Coronavirus – Vaccine greatly reduces risk of transmission


A Belgian study shows that having received two doses of Moderna or Pfizer, the probability of infecting someone else drops by more than half.

Between two fully vaccinated people, the risk of transmission is up to 90% lower.

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Getting vaccinated against the coronavirus first and foremost helps to avoid developing serious forms of the disease. But this does not prevent you from being infected all the same, nor from being able to infect someone else. The first observations nevertheless seemed to show that this risk of transmission was lower in the vaccinated. A Belgian study which has just been published shows this, with initial figures to support it.

This research, carried out by the Scientific Institute of Public Health Sciensano was carried out on 290,000 contact cases in Belgium between January and June 2021. In the country, anyone who had been in contact with someone infected had to quarantine themselves and carry out two PCR tests. It is the results of these tests that have been studied. 8,000 of these contact cases were vaccinated, 282,000 were not.

First observation on the effectiveness of vaccines (Pfizer and Moderna were the two mainly administered): people who received the two doses had 74 to 85% less risk of being infected. That’s lower than the efficacy figures given by the vaccine makers, but that’s to be expected because they talk about reducing the chances of developing severe symptoms, not getting infected. And there were indeed in this Belgian study people who were positive asymptomatic.

Between vaccinated, we get infected very, very little

So yes, the vaccine reduces the risk of being infected, but if we are still, can we transmit the virus? Tests revealed that 990 fully vaccinated people still caught the coronavirus. But by studying their contact cases, it turned out that they had between 52 and 62% less risk of infecting someone other than an unvaccinated person. And when a fully vaccinated but positive person encounters another fully vaccinated person, they have a 90% lower risk of infecting them.

This is reassuring, but the study has certain limits. It was carried out before the more virulent Delta variant. If the vaccines still protect against it, would a vaccinee who would still catch it have a little more risk of transmitting it than the Alpha variant? Data is still lacking on this point. As the tests in this study were not performed in nursing homes, vaccine protection may also be less effective in the elderly than the figures reported here. But again, we don’t know. Finally, few people outside retirement and care homes were vaccinated during the period in question. But still, among the 8,000 who were, the numbers of infection and contamination of others were very low.

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