Paris (AFP)
To speak of the effectiveness of vaccines against the coronavirus or its variants is to pile up percentages as spectacular as they are difficult for the general public to interpret.
Instructions to better understand.
- How to measure efficiency?
When a vaccine is said to be X% effective against Covid-19, it means that it reduces the risk of catching the disease by X%, according to calculations by its designers based on clinical trials.
Among the tens of thousands of volunteers in the trials, one part receives the vaccine and another a placebo.
During the test, all lead normal lives and some will catch Covid-19.
If the vaccine is effective, the number of people infected will be lower among vaccinated participants.
To illustrate this calculation, we can take the example of the Russian vaccine Sputnik V, the last to have published results.
Published on February 2 in the medical journal The Lancet, they show that the vaccine is 91.6% effective against symptomatic forms.
The Russian researchers came up with this figure because 16 of the 14,900 volunteers who had received the two doses (three weeks apart) tested positive, compared with 62 of the 4,900 who had received the placebo.
At this stage, the best performing vaccines are those from Pfizer / BioNTech and Moderna, which use messenger RNA technology.
They are 95% and 94.1% effective (with 21 and 28 days between doses, respectively).
- What levers?
Effectiveness is not a fixed figure and depends on several criteria, such as how the vaccine is administered.
For example, AstraZeneca and its partner, the University of Oxford, claimed that the effectiveness of their vaccine varied significantly with the spacing between doses.
"Efficacy reached 82.4% in volunteers with an interval of 12 weeks or more between the two doses", against 54.9% with less than 6 weeks, write their researchers in a study released on February 1.
They are based on the results recorded in subgroups of volunteers from their trials.
The European Medicines Agency (EMA), for its part, has retained the general figure "of about 60% effectiveness" (with an interval of 4 to 12 weeks between doses) to authorize this vaccine on January 29.
In addition, the effectiveness depends on the criterion chosen to measure it.
So, that of Johnson & Johnson's vaccine is 66% overall, according to a manufacturer's announcement on Jan. 29.
But specialists insist above all on the fact that it rises to 85% when we focus on the prevention of severe forms of the disease.
Finally, scientists are wondering whether we could not increase the effectiveness of vaccination by using a vaccine different from the first for the second dose.
The University of Oxford has just launched a study on 820 volunteers over 50 years old to test this hypothesis with vaccines from Pfizer / BioNTech and AstraZeneca.
- And against variants?
The question torments specialists.
While the expansion of the variant initially detected in England is worrying, vaccine-related fears mostly focus on another variant, which has emerged in South Africa.
One of its mutations (called E484K) seems likely to make vaccines less effective.
South Africa even decided on Wednesday to use Johnson & Johnson's vaccine rather than AstraZeneca's, after a study questioning its effectiveness against the South African variant.
Carried out in 2,000 people, the study claims that AstraZeneca's vaccine offers "limited protection against moderate forms of the disease caused by the South African variant in young adults."
Many experts, however, have warned against any definitive conclusion, and the WHO recommended Wednesday to continue using this vaccine "even if variants are present".
The South African study is "undeniably (...) worrying", but it has "important limits", underlined Monday the boss of the WHO, Tedros Adhanom Ghebreyesus.
He pointed to its "small size", which limits its statistical relevance, and the fact that the volunteers were young and in good health, therefore not representative of the general population.
"It is important to determine whether or not this vaccine remains effective in preventing more severe forms of the disease," he added.
For their part, Pfizer and BioNTech ensure that their vaccine remains effective against the main mutations of the English and South African variants.
They are based on an in vitro study by their researchers made public on January 27 and published Tuesday in the journal Nature Medicine.
Finally, Moderna said at the end of January that its vaccine was effective against the British variant and, to a lesser extent, against the South African.
The company therefore wants to develop a booster dose specifically directed against the South African variant.
Published Wednesday in the journal Nature, a study shows that faced with variants, the vaccines from Pfizer / BioNTech and Moderna lose a "reduced but significant" part of their effectiveness.
“Messenger RNA vaccines may need to be changed periodically to avoid loss of efficacy” against variants, the researchers conclude.
The EMA also asked all manufacturers on Wednesday "to do research to determine whether their vaccine offers protection against the new variants".
- Effective for whom?
It is not known whether these vaccines are also effective in the elderly, as the immune response decreases with age.
This question is particularly acute for the AstraZeneca vaccine, which several European countries only administer to those under 65, or even 55, because of a lack of data on its effectiveness in the elderly.
However, the WHO ruled on Wednesday that it was also valid in this age group.
Finally, it remains to be seen whether these vaccines prevent the transmission of the disease, in addition to protecting the vaccinated person.
In recent days, Israeli scientists have claimed on social media that this appears to be the case at least for Pfizer / BioNTech, which is administered on a large scale in Israel.
© 2021 AFP