The US Department of Health and Human Services will cancel contracts and pull funding for some vaccines that are being developed to fight respiratory viruses such as COVID-19 and the flu.

Robert F. Kennedy Jr announced overnight that 22 projects, totaling $US500 million ($770 million), to develop vaccines using mRNA technology will be halted.

Kennedy’s decision to terminate the projects is the latest in a string of decisions that have put the longtime vaccine critic’s doubts about shots into full effect at the nation’s health department.

Kennedy has pulled back recommendations around the COVID-19 shots, fired the panel that makes vaccine recommendations, and refused to offer a vigorous endorsement of vaccinations as a measles outbreak worsened.

The health secretary criticised mRNA vaccines in a video on his social media accounts, explaining the decision to cancel projects being led by the nation’s leading pharmaceutical companies, including Pfizer and Moderna, that offer protection against viruses such as the flu, COVID-19 and H5N1.

“To replace the troubled mRNA programs, we’re prioritising the development of safer, broader vaccine strategies, like whole-virus vaccines and novel platforms that don’t collapse when viruses mutate,” Kennedy said in the video.

Infectious disease experts say the mRNA technology used in vaccines is safe, and they credit its development during the first Trump administration with slowing the 2020 coronavirus pandemic.

Future pandemics, they warned, would be harder to stop without the help of mRNA.

“I don’t think I’ve seen a more dangerous decision in public health in my 50 years in the business,” Mike Osterholm, a University of Minnesota expert on infectious diseases and pandemic preparations, said.

He noted mRNA technology offered potential advantages of rapid production, crucial in the event of a new pandemic that required a new vaccine.

The shelving of the mRNA projects is short-sighted as concerns about a bird flu pandemic continue to loom, Dr Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia, said.

“It’s certainly saved millions of lives,” Offit said of the existing mRNA vaccines.

Scientists are using mRNA for more than infectious disease vaccines, with researchers around the world exploring its use for cancer immunotherapies.

At the White House earlier this year, billionaire tech entrepreneur Larry Ellison praised mRNA for its potential to treat cancer.

Traditionally, vaccines have required growing pieces of viruses, often in chicken eggs or giant vats of cells, then purifying that material.

The mRNA approach starts with a snippet of genetic code that carries instructions for making proteins. Scientists pick the protein to target, inject that blueprint and the body makes just enough to trigger immune protection — producing its own vaccine dose.

In a statement, HHS said “other uses of mRNA technology within the department are not impacted by this announcement”.

The mRNA technology is used in approved COVID-19 and RSV shots, but has not yet been approved for a flu shot.

Moderna, which was studying a combination COVID-19 and flu mRNA shot, had said it believed mRNA could speed up production of flu shots compared with traditional vaccines.

The abandoned mRNA projects signal a “shift in vaccine development priorities,” the health department said in its statement, adding that it would start “investing in better solutions”.

“Let me be absolutely clear, HHS supports safe, effective vaccines for every American who wants them,” Kennedy said in the statement.

Speaking hours later at a news conference in Anchorage, Alaska, alongside the state’s two Republican US senators, Kennedy said work was under way on an alternative.

He said a “universal vaccine” that mimicked “natural immunity” was the administration’s focus.

“It could be effective – we believe it’s going to be effective – against not only coronaviruses, but also flu,” he said.