The U.S. Food and Drug Administration is warning health care workers that any changes to the authorized dosing schedules of COVID-19 vaccines currently being administered will significantly place public health at risk and undermine “the historic vaccination effort to protect the population” from the coronavirus pandemic.

The first and second doses of the Pfizer-BioNTech vaccine are approved to be delivered within a 21-day window, while the Moderna injections should be spread over 28 days. When given at those intervals, both vaccines are about 95% effective, according to the respective drugmakers.

But the FDA has heard suggestions that the number of limited doses could be stretched by cutting them in half, extending the length of time between doses, or mixing and matching vaccines in order to immunize more people against COVID-19.


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