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.

Inactivated influenza vaccine effectiveness against influenza in recent meta-analyses and systematic reviews ranges from 59% (95% confidence interval 51–67%) to 73% (95% confidence interval 54–84%) in healthy adults for years when circulating and vaccine strains are well matched. Vaccine effectiveness may not be as high in the elderly and those with high-risk conditions. A re-analysis of the Cochrane Review, Vaccines for preventing influenza in the elderly, applying a biological perspective to the same information found that influenza vaccination of 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 aged under 6 months whose
mothers received an influenza
vaccination during pregnancy
Efficacy against laboratory-confirmed influenza 41%-49%
Healthy children aged under 2 years 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 aged under 16 years Effectiveness against influenza requiring
hospitalisation
56% (12%-78%)
Healthy adults (aged 18–64 years) Effectiveness against influenza-like illness
requiring a general practitioner (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
Effectiveness against laboratory-confirmed
influenza
81% (31–95%)64
65% (3–87%)64

50% (15–71%)59

Adults aged 65 years or older
(Cochrane Review 2010)
Effectiveness in preventing influenza,
influenza-like illness, hospitalisations,
complications and mortality
Inconclusive due to poor quality of studies
Adults aged 65 years or older
(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%)
39% (35%-43%)
49% (33%-62%)

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. 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.

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. For 2017, the seasonal level of influenza was too low to allow robust estimates of influenza vaccine effectiveness at preventing influenza-like illness presentations to general practice and influenza-related hospitalisations.

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.

Influenza and older people

As we age our immune system becomes less efficient at preventing infectious diseases and associated complications. Older people (aged 65 years or older) with influenza are more likely to require hospitalisation and significantly more likely to die than adults with influenza who are aged under 65 years. The presence of chronic conditions such as diabetes or heart, kidney, neurological or respiratory diseases adds to their risk of influenza-related complications.

Although older people may have a reduced immune response to influenza vaccine compared with younger adults, they may still benefit from influenza vaccination. Influenza vaccination has been shown to reduce symptom severity in older adults who get influenza despite having been vaccinated.

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.