. Inactivated influenza vaccine effectiveness is around 60% against laboratory-confirmed-influenza and just below 20% against influenza-like illness. Vaccine effectiveness may not be as high in older people. However, older people who have been vaccinated but subsequently get influenza are less likely to have severe disease, complications, including cardiovascular events, hospitalisation, increased disability or frailty or influenza-related death. The table below summarises selected current estimates of inactivated influenza vaccine efficacy and/or effectiveness against a range of clinical outcomes.

Pooled New Zealand data from the Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance (SHIVERS) study have shown that influenza vaccine effectiveness over 2012–2015 was around 46% (95% confidence interval 35–55%) preventing influenza-like illness presentations to general practice and 52% (41–62%) preventing influenza-related hospitalisations. Low influenza activity in 2018 caused imprecision in the assessment of influenza vaccine effectiveness. Estimated vaccine effectiveness in preventing influenza-like illness presentations to general practice was 38% (95% confidence interval 1–61%) and in preventing influenza-related hospitalisations was 35% (95% confidence interval 12–52%).

Key: LCI: laboratory-confirmed influenza; ILI: influenza-like illness.

Some studies have suggested residual protection from prior season influenza vaccination influences current season vaccine efficacy and/or antibody waning.

Pooled analyses of these studies are inconclusive and have highlighted an area for further research.

In contrast, an increasing body of evidence supports the significant role of prior and current season influenza vaccination in reducing the risk of influenza related hospitalisation with severe illness, ischaemic stroke, heart failure, and acute coronary syndrome, acute myocardial infarction,  or respiratory disease. A protective effective of cumulative influenza vaccination has also been identified through an improved survival rate in vaccinated adults with heart failure.