The top U.S. drug regulator is resisting calls to tinker with how COVID-19 vaccines are administered, sticking to the scientific evidence and its own procedures after being more flexible in the face of outside pressure earlier in the pandemic.

There have been increasing calls worldwide to experiment with dosing regimens to speed up inoculation efforts and arrest a worsening pandemic. However, the Food and Drug Administration’s leadership pledged in a statement late Monday “to make decisions based on data and science.”

Extending the length of time between shots, or cutting doses by half, hasn’t been studied in clinical trials and could put “public health at risk,” said FDA Commissioner Stephen Hahn and Peter Marks, director of the agency’s office that oversees vaccines, in the statement.

While the FDA has been criticized for its earlier stand on treatments touted by President Donald Trump, its strong statement this week is emblematic of how the agency has handled vaccine reviews, demanding safety data that rejected Trump’s push for a vaccine by the November election and insisting on a public airing of that data while Trump clamored for authorization.

“The FDA has acquitted itself really well with vaccines,” said Jason Schwartz, an assistant professor in the Department of Health Policy and Management at the Yale School of Public Health. “There’s a clear lesson learned.”

While Trump administration officials had forecast that 20 million people would get shots by the end of 2020, just 4.73 million were delivered as of Tuesday, according to Bloomberg’s vaccine tracker. That shortfall has spurred speculation by some researchers, politicians and academics on how to get more shots into arms more quickly.

On Sunday, Moncef Slaoui, Operation Warp Speed’s scientific adviser, said the U.S. government is considering cutting doses of the Moderna Inc. shot given to those age 18 to 55 by half. He said there is evidence showing the half-dose provides the same level of protection for that age group, but added that such a decision would be in the FDA’s purview.

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