Preventing influenza: Cochrane evidence on vaccination

In the second of two blogs from public health doctors on preventing influenza (‘flu), Emma Plugge takes a look at some of the Cochrane evidence on preventing influenza through vaccination.

“Never since the Black Death has such a plague swept over the face of the world [and] never, perhaps, has a plague been more stoically accepted.”

The Times, December 1918, quoted in ‘Spanish influenza redux: revisiting the mother of all pandemics’. Mark Honigsbaum.  Lancet. 2018.

When we think of an infectious disease that’s going to wipe out the population, what springs to mind? Ebola? Monkey pox? An extra-terrestrial organism brought by space debris? And we do think of such things – well, it’s certainly a subject that Hollywood likes to visit frequently. Films such as ‘Contagion’, ‘Doomsday’ and ‘Outbreak’ are but a few in a long list of disaster movies where a virus wipes out most of the population, leaving a few plucky souls to find a cure and tend to the sick. The heroes and the plots may vary but what they all have in common is that the dreaded virus is never influenza. Influenza, ‘flu’, it would seem, is far too mundane…

But let’s travel back 100 years ago (almost exactly) and the ‘Spanish flu’ was sweeping across the world, killing at least 50 million people, a number hard to comprehend. This disease killed three times as many people than the First World War which was drawing to a close as the flu pandemic began. So whilst we mark the centenary of the end of this terrible war, let us not forget influenza, nor accept it, however ‘stoically’.

influenza poster 1918

“There is no medicine which will prevent it”. The stark reality of influenza in 1918.
By Provisional Board of Health, Alberta (Glenbow Museum, Reference No. NA-4548-5 via [1]) [Public domain or Public domain], via Wikimedia Commons

Influenza can be prevented

There are measures we can all take such as good respiratory and hand hygiene practices, encapsulated in the NHS ‘catch it, bin it, kill it’ campaign. Vaccination is also important. In yesterday’s blog on influenza vaccines, David Roberts has highlighted the issues with trying to develop the most effective vaccines – it is problematic. This highlights the importance of a series of recent Cochrane Reviews that has carefully examined and set out the evidence for influenza vaccination for a range of population subgroups. It is very important that we know how effective influenza vaccination is and what the adverse effects are when millions of doses are given in the UK each year.

Across the UK, flu vaccination is recommended each year for all those aged 65 years or over (‘older adults’) and for pregnant women and Cochrane Reviews have examined effectiveness in both these groups. A review looking at vaccination in older adults identified eight randomised controlled trials (RCTs) with over 5000 participants and found that older adults receiving the influenza vaccine may experience less influenza over a single season compared with placebo, from 6% to 2.4%. However the evidence was considered to be low certainty because of ambiguity over the diagnosis of influenza. Similarly the evidence relating to complications was ‘of poor quality, insufficient, or old’.  The evidence of effectiveness of influenza vaccination in pregnant women was also limited, identifying only one RCT and one controlled clinical trial: no data were available on effectiveness against seasonal influenza during pregnancy.

The findings of a review on influenza vaccination for healthy adults included 52 clinical trials of over 80,000 people and provided moderate-certainty evidence that influenza vaccines probably reduce influenza in these people from 2.3% without vaccination to 0.9%, during a single influenza season.

A separate but complimentary review looked at the evidence on influenza vaccination in healthy children. This review included 41 clinical trials and over 200,000 children. This showed with moderate-certainty evidence that, when compared with placebo or doing nothing, influenza vaccines probably reduced the risk of influenza infection in children aged 3 to 16 years from 18% to 4% over a single season. Somewhat surprisingly, the evidence from these studies on the adverse effects of vaccination (such as time off school or work) and the positive consequences (such as a reduction in complications like pneumonia) was lacking both in quality and quantity.

These reviews have undoubtedly been helpful, identifying vaccine effectiveness in adults and children but also recognising important research gaps in vulnerable populations such as older adults and pregnant women. It’s clear that in 2018 we need to move forward by conducting more research in this area, building the evidence and examining it critically, not accepting our fate ‘stoically’.

Join in the conversation on Twitter with @CochraneUK or leave a comment on the blog. 


Emma Plugge has nothing to disclose.

Preventing influenza: Cochrane evidence on vaccination by Emma Plugge

is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International

7 Comments on this post

  1. So the evidence was moderate, at best, the efficacy small, and the adverse effects, not really known. In the light of this evidence, the author recommends being vaccinated?

    Am I missing something here?🤷‍♂️

    Hisham / Reply
  2. The article does not evaluate the risks of ‘side affects’. What are the effects due to chemicals included in the vaccine? Does the influenza vaccine increase susceptibility to other viral diseases. Does it reduce the immunity memory and make humans more dependent on vaccines instead of their natural responses to future infections?

    Michael Scott / Reply
  3. In this country, no one is forced to have the influenza vaccine and so this review is helpful in informing people the possible benefits to them as individuals and helping them decide whether or not to be vaccinated. However, it’s also important to remember that vaccination helps reduce transmission and so potentially protects others. This is particularly important for health service staff who are likely to be working with vulnerable people who are at higher risk of serious complications or death if they are infected and are working in a ‘closed community’ where Influenza is likely to spread more rapidly.

    Emma Plugge / Reply
    • Erm, from what I read, the evidence was not strong, and the benefit not large. The adverse effects are not known because the quality of the evidence was insufficient. Why would I be persuaded to have a vaccine on this evidence?

      Hisham / (in reply to Emma Plugge) Reply
  4. I think that giving the straight risk numbers and avoiding the use of the absolute risk reduction helps avoid:
    1) using relative risk and being accused of trying to fool people. So many sensationalist reports of studies grab headlines by using the highly misleading “relative risk”
    2) If the absolute risk reduction were actually specified directly, people would see a really low number and be far less convinced of it’s effectiveness. It’s clear that public health officials want us to get the vaccine no matter how unlikely it is to help us as individuals. Why? um, 1-2% less flu serves whom? Or…?

    Steve / Reply
  5. I had my flu jab last week (I am “healthy” but work in a UK NHS hospital that encourages us to have it ) – so was interested in reading this.

    In the Dimecheli review implications for practice it states ” Healthy adults who receive inactivated parenteral influenza vaccine rather than no vaccine probably have a 1% lower risk of experiencing influenza over a single influenza season (2.3% versus 1%, moderate‐certainty evidence) and probably have a 3.4% lower risk of experiencing influenza‐like illness (ILI) (21.5% versus 18.1%, moderate‐certainty evidence). ”

    Shouldn’t they specifically state “absolute” risk more clearly than just saying risk and why round the 1.3% down to 1% when they don’t for the 3.4% ILI risk?

    Gerry Stansby / Reply

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