how the effectiveness of a vaccine is calculated and other scientific news

Published on : 19/11/2020 – 07:55Modified : 19/11/2020 – 12:20

As last week, several results of clinical trials of candidate vaccines against Covid-19 have been made public in recent days. Moderna thus announces 94.5% efficiency while Pfizer re-evaluates its own: it goes from 90% to 95%. Nevertheless, the succession of announcements and these high figures can question: is there not a race to which will announce the best rate?

If the chain of press releases may come as a surprise, there is nevertheless a simple reason for their succession: the phase 3 clinical trials conducted by Pfizer and Moderna were the first to start, which moreover is around the same time. . They are therefore expected to deliver their results at the same time. The same goes for the advertised efficiency rates. The two vaccine candidates are based on the same technology: messenger RNA. It is therefore, here too, logical that their results are in the same order of magnitude. Too marked a difference would have, on the contrary, interrogates.

How are these efficiency rates calculated?

To obtain efficacy and safety data, the laboratories carry out large-scale clinical trials. They recruit tens of thousands of volunteers: over 30,000 for Moderna, over 40,000 for Pfizer.

These cohorts are divided into two groups: one receives the vaccine candidate, the other a placebo. The participants then return home and a number of Covid-19 contaminations are expected to be observed. It is then sufficient to look at how these infections are distributed between the two groups to deduce the effectiveness of the candidate vaccine.

Thus, in its latest announcement, Pfizer reports that 28 days after the administration of the first dose, there were 170 confirmed cases of Covid-19; 162 in the placebo group, 8 in the vaccine group. This therefore gives 95% efficiency.

There is a little less hindsight for theModern essay, since the presented analysis was carried out when only 95 people were infected. However, the distribution is similar: 90 in the placebo group, 5 in the vaccine group for 94.5% efficacy.

Gross efficiency is not the only factor to consider, however. Among those infected, we can also count severe cases. In the data published by Pfizer, out of the 170 contaminations, there were 10 serious cases, including one in the vaccine group. In those of Moderna, all the serious cases are in the control group. The vaccine therefore appears to be effective in preventing them. This is interesting from the point of view of management of the epidemic, in particular for hospital pressure and the resulting confinements. However, we will have to wait for this data to be consolidated, especially when the Moderna trial will be as advanced as that of Pfizer, in about a week.

Targeted containment tracks

In France at least, the confinement seems to show its effectiveness, according to the authorities, and we are already starting to talk about its end. A study published in the journal Nature gives some ideas for better lifting.

The authors carried out massive modeling work in the 10 largest cities in the United States. By studying the movements of the population from residential neighborhoods to places of interest such as restaurants or shops, they were able to determine which were the places where the virus circulated the most. Thus, the data of nearly 100 million people were analyzed to determine that only 10% of places of interest are responsible for 85% of contamination. By carrying out targeted closures, we can therefore have an impact almost similar to generalized containment. Bad news, however, for the places concerned, mainly bars and restaurants, places of worship and hotels; following this logic, they would be the last to reopen their doors.

What happens to cured patients? If the occupancy rate of hospital beds is linked to the epidemic dynamic, it is nevertheless possible to anticipate a part of it. We read in the weekly report of the American Center for Disease Control (CDC) a study in which the authors come back to these patients cured of Covid-19 who must be hospitalized a second time because of the after-effects left by the virus. This can indeed leave damage such as inflammation in the heart, lungs or neurological.

By studying the records of more than 100,000 hospital patients, the researchers calculated that 9% of them had to be again within two months of their first discharge. These are mainly patients over 65 years of age or with co-morbidities.

We can do the same calculation for France. Around 30,000 patients are now hospitalized due to coronavirus. Statistically, we therefore know that 2,700 of them will cross the hospital doors again in the next two months, which will add significant additional pressure on the hospital system.


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