The Trump administration has delivered its latest blow to vaccines, canceling a nearly $600 million contract to the drugmaker Moderna that was intended to develop a shot for humans against bird flu.

The decision also forfeited the U.S. government’s right to purchase doses before a pandemic, and canceled an agreement set up by the Biden administration in January to prepare the nation for a potential bird flu pandemic. The Moderna contract built on a previous government investment of $175 million last year.

The move was not entirely unexpected. The Department of Health and Human Services said earlier this year that it was reviewing the contract. And Health Secretary Robert F. Kennedy Jr. has repeatedly questioned the safety of mRNA technology, which is used in Moderna’s COVID vaccine.

First used for the COVID vaccines made by Pfizer-BioNTech and Moderna, mRNA shots instruct the body to produce a fragment of the virus, which then sets off the body’s immune response.

Andrew Nixon, a Health and Human Services spokesperson, said: “After a rigorous review, we concluded that continued investment in Moderna’s H5N1 mRNA vaccine was not scientifically or ethically justifiable.”

Moderna said it would explore alternatives for developing the vaccines covered by the contract, which were to be designed for several types of flu viruses that have the potential to cause a pandemic.

Christopher Ridley, a spokesperson for Moderna, defended the mRNA technology. “Results during the pandemic speak for themselves, including demonstrated efficacy and a safety profile established in over a billion people worldwide,” he said in a statement Thursday.

For several years, a type of avian flu known as H5N1 has circulated around the world, killing wild birds and domestic flocks, and spreading to a range of other species including bears and sea mammals.

It arrived in the United States in 2022, and has resulted in the culling of more than 173 million birds, frequently devastating commercial poultry flocks.Last year, bird flu also spread to dairy cattle. It has since struck more than 1,000 herds in 17 states and sickened 70 people, most of them dairy or cattle workers. In January, Louisiana reported the death of an older adult who had interacted with sick backyard birds, the first such fatality in the United States.

So far, the virus does not seem to spread easily among people. But scientists have long worried about a bird flu pandemic because flu viruses can rapidly mutate and acquire new abilities.

The national stockpile holds a few million doses of an existing H5N1 vaccine to protect humans. But it is unclear whether the shots would continue to protect Americans if the virus were to change significantly. The government has three other avian flu contracts, according to the health department.

Many scientists regard mRNA vaccines, which can be quickly altered to match the newest versions of virus, as the best option for protecting Americans in a fast-moving outbreak.

“When the next flu pandemic occurs, there is not going to be enough vaccine for everyone who wants it unless we invest to broaden the types of flu vaccines being made and the number of companies that make them,” said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health.

“We shouldn’t let politically motivated attempts to unfairly brand mRNA vaccines as dangerous stand in the way of ensuring everyone who wants a pandemic vaccine can get one,” she said.