By Ankur Banerjee and Vishwadha Chander

(Reuters) – COVID-19 vaccines developed by Pfizer Inc with BioNTech SE and Moderna Inc reduced the risk of infection by 80% two weeks or more after the first of two shots, according to data from a real-world U.S. study released on Monday.

The risk of infection fell 90% by two weeks after the second shot, the study of just under 4,000 vaccinated U.S. healthcare personnel and first responders found.

The study by the U.S. 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.

The findings from of the real-world use of these messenger RNA (mRNA) vaccines confirm the efficacy demonstrated in the 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,” CDC Director Rochelle Walensky said in a statement.

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.

The CDC study comes weeks after real-world data from Israel suggested that the Pfizer/BioNTech vaccine was 94% effective in preventing asymptomatic infections.

Some countries, including Britain and Canada, are allowing extended gaps between doses that differ from how the vaccines were tested in clinical trials in order to alleviate supply constraints. In the trials, there was a three-week gap between Pfizer shots and four weeks for the Moderna vaccine.

In Britain, authorities said in January that data supported its decision to move to 12 weeks between the first and second Pfizer/BioNtech shots. Pfizer and its German partner have warned that they had no evidence to prove that.

(Reporting by Ankur Banerjee and Vishwadha Chander in Bengaluru; Editing by Peter Henderson and Bill Berkrot)