People who receive Covid-19 vaccines by Pfizer and Moderna are up to 91 per cent less likely to develop the disease, according to a US study which also suggests that the preventives reduce the severity of symptoms and duration in those who still get an infection.

The research, published in the New England Journal of Medicine on June 30, is among the first to show the benefits of mRNA vaccines even among those who experience breakthrough infections -- testing positive after immunisation.

The mRNA vaccines by Pfizer and Moderna contain genetic instructions for our cells to make the spike protein of the SARS-CoV-2, which the virus uses to infect and enter the human cells.

Our immune system then builds an immune response against the spike protein, and learns how to fight off the coronavirus if we encounter it in future.

"One of the unique things about this study is that it measured the secondary benefits of the vaccines," said study co-author Sarang Yoon, an assistant professor at the University of Utah, US.

The study was designed to measure the risks and rates of infection among those on the front lines of the pandemic -- doctors, nurses, and first responders.

"These are the people who are getting exposure to the virus day in and day out, and the vaccine protected them against getting the disease. Those who unfortunately got Covid-19 despite being vaccinated were still better off than those who didn't," he added.

The study found that mRNA Covid-19 vaccines were 91 per cent effective in reducing risk for infection once participants were "fully" vaccinated, two weeks after the second dose.

The researchers also found that the vaccines are 81 per cent effective in reducing risk for infection after "partial" immunisation, two weeks after the first dose was given.

The study recruited 3,975 participants at eight sites in the US. Participants submitted samples for Covid-19 testing on a weekly basis for 17 weeks between December 13, 2020 and April 10, 2021.

Only 204 (5 per cent) of the participants eventually tested positive for SARS-CoV-2, the virus that causes Covid-19. Of these, 156 were unvaccinated, 32 had an indeterminate vaccine status, and 16 were fully or partially vaccinated.

The fully or partially vaccinated participants who developed breakthrough had milder symptoms than those who were unvaccinated.

In those who developed a breakthrough infection, the presence of fever was reduced by 58 per cent per cent among vaccinated people, and the days spent sick in bed were reduced by 60 per cent.

Detection of the virus was reduced by 70 per cent percent among those with breakthrough infections, from 8.9 days to 2.7 days.

These findings also suggest that fully or partially vaccinated individuals who get Covid-19 might be less likely to spread the virus to others.

The researchers found that infected study participants who had been fully or partially vaccinated when infected had 40 per cent less detectable virus in the nose and did so for six fewer days compared to those who were unvaccinated.

"I hope these findings reassure the public that mRNA Covid-19 vaccines are safe and protect us from this severe disease," Yoon added.