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%).

How long after vaccination does it take for antibodies to be produced?

It can take up to two weeks for the vaccine to provide the best influenza protection. However, influenza vaccinations can be given when influenza virus activity has been identified as protective antibody levels have been observed to develop rapidly from four days after vaccination.

 

Population Type of Outcome Level of protection
(95% confidence intervals)
Infants aged under 6 months whose mothers received an influenza
vaccination during pregnancy

Effectiveness against LCI

 

Effectiveness against LCI-related hospitalisation

41% (7–63%)

49% (12–70%)

47% (12–68%)

Healthy children aged under 2 years Effectiveness against LCU Insufficient data
66% (9–88%)
Healthy children aged 6-35 months Effectiveness against LCI 66% (29–84%)
Healthy children aged 6 months to under 18 years

Effectiveness against LCI-related death:

– Healthy, no high-risk condition

– With a high-risk condition

 

65% (47–78%)

51% (31–67%)

Healthy children aged 2 years to under 16 years

Efficacy against LCI

Effectiveness against LCI-related hospitalisation

Effectiveness against ILI

64% (52–72%)

56% (12–78%)

28% (21–35%) to 47% (33–58%)

Healthy adults (aged 18–64 years)

Effectiveness against LCI generally or

LCI-related hospitalisation in NZ

Effectiveness against ILI generally or

ILI-related general practitioner (GP) visit in NZ

59% (53–64%) to 66% (55–75%)

61% (34–77%)

16% (5–25%) to 18% (2–31%)

55% (24–73%)

Pregnant women

Effectiveness against LCI

Effectiveness against acute respiratory illness requiring an emergency department visit or

hospitalisation

50% (15–71%)

81% (31–95%)

65% (3–87%)

Adults aged 65 years or older
(Cochrane Reviews 2010 and 2018)

Effectiveness in preventing LCI, ILI, influenza-related hospitalisations, complications or mortality

 

Effectiveness against LCI

Effectiveness against ILI 

Inconclusive due to poor quality of studies

Reanalysis of randomised control trial studies from 2010 Review:

58% (34–73%)

41% (27–53%)

Adults aged 65 years or older
(Re-analysis of Cochrane Review 2010 information)

Effectiveness against LCI

Effectiveness against ILI

Effectiveness against non-fatal and fatal complications

49% (33–62%)

39% (35–43%)

28% (26–30%)

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.

Recommend annual influenza vaccination to your eligible patients for better protection against serious influenza complications