Effectiveness of inactivated influenza vaccines

How effective is the trivalent inactivated influenza vaccine?

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

Inactivated seasonal influenza vaccine effectiveness against influenza in recent meta-analyses and systematic reviews ranges from 59% (95% CI 51-67) to 73% (54-84) in healthy adults for years when circulating and vaccine strains are well matched. Recently a re-analysis of the Cochrane Review on effectiveness of influenza vaccines in the elderly was conducted using different methods that enabled the utilisation of the same information, but viewed from a biological perspective, and were less likely to miss a real effect finding that influenza vaccination in the elderly is often protective.

The following table summarises selected current estimates of both vaccine efficacy and vaccine effectiveness against a range of clinical outcomes.

Population Type of Outcome Level of protection
(95% confidence intervals)
Infants less than 6-months whose mothers received an influenza vaccination during pregnancy Efficacy against laboratory confirmed influenza 41%-48%
Healthy children less than 2 years of age Effectiveness against laboratory confirmed influenza Insufficient data
66% (9%-88%)
Healthy children aged 6-35 months Effectiveness against laboratory confirmed influenza 66% (29%-84%)
Healthy children less than 16 years of age Effectiveness against influenza requiring hospitalisation 56% (12%-78%)
Healthy adults (18-64 years) Effectiveness against influenza like-illness requiring a GP visit or hospitalisation in NZ
Effectiveness against laboratory confirmed influenza
30%-60%
59% (51%-67%)
Pregnant women Effectiveness against acute respiratory illness requiring: an emergency department visit, or hospitalisation 81% (31%-95%)
Elderly aged 65 years and over (Cochrane Review 2010) Effectiveness in preventing influenza, influenza-like illness Inconclusive due to poor quality of studies
Elderly aged 65 years and over (Re-analysis of Cochrane Review 2010 information) Effectiveness against non-fatal and fatal complications
Effectiveness against influenza-like illness
Effectiveness against laboratory confirmed influenza
28% (26%-30%)
49% (33%-62%)
39% (35%-43%)

Influenza vaccines are effective in children; however less evidence is available for children less than 2 years of age. In healthy adults, influenza vaccines are effective in reducing cases of influenza particularly when the vaccine and circulating virus
strains are well matched. Evidence suggests the effectiveness of influenza vaccination in the community-dwelling elderly is modest. There is some evidence that in long-term care facilities, influenza vaccination is effective against complications.

New Zealand data from the SHIVERS study have shown that the influenza vaccine used during the 2014 influenza season was around 53% effective (95% confidence intervals, 28–70%) at preventing influenza-related presentations to general
practice and 52% effective (27%–68%) preventing hospitalisations overall. In the 2015 season the influenza vaccine was around 36% effective (95% confidence interval, 11–54%) at preventing influenza-related presentations to general practice
and 50% effective (20%–68%) preventing influenzarelated hospitalisations overall.

Is the vaccine useful in older adults?

Although older people (65 years plus) may have a reduced immune response to influenza vaccine compared with younger adults, they may still benefit from influenza vaccination. Older people are more likely to have a condition that places them at higher risk of complications from influenza. 

Influenza vaccination is recommended (although not funded) for those who are in close contact with older people and individuals at high-risk of influenza infections to reduce the spread of disease to those who are more vulnerable and also may be less likely to mount a strong immune response to the vaccine.

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

It takes up to two weeks for the vaccine to start providing protection. Some studies have observed protective levels of antibodies developing as early as four days after vaccination.

Why does a child need two doses if vaccinated for the first time?

Children less than 9 years of age have a better immune response after two doses for the first influenza vaccination. This may be because they are more likely to be immunologically naive to influenza.