COVID-19 vaccines developed by Pfizer Inc with BioNTech SE and Moderna Inc 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.

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 Centers 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 authorizations from the U.S. Food and Drug Administration.

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.

"The authorized 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," Walensky said.

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 recognize and attack the actual virus.