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.

That will impact 22 projects being led by major pharmaceutical companies, including Pfizer and Moderna, for vaccines against bird flu and other viruses, HHS said.

Health Secretary Robert F Kennedy Jr, a vaccine sceptic, announced he was pulling the funding over claims that "mRNA technology poses more risks than benefits for these respiratory viruses".

Doctors and health experts have criticised Kennedy's longstanding questioning of the safety and efficacy of vaccines and his views on health policies.

The development of mRNA vaccines to target Covid-19 was critical in helping slow down the pandemic and saving millions of lives, said Peter Lurie, a former US Food and Drug Administration official.

He told the BBC that the change was the US "turning its back on one of the most promising tools to fight the next pandemic".

In a statement, Kennedy said his team had "reviewed the science, listened to the experts, and acted". "[T]he data show these vaccines fail to protect effectively against upper respiratory infections like COVID and flu," he said.

He said the department was shifting the funding toward "safer, broader vaccine platforms that remain effective even as viruses mutate".

Kennedy also claimed that mRNA vaccines can help "encourage new mutations and can actually prolong pandemics as the virus constantly mutates to escape the protective effects of the vaccine".

Health experts have said that viruses mutate regardless of whether vaccines exist for them.

This was true every year for the flu virus, for example, said Dr Paul Offit, the director of the Vaccine Education Center at Children's Hospital of Philadelphia.

Dr Offit said mRNA vaccines were "remarkably safe" and a key to helping prevent against severe infections from viruses like Covid-19.

HHS said the department that runs the vaccine projects, Biomedical Advanced Research and Development Authority (BARDA), would focus on "platforms with stronger safety records and transparent clinical and manufacturing data practices".

While some vaccines use an inactivated virus to trigger an immune response, mRNA vaccines work by teaching cells how to make proteins that can trigger an immune response. Moderna and Pfizer's mRNA vaccines were tested in thousands of people before being rolled out and were found to be safe and effective.

Dr Offit, who invented the rotavirus vaccine, said the funding cancellation could put the US in a "more dangerous" position to respond to any potential future pandemic. He noted mRNA vaccines have a shorter development cycle, which is why they were crucial to responding to the Covid-19 pandemic.

Since taking office, Kennedy has taken a number of steps to transform how the nation's health department develops and regulates vaccines.

In June, he fired all 17 members of a committee that issues official government recommendations on immunisations, replacing them with some people who have criticized the safety and efficacy of vaccines.

He also removed the Covid-19 vaccine from the Center for Disease Control and Prevention's recommended immunization schedule for healthy children and pregnant women.