Pfizer, Moderna vaccines cut infection risk by 91%: US CDC

Taking two doses of the Pfizer-BioNTech and Moderna's mRNA-based vaccines, authorised by the Food and Drug Administration, can reduce the risk of coronavirus infection by 91 per cent, according to a new study by the US Centers for Disease Control and Prevention (CDC).

Even a single dose of the mRNA vaccines can reduce the risk of infection by 81 per cent. These estimates included symptomatic and asymptomatic infections.

"Covid-19 vaccines are a critical tool in overcoming this pandemic," said CDC Director Rochelle P Walensky.

"Findings from the extended timeframe of this study add to accumulating evidence that mRNA Covid-19 vaccines are effective and should prevent most infections -- but that fully vaccinated people who still get Covid-19 are likely to have milder, shorter illness and appear to be less likely to spread the virus to others. These benefits are another important reason to get vaccinated," Walensky added.

The study also showed that mRNA vaccination benefits people who get Covid-19 despite being fully vaccinated (14 or more days after dose 2) or partially vaccinated (14 or more days after dose 1 to 13 days after dose 2).

Fully or partially vaccinated people who developed Covid-19 spent on average six fewer total days sick and two fewer days sick in bed. They also had about a 60 per cent lower risk of developing symptoms, like fever or chills, compared to those who were unvaccinated. Some study participants infected with SARS-CoV-2 did not develop symptoms.

People who were fully or partially vaccinated and then got Covid-19 had 40 per cent less detectable virus in their nose (that is, a lower viral load), and the virus was detected for six fewer days (that is, viral shedding) compared to those who were unvaccinated when infected. This means they were also less likely to spread the virus to others.

In addition, people who were partially or fully vaccinated were 66 per cent less likely to test positive for SARS-CoV-2 infection for more than one week compared to those who were unvaccinated.

While these indicators are not a direct measure of a person's ability to spread the virus, they have been correlated with reduced spread of other viruses, such as varicella and influenza, the CDC said.

For the study, 3,975 participants completed weekly SARS-CoV-2 testing for 17 consecutive weeks (from December 13, 2020 to April 10, 2021) in eight US locations.

Participants self-collected nasal swabs that were laboratory tested for SARS-CoV-2. If the tests came back positive, the specimens were further tested to determine the amount of viral load and viral shedding.