This speeding up of the production process is one of the key reasons mRNA vaccines have "long been distrusted by vaccine sceptics". Kennedy himself referred to mRNA vaccine technology as "troubled", and called older "whole-cell" vaccine technology "safer".

The US Department of Health and Human Services (HHS) plans to cancel $500m (£376m) in funding for mRNA vaccines being developed to counter viruses that cause diseases such as the flu and Covid-19.

The targeting of the vaccine technology once considered revolutionary follows intense criticism by conservatives and anti-vaccine activists.