Effectiveness of flu jab studied

“The annual flu jab given to hundreds of thousands of people this winter provides only limited protection against the illness,” reported The Independent.

This news story is based on a systematic review of studies looking at the effectiveness of two common flu vaccines in the prevention of seasonal flu infection. The vaccines assessed were:

  • trivalent inactivated vaccine (TIV): the most widely used vaccine in the UK
  • live attenuated influenza vaccine (LAIV): which is less commonly used

Over 5,550 studies were screened resulting in 31 studies being included in the review.

Pooling the results of these studies showed that TIV was effective at preventing seasonal flu in 59% of adults aged 18-65 years while LAIV was effective in 83% of children aged six months to seven years. Information on other age groups was not available.

It is a well-known fact that current flu vaccines are not 100% effective. They do however, remain the best available protection for groups who are more vulnerable to the effects of seasonal flu such as people aged over 65 years, people with a serious medical condition and pregnant women.

What this study highlighted is the need for further research into the effectiveness of the vaccines in some groups, especially the over 65s. It also highlighted the need for the development of more effective vaccines in the future. While the LAIV vaccine is reported as being more effective, there is currently no high quality data on its effectiveness in age groups other than children aged six months to seven years.

Those recommended to have a flu jab by their GP should have no reservation in doing so based on this study.

Where did the story come from?

The study was carried out by a collaboration of researchers led by the Centre for Infectious Disease Research and Policy at the University of Minnesota. Funding was provided by the Alfred Sloan Foundation.

The study was published in the peer-reviewed medical journal The Lancet Infectious Diseases.

The coverage of this story in the media was generally balanced, although many media outlets presented the finding that the flu vaccine used in this country is not 100% effective as new. It is a well-documented fact that the vaccine is not completely effective and many studies have looked into this before now finding different levels of effectiveness. The difference with this study is that it reviewed the body of research to date and carried out an overarching analysis of the findings from these studies.

What kind of research was this?

This systematic review and meta-analysis pooled the results of 31 studies assessing the effectiveness of the two most common types of flu vaccine used in the US and the UK. The researchers included studies that looked at vaccine effectiveness in any age group that had confirmed the presence or absence of flu using reliable laboratory testing methods.

Systematic reviews aim to identify all relevant studies on a specific topic with the aim of reaching an overall conclusion based on all the evidence available.

The authors of this systematic review suggest that previous systematic reviews on flu vaccines have included many studies that did not adequately diagnose or confirm flu infection in participants. They aimed to address this weakness in this current systematic review by including studies with a more reliable assessment of flu infection.

What did the research involve?

The researchers looked at databases of published scientific studies to identify randomised controlled trials (RCTs) and observation studies that looked at the effectiveness of flu vaccines (how well the vaccine works in the real world).

To be eligible, studies had to be published between 1967 and 2011, and look at how successful the vaccines were at preventing flu infection against all circulating influenza viruses (of which there are many) during individual flu seasons. The researchers also included studies assessing the effectiveness of flu vaccine against pandemic flu.

Two common flu vaccines were assessed:

  • the trivalent inactivated vaccine (TIV): which can be given to  most people aged six months or older
  • the live attenuated influenza vaccine (LAIV): which can be given to healthy non-pregnant individuals aged 2-49 years

TIV is the most commonly used flu vaccine in the UK and accounts for about 90% of the vaccine given in the US.

Studies were only included if they used either a method of measuring cases of flu called real-time PCR (RT-PCR) or viral culture (a culture grown in a lab typically from a nasal swab) or both. These methods of flu detection were reported by the authors to be more accurate than other tests commonly used, such as a blood serum test.

The researchers screened 5,707 potentially eligible studies and identified 31 that were eligible for inclusion in the systematic review. This included 17 RCTs and 14 observational studies. The 17 RCTs contained data for 24 influenza seasons and 53,983 participants from 23 countries.

Where sufficient data were available, the researchers performed a meta-analysis, a method of statistically pooling the results of many studies. This was an appropriate method to summarise the overall effect of flu vaccine based on multiple studies.

What were the basic results?

The pooled results showed that TIV prevented seasonal flu in 59% (95% confidence interval [CI] 51 to 67%) of adults aged 18-65 years. Individual studies showed that this protection varied considerably between seasons. No eligible studies were available for TIV effectiveness for children aged 2-17 years or adults aged 65 years or over.

LAIV showed better results and was effective in 83% (95% CI 69 to 91%) of children aged six months to seven years. No pooled data was available for other age groups, and individual studies included in the analysis again showed significant variation in effectiveness between seasons.

Five studies assessed the effectiveness of flu vaccine for the prevention of pandemic flu strain H1N1 (swine flu). The pooled results showed that the vaccine prevented influenza in an average (median) of 69% of cases (range 60-93%). Comparatively few cases of influenza occurred in individuals aged 65 years or older.

How did the researchers interpret the results?

The authors concluded that ‘influenza vaccines can provide moderate protection against virologically confirmed influenza, but such protection is greatly reduced or absent in some seasons’.

The authors acknowledge that because they used quite strict inclusion criteria for their systematic review (to minimise the effect of bias and any effect of other influencing factors) there were ‘substantial gaps in the evidence base for some age groups.’ Most notably there was a lack of information on the 65-year and older age group, the main group recommended to have the flu jab in the UK.


This systematic review screened over 40 years’ worth of published studies. It serves to highlight a relative lack of high quality evidence on the effectiveness of flu vaccine in certain age groups, one of which is the over 65s. The review provides good evidence that the flu vaccine provides moderate protection against infection and illness for the majority of adults, but that it is not 100% effective.

One strength of the study is that it only included studies that used accurate methods of assessing flu infection. By doing this, the authors believe they have produced the ‘most accurate estimates of the efficacy and effectiveness of influenza vaccines’. While it is not possible to verify this claim, these results do provide valuable information to inform the current debate on how effective flu vaccines are, which is not a clear-cut issue.

The study has many strengths but also some limitations. These include:

  • There were no RCTs showing efficacy of TIV in people aged 2-7 years or adults aged 65 years or older. For LAIV, there was no data showing efficacy for people aged 8-59 years. Hence, it is still unclear how effective the vaccines are in these groups. Another recent systematic review also concluded that there is a lack of evidence around the effectiveness of flu vaccine in the over-65 age group. Further studies assessing the effectiveness of currently licensed vaccines in the over-65 age group are needed to address this information gap.
  • This study only looked at how well the vaccine prevented flu infection. It did not assess the effect of flu vaccine on preventing death or serious complications due to flu. The authors state this has been well researched previously, concluding that vaccines might prevent some serious complications of flu in the over 65s. However, estimates of exactly how many complications are likely to be prevented may have been overestimated in earlier studies because of methodological weaknesses.

Current flu vaccines are already known to not be 100% effective, but they remain the best intervention available for seasonal influenza at present, and people in at-risk groups should still be vaccinated. What this study highlighted is the need for vaccine developers to continue trying to produce more effective vaccines in the future and a lack of good quality evidence for the effectiveness of flu jab in people aged 65 years and over.

NHS Attribution