The federal government announced Wednesday that it is cancelling a contract to develop a vaccine to protect people against flu viruses that could cause pandemics, including the bird flu virus that’s been spreading among dairy cows in the U.S., citing concerns about the safety of the mRNA technology being used.

The Department of Health and Human Services said it is terminating a $766 million contract with the vaccine company Moderna to develop an mRNA vaccine to protect people against flu strains with pandemic potential, including the H5N1 bird flu virus that’s been raising fears.