Aside from the usual controversy around vaccinations in general, some pro-vaccinators still wonder about the effectiveness of the flu shot itself. Does it really work? And how well does it work?
Fueling this concern has been the results of a recent meta-review in Cochrane Library (aka a report that took a large amount of similar studies about the flu vaccination and tried to compare them as best as possible). This review stated that the flu vaccine drops flu rates by a mere less than 1%! -Shocking!
The review by Demicheli V, Jefferson T, Ferroni E, Rivetti A & Di Pietrantonj C reported that the exact drop in flu rates was from just over 2% to just under 1% (moderate-certainty evidence), and was released in February 2018.
Sounds underwhelming doesn’t it? Especially in the face of last year being the first year the Queensland Government has subsidised all children under 5 years of age to have the flu shot.
So why the seeming discordance in opinion of value toward this immunisation?
Statistics can be misleading by how they are presented.
A recent ABC report delved into this controversial flu stat when a local Professor, Dr Price, called into question the effectiveness of the flu shot by referencing this meta-review. https://www.abc.net.au/news/2018-07-05/fact-check-flu-vaccine/9906284
Dr David Price, a biostatistician at the University of Melbourne, told Fact Check: “What these numbers mean, is that for every 1000 people, 23 would get flu without a vaccine programme. However, with a vaccine programme, the number of people who would get flu reduces to 9 in 1000.”
The difference in these values, Dr Price explained, showed that for every 1000 individuals vaccinated, 14 flu cases were prevented.
“It appears as though Professor Del Mar may have misinterpreted this figure as the vaccine effectiveness, which it is not. The vaccine effectiveness is actually around 59 per cent,” Dr Price said.
“A fairer statement in referring to the difference between 0.9 per cent and 2.3 per cent would be to refer to a reduction of 1.4 percentage-points. When thinking about this difference as a relative difference… we can see that the total number of cases as a result of the vaccine has more than halved, and thus sounds far more effective than stating that the vaccine only reduces influenza by one to two per cent.”
There is a difference between relative risk and absolute risk reduction, and vaccine effectiveness has therefore been misportrayed.
Imagine checking your lotto ticket and the newsagent staff might say “Congrats! You have doubled your winnings from last month”… and then you recall you only won $4 last month.
The ‘doubling’, ‘halving’ or giving % values often are a misleading way to represent true numbers, especially when the true numbers might give us better context. True numbers are the ‘absolute’ you will see when data is portrayed (e.g. $4 up to $8 lotto win which is an absolute increase of 4 dollars). Switching from relative to absolute numbers (and vice versa) can completely change the perception of the data entirely.
In this initial meta-review, the data was presented underwhelmingly by stating the effectiveness of the vaccine caused a drop in flu rates by presenting the data in ‘absolute’ terms, and not ‘relative’ terms. A drop by only 2% to 1% is unflattering, where alternately, the relative drop in cases (being around 60% reduction) is far more reassuring.
So how does a 60% reduction in incidence look in the community?
Back to Dr Price’s comments above: “Without immunisation the apparent base incidence of influenza would be 23 per 1000”. This 23 per 1000 value clearly has significant impacts on the community each winter if we recall over 110 000 Australians lost their lives in 2017 to this contagious disease. Whatever incidence can stand for, the flow on effects of the disease can be irreversible and far-reaching.
So as mentioned by Dr Price above, that with vaccine effectiveness actually more like 60%, this theoretically could have meant that approximately 60% of flu cases might have never happened if all people had undertaken vaccination. And what would this have meant for the 110 ooo individuals who lost their battle in 2017? How many lives might have been saved from a 60% success rate if the uptake of vaccination was done by most?
So having your flu shot is also about 60% effective in reducing your personal chances if we extrapolate this as a personal estimate from the data (which is the best we can do, because we can’t run individual studies effectively with everyone being so special and unique – studies and evidence don’t work well like that).
Flu shot: worth it?
It’s not perfect or 100% effective; but it’s the best protection going for an illness most would rather do without, and is still better than taking a ‘flip coin’ chance.
If you’d like a more in depth review of the articles in question above, the first article (the meta-review from Cochrane) is here: Demicheli V, Jefferson T, Ferroni E, Rivetti A, Di Pietrantonj C. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews 2018, Issue 2. Art. No.: CD001269. DOI: 10.1002/14651858.CD001269.pub6