Studies on the effectiveness of vaccines against Covid-19 follow one another ... and the results are not always the same.
New work, led by Imperial College London, was made public on Wednesday.
It shows that the effectiveness of two doses of vaccine would be nearly 50% against infection, and nearly 60% against symptomatic forms.
This is less than for previous surveys showing higher rates, but this can be explained for different reasons.
Here's what to remember.
How was this study conducted?
This survey was conducted as part of the React-1 program, developed by Imperial College London.
But this is the first time that she has looked at the effectiveness of vaccines.
A random sample of nearly 100,000 UK residents was examined over the period June 24 to July 12.
527 of them, or 0.63%, tested positive.
This prevalence is "similar to that observed at the beginning of October 2020 and at the end of January 2021" and it is 4 times higher than that obtained during the previous phase of the survey, from May 20 to June 7.
This finding is logical, since the country was faced with a very significant epidemic wave during this period - a peak was noted on July 20, so the study does not take it into account.
At the start of this summer, the Delta variant had already become a very large majority across the Channel.
What are the results ?
By comparing the vaccinated (with two doses) and the unvaccinated, the researchers arrive at an efficacy of the vaccination of 49% against the infection, and of 59% against the symptomatic forms.
3.84% of individuals who received two doses of vaccine, or nearly 1 in 25, were positive, compared to 5.86% of those who received a single dose and 7.23% of those who received none. .
To arrive at these figures, the authors of the study limited themselves to the 57,457 participants aged 18 to 64 years.
Indeed, the elderly involve biases because they are more vaccinated and they can have different behaviors, in particular more respect the barrier gestures, than the young people.
Without this methodological correction, the risk of being infected was divided by 3 - and not by 2 - in vaccinated people.
In addition to age, gender, region, and ethnicity were also taken into account to adjust the data.
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The 59% efficacy against symptomatic forms is lower than that obtained by the majority of studies already known. Also in England, two doses of vaccine would be effective against the Delta variant up to 79%, according to data from Public Health England. The difference is explained by the method used. Since Imperial College's work involves "a random sample of the general population, [this] includes asymptomatic individuals," said Professor Paul Elliott, director of the React-1 program and professor of epidemiology and epidemiology. public health medicine at Imperial College London. However, these can be quite numerous in the vaccinated. And real population studies like Public Health England's focus overwhelmingly on symptomatic cases.
In addition, according to the results of Imperial College, positive people vaccinated had a much lower viral load than positive unvaccinated cases.
This is contradictory with the data from the American Centers for Disease Control and Prevention (CDC) communicated on Friday, suggesting on the contrary that the viral load would be equivalent whether one is vaccinated or not (without being able to if this implies the same contagiousness).
Can we transpose them for France?
No. The main reason is that the study did not distinguish between people vaccinated with AstraZeneca and those vaccinated with Pfizer. All the scientific data agree to show that the first of the two is less effective than the second, in particular against the symptomatic forms. The 79% of Public Health England mentioned above is actually an average, and the measured effectiveness rate is 88% for Pfizer and 60% for AstraZeneca. This therefore “brings” the final score down. However, the majority of vaccinated across the Channel (24 out of 46 million on July 21) are with "AZ", developed in partnership with the British University of Oxford. In France,only 10% of people who have received at least one dose of the vaccine have been “stung” with AstraZeneca (and almost no one has received it as their first injection for several weeks).
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Moreover, this study does not give efficacy against the severe forms specifically.
However, as the infectious disease specialist Anne-Claude Crémieux recently told us, "we expect the vaccine, and in particular after two doses, that it protects especially against hospitalizations".
Finally, like any study, this one should not be interpreted in isolation.
Other work may lead to slightly different results, depending on the period examined, the epidemic situation in the country, etc.
The main lesson to be learned is "vaccination is still very effective against Delta," according to Paul Elliott. But also that "despite an extremely high vaccination coverage, we are very likely to have a new wave of infections in the fall", analysis in the Financial Times Tom Wingfield, lecturer at the Liverpool School of Tropical Medicine. 73% of adults are fully vaccinated in the UK, compared to 66% in France.
