Even after just the first of two shots of either Pfizer-BioNTech or Moderna's COVID-19 vaccines, the risk of coronavirus infection drops by 80%, data showed Monday from a real-world analysis of vaccinated U.S. health care personnel and first responders.

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

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

The findings from 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 health care 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 coronavirus. That teaches the immune system to recognize and attack the actual virus. Originally designed to fight cancer, BioNTech is using mRNA technology to end the pandemic.

The European Medicines Agency (EMA) recently approved new manufacturing sites for COVID-19 vaccines made by Pfizer-BioNTech, Moderna and AstraZeneca – another sign that inoculation efforts may stem the tide of the pandemic.