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 United States healthcare personnel and first responders.

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

The study by the US Centres 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.