SCIENCE – There was no longer any real surprise as the information had been circulating for 24 hours, but it is now official: all adults will have the right to receive a third dose of the Covid-19 vaccine from November 27.
During a press conference, Olivier Véran also specified that this reminder could be carried out as early as 5 months after the second dose and must be done before 7 months, otherwise the health pass will no longer be valid from January 15. A clear injunction, coupled with a reduction in the validity of tests from 72 to 24 hours to push the last unvaccinated to take the plunge.
The Pfizer vaccine will be available to all adults and Moderna, the other messenger RNA vaccine, will be available for those over 30 years old. In 11 months, more than 7 billion doses have been injected into more than 4 billion people. The benefit-risk balance of the first and second doses of vaccines has been proven over and over again (for more details, see our articles on this topic below).
But what about this third dose, especially for the less young and less at risk of severe forms of Covid-19? The answers are starting to pile up.
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A decrease in relative efficiency
Before talking about the third dose, we must take stock of the first two. We know that vaccines, especially messenger RNA such as Pfizer and Moderna, are very effective against Covid-19 (around 90%). Both to protect against serious forms, but also to prevent infection.
But we also know that the Delta variant somewhat decreases this efficacy for infection and, above all, that it decreases over time. Against severe forms, the efficacy of vaccines remains very high according to multiple studies. On the other hand, a slight decrease in the elderly was recorded.
This is why the booster dose was first offered to people at greatest risk and for whom a slight decrease in immunity could have serious consequences.
A third dose that works for everyone
There are few studies that actually calculate the effectiveness of this third dose in young adults. One of the most significant has been published October 29. Researchers analyzed more than 1.4 million Israelis as the recall was offered to the general population to deal with a surge in Covid-19. Half received a third dose, the other half did not.
And the comparison is clear: a new injection is 93% effective in reducing the risk of hospitalization compared to a full vaccination schedule. 231 hospitalizations were recorded in the group with a double dose against 29 in the one with a third dose.
But if we look at the detail by age group, the difference is much less nuanced for the youngest. For the 16-39 year old group, 6 hospital admissions were recorded in the “double dose” group, against one in the “booster dose” group. Each group is made up of 288,000 people.
Clearly, there are very few hospitalizations among vaccinees, with or without a booster dose, and if the third dose seems effective, it is difficult to know statistically how much the risk has been reduced. For 40-69 year olds, 73 people were hospitalized in the “double dose” group against 10 in the “single dose” group. A sufficient sample to say that the third dose has clear utility.
A way to curb the epidemic?
We could therefore say that for a healthy young adult, the third dose is a plus, but not a necessity. Moreover, in the United Kingdom, it is not yet accessible to those under 40 years old and the American health agencies specify that the oldest “should” be vaccinated, but that the youngest “can” do it.
However, it should not be forgotten that a vaccine allows you to protect yourself, but also to protect others. The fewer people who are infected, the less likely they are to transmit the virus.
As the effectiveness of the vaccine in the face of simple infection drops sharply over time, a booster dose could make it possible to limit the number of infections, and therefore the spread of the virus.
In his opinion paving the way for this generalization of the third dose, the High Authority for Health (HAS) evokes new models from the Institut Pasteur which should be published shortly, calibrated with data “before the recent acceleration of the epidemic” . These models, as always, are not predictions of the future at all. Moreover, the HAS specifies that “the most pessimistic scenario underestimates the current epidemic situation”.
But they are interesting to show us the interest, in theory, of a measure like the widening of the third dose. Thus, according to calculations by the Institut Pasteur, a booster in those over 65 would reduce the hospital peak of the fifth wave by 20%, while a third dose for the entire population would decrease it by 40%. Why? Because theoretically, by reducing the circulation of the virus, the epidemic progresses less quickly and the peak is less strong.
Well-known and well-supervised side effects
Clearly, the third dose makes it possible to further reduce the risk (already low for the youngest) of being hospitalized because of a serious form of Covid-19 and, above all, greatly lowers the risk of being infected. Which, if the majority of the population gets vaccinated, could decrease the peak of the fifth wave.
But what are the risks? Here too, variousstudies were carried out, both as part of original clinical trials but also in “real life”, notably in Israel.
The side effects observed are similar to those well known for the first two doses. There would be more than the first, but less than the second. The majority are light and transient (fever, pain in the arm, etc.). Some, more rare, are known, such as myocarditis in young men.
This is why HAS advises against using Moderna for men between 18 and 30 years old, because the risk is higher than with Pfizer. Remember that this risk is very low: 131 cases per million doses injected for Moderna, 26 for Pfizer.
See also on The HuffPost: Olivier Véran opens the third dose of anti-Covid vaccine for all adults
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