Pfizer, Moderna Covid-19 vaccines highly effective even after first shot in real-world use

The vaccines effectively prevented coronavirus infections, not just illness, with substantial protection evident.
The vaccines effectively prevented coronavirus infections, not just illness, with substantial protection evident.PHOTO: REUTERS

BENGALURU (REUTERS, BLOOMBERG) - Covid-19 vaccines developed by Pfizer with BioNTech and Moderna reduced the risk of infection by 80 per cent two weeks or more after the first of two shots, according to data from a real-world study of vaccinated US healthcare personnel and first responders released on Monday (March 29).

The risk of infection fell 90 per cent by two weeks after the second shot, the study of just under 4,000 people found.

The study by the US Centres for Disease Control and Prevention (CDC) evaluated the vaccines’ ability to protect against infection, including infections that did not cause symptoms. Previous clinical trials by the companies evaluated their vaccine’s efficacy in preventing illness from Covid-19.

“This study shows that our national vaccination efforts are working,” CDC Director Rochelle Walensky said in a statement.

The findings from 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. Earlier clinical trials had established that the shots also prevent illness, hospitalisations and deaths.

The study looked at the effectiveness of the mRNA vaccines among 3,950 participants in six states over a 13-week period from Dec 14, 2020 to March 13, 2021.

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 authorised mRNA COVID-19 vaccines provided early, substantial real-world protection against infection for our nation’s healthcare personnel, first responders, and other frontline essential workers,” Dr Walensky said. 

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.

The new mRNA technology is a synthetic form of a natural chemical messenger being used to instruct cells to make proteins that mirror part of the novel coronavirus. That teaches the immune system to recognise and attack the actual virus.