Covid: once inoculated, does vaccine prevent from spreading the virus?

Published by Laurent P. · Published on 6 January 2021 at 16h39 · Updated on 6 January 2021 at 19h22
According to data already made available Covid vaccines on the market or being approved protect from being contaminated, but do they prevent from transmitting the virus to somebody else? We try to answer this question.

How would Covid vaccines on the market or being studied protect from the disease? A question asked by many people, not because of the effectiveness to fight against symptoms (data are widely enough to show it), but its capacity to prevent from transmitting the virus once injected.

Does the vaccine prevent from contaminations? Scientists hope so, but so far, data on the matter are not enough. How come? Scientists rather focused – as reminded by Le Monde – on symptoms, that can be easily spotted when clinical trials were started, as well as less expensive to study, unlike conducting weekly virological tests enabling to see if the virus keeps on spreading once people are vaccinated.

Especially when it comes to public health, sick people are the issue, especially when developing severe forms. If Covid-19 only looked like a standard cold, we would need no vaccineLa Pitié-Salpêtrière hospital immunology and infectious disease center managed Guy Gorochov explains. Then, this is a logical way to overcome this disease and avoid massacre.

Determining Covid protection in the middle of the vaccine strategy

As countries have started to vaccinate people, they are wondering about protection granted and that could change vaccine strategies around the world. For instance, still according to Le Monde, “if a vaccine truly is a barrier against the virus, it may seem right to immunize potential super-contaminators, those meeting a lot of people – doctors, bus drivers, supermarkets cashiers”. On the contrary, if the vaccine can only curb symptoms and the degradation of cases, vaccine priority changes, leading to what the government intends to do when it comes to France’s vaccine campaign.

A change in strategy that “if we get new data” would “make our priorities evolveHaute Autorité de Santé vice-president of vaccine technique commission Daniel Floret told Le Monde. Anyway, several studied are going on to determine if spreading remains possible – or at least weaker – once vaccinated: as shown by a trial from Moderna Therapeutics, led on monkeys and consisting in assessing nose swabs (PCR tests) after injecting one dose of vaccine. A study leading to promising results, since in two days’ time, the virus almost entirely disappeared from the swabs, even after Covid exposure.

Data still not enough

Results that should be handled with care. “Monkeys weigh 8kg, ten times less than us. But, when they have been given 10 micrograms of the product, that proportionally matches Moderna vaccine to humans, it did not lead to more effects that test physiological saline…” Créteil Henri-Mondor hospital Clinical immunology and infectious disease center head Jean-Daniel Lelièvre said.

The experience has been tried again, during phase 3 of the clinical trial that led to the creation of the vaccine, on 28,000 – human – volunteers, before injecting the vaccine. Results are more mixed: among people PCR-tested negative before vaccine, 52 of them have tested positive and asymptomatic one month later. As for these 52 people positive, 14 of them had been given the vaccine, and 38 have been given placebo.

The data were not sufficient to assess asymptomatic infection, although our results from a preliminary exploratory analysis suggest that some degree of prevention may be afforded after the first dose” the trial authors wrote on December 30 in New England Journal of Medecine. “There are not enough samples and too big margins of errorLelièvre said. The question remains without an answer for now.

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