Covid vaccine: what does 70%, 80% or 90% effective mean?

Published by Laurent P. · Published on 10 December 2020 at 18h13 · Updated on 11 December 2020 at 16h27
As several vaccines are beginning to share their clinical trial results, their effectiveness can be very different. 70%, 80%, 90%... What do these number mean? Keep reading to find out more!

Covid vaccine is soon to be available everywhere around the globe… Some countries even started to vaccinate their inhabitants with different products from recently-completed clinical trials. And results are striking: Pfizer laboratory vaccine is 94% effective, AstraZeneca is 70%, and Moderna Therapeutics is 94.5% effective. The future sounds reassuring and coronavirus-free, if we dare hope so. But what does this percentage mean? Does a vaccine that is 70% effective protect as well as vaccine that is 95% effective? What are the consequences for people vaccinated? We try to answer these questions.

How is the effectiveness percentage calculated? It is based “on the number of people catching the virus in the group of people that have been vaccinated, which is then compared to the group that has been given placebo” our peers from LCI explain. Effectiveness is not the outcome of the final results from the clinical trials, but from preliminary results. We shall wait for the last analyses to be settled. As for Lille University Hospital head of the Clinical investigation center Dominique Deplanque, he calls for prudence when it comes to reading the percentages: “Before we talk about numbers, we shall agree on the effectiveness feature” he says. He goes on, saying that “depending on the studies, there are several levels of effectiveness”.

As Dominique Deplanque reminds, it is necessary to first “show effectiveness of the onset of the disease itself”, then vaccinating someone to check if they can be contaminated by the virus or not, and then “the intensity of the symptoms to “limit the severe forms”” LCI explains. “When trials are conducted, the main goal and the secondary goal are usually targeted and amid the current pandemic, both are complementary, the goal behind it all is to limit the level of the pathology and even eradicate itLille University Hospital head of the Clinical investigation center goes on.

It is possible for a vaccine to be 100% effective? Once again, Dominique Deplanque answers the question: “the probability to get 100% of subjects vaccinated not to develop the disease ever, is almost impossible”. How come? “A time factor that involves the fact that the longer time goes by, the more fragile immunity is”. This result does not only apply to Covid vaccine, but to all product fight against other diseases.

Does a vaccine that is 70% effective protect as well as a vaccine that is 95% effective? The answer is no as well: “70%, when we know herd immunity is reached from the moment there are enough people that have been in contact with the virus, whatever happens it enables to go quicker than with natural immunityPr. Deplanque says. He goes on: “As of today, the protein targeted by the candidate vaccine seems stable enough for the immune response to be relevant”. What seems to be really relevant is not the effectiveness level in comparison with others, but the result on the population. The more people are vaccinated, whether the vaccine is 70% or 90% effective, the slower the virus will spread and easier to eradicate.

And what about a vaccine that is less than 70% effective? As calculated by the American Journal of Preventive Medicine, “To extinguish an ongoing epidemic, the vaccine efficacy has to be at least 60% when coverage is 100%”. The effectiveness would be higher if fewer people are vaccinated… For instance, if only 75% of the population is vaccinated, the vaccine will have to be at least 80% effective to generate herd immunity.

For the record, the most effective vaccine at the moment is the measle vaccine, that is 90% effective (95% for booster shoots). As for the flu vaccine, it is around 70% effective.

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