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Pfizer, Moderna vaccines reduce Covid-19 infection risk by 90% after second dose

BENGALURU (REUTERS, BLOOMBERG) – The Covid-19 vaccines developed by Pfizer with BioNTech and Moderna are highly effective and reduced the risk of infection by 90 per cent by two weeks after the second shot among healthcare personnel and first responders, according to a US study released on Monday (March 29).

The findings from a study by the US Centres for Disease Control and Prevention of the real-world use of these messenger RNA (mRNA) vaccines confirm what was seen in large controlled clinical trials conducted before they received emergency use authorisations from the US Food and Drug Administration.

The vaccines effectively prevented coronavirus infections, not just illness, with substantial protection evident, government researchers said. Earlier clinical trials had established that the shots also prevent illness, hospitalisations and deaths.

The study adds to evidence that new vaccines made with messenger RNA technology actually reduce the spread of the virus in real-world conditions. An earlier study in Israel found a single dose of the Pfizer vaccine reduced infections by as much as 85 per cent.

The CDC studied a group of about 4,000 front-line workers, including healthcare personnel, first responders, teachers and service workers from mid-December to mid-March as vaccines rolled out widely. These groups were among the first to be vaccinated, along with the vulnerable elderly, because of their risk of exposure to the virus.

Participants were tested for Covid-19 weekly and surveyed for reports of symptoms. The researchers compared the frequency of Covid infections before and after vaccinations to estimate how effective the shots were at preventing Sars-CoV-2 spread, regardless of whether people felt sick or not.

Both the vaccines from Pfizer-BioNTech and Moderna require two doses spaced weeks apart. Two weeks after the first dose, the shots appeared to prevent 80 per cent of infections; that rose to 90 per cent two weeks after the second dose, when people were considered fully immunised.

The study included a mix of participants who were tested after developing symptoms as well as infections that were picked up by weekly tests when people were still feeling well. The results did not break out in detail the vaccine’s ability to prevent asymptomatic or minimally symptomatic infections.

“The study demonstrates that these two mRNA vaccines can reduce the risk of all Sars-CoV-2 infections, not just symptomatic infections,” the CDC said in a statement.

Estimates of how well the shots prevent infection should be interpreted cautiously, due to a relatively small number of infections confirmed, the CDC said. The agency also did not have enough data in this study to distinguish between the two vaccines. The study took place before a third vaccine from Johnson & Johnson became widely available.

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