How well do vaccines prevent transmission of the virus?

How well do vaccines prevent transmission of the virus?
How well do vaccines prevent transmission of the virus?

With deconfinement will inevitably come an increase in the number of close contacts. People who have received zero, one or two doses of the vaccine will see people who have received zero, one or two doses of the vaccine. Vaccines are known to protect very well against COVID-19 infections, and even better against severe cases. However, in the rare cases that the vaccinees catch the virus (even without developing symptoms), at what point do they pass it on to the next?

To this thorny question, already addressed in our newsletter in February, we find new answers week by week. According to studies in a few countries, vaccines cut the risk of COVID-19 transmission by about half from people vaccinated to their contacts. A reassuring observation, which nevertheless calls for maintaining a certain caution as long as the virus is circulating strongly in the communities.

At the end of April, the English public health agency informed of the results of a large study of more than 365,000 half-vaccinated households where at least one case of COVID-19 had been detected. When the “index case”, that is to say the person who imported the virus into the household, had been vaccinated for 21 days or more, the risks of this one producing “secondary transmission” among relatives were 40-50% lower. The results were valid for both Pfizer’s vaccine and AstraZeneca’s.

Very recently, a study from Finland pointed in the same direction. From two to ten weeks after the first dose, the risk of secondary infection gradually decreased to approximately 40%. Just under half of the study group received their second dose after four weeks; the rest of the group still only had one dose after ten weeks. This pre-publication, not yet peer-reviewed, was filed online on May 29.

The mechanism involved in cutting off secondary transmission is not rocket science: the virus simply multiplies less in vaccinated people who develop an infection. A study from Israel, published in late March, showed that the viral load of infections occurring 12 to 37 days after a first dose of vaccine was “substantially reduced”. “These reduced viral loads suggest a potentially lower infectivity, which contributes to the effect of the vaccine on the spread of the virus”, explained the authors of the publication published in Nature Medicine.


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