How much does the third dose of vaccine reduce the chance of testing positive?

In a large study carried out on more than 300,000 people, researchers were interested in the effects of the third dose of the vaccine against Covid-19.

One, two, then now three doses: the vaccination schedule recommended by the French government within the framework of the covid has evolved well in a few months. But do these three doses have the desired effects? Do they at least improve the overall situation? Many results seem to indicate so, but it is still necessary to know how much. A group of Israeli researchers recently took an interest in this question in work spotted by Futura.

To find out, they looked at a set of data collected on 306,710 Israelis aged 40 or over, and never having contracted the disease. This origin is not trivial. Indeed, the country has already adopted the third dose of vaccine since August 24, 2021. This means that the Israeli population has three additional months of decline compared to France; in France, access to the third dose was validated for adults on October 27.

Among these individuals, the researchers identified two groups: a first group vaccinated twice, and a second who received a third dose. These two categories were followed for a little over two months, during which time the authors painstakingly recorded the results of all PCR tests. At the end of the experiment, these 306,710 people had accumulated 500,232 PCR tests. Or a sample large enough to obtain conclusive results.

The third dose reduces the chances of being tested positive

In the two-dose group, 6.6% of the tests came back positive; on the other hand, for the triply vaccinated group, 1.8% of the tests came back positive. After two weeks, the third injection would therefore reduce the risk of being tested positive by 86% compared to a person who would have received only two doses. In addition, the probability of being hospitalized would be 92% to 97% lower among “triple vaccinated”, again compared to those who received two doses.

The chances of contracting the disease with three doses of the vaccine are therefore very real, but significantly lower than with two injections. The other good news is that this protection apparently persisted for the duration of the study, which was 65 days. Rather encouraging data; but the objective of this study was to study precisely the rate of infection, and not the duration of protection.

A clear trend, but also some limits

Encouraging results, but which must however be interpreted with great caution. Initially, this study is satisfied with a purely quantitative statistical evaluation, and based on the number of positive tests. It does not distinguish between more or less severe forms of the disease. The same goes for the spectacular drop in hospitalizations; even if it is obviously necessary to be satisfied with it, the researchers themselves explain in their paper that they still lack data to provide a solid and argued interpretation of this phenomenon.

To be able to get rid of these limits, it will therefore be necessary to scaffold new studies over longer durations. Because if their results suggest a fairly durable protection after the third dose, the team recalls in its paper that it will be “also important to monitor the decline in immunity after the third dose”Well beyond the 65 days covered by this study. This is data that will be essential in deciding whether the strategy of the third dose actually produces the expected effects on a global scale, or whether it is a short-term statistical anomaly.

The text of the study is available here.

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