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US government scientists are pushing back against calls for one-dose regimens for the COVID-19 vaccines by Moderna and Pfizer, saying there isn’t enough evidence that a single dose provides long-term protection, The Wall Street Journal reported on Saturday.

“It is essential that these vaccines be used as authorized by FDA in order to prevent COVID-19 and related hospitalizations and death,” Peter Marks, director of the Food and Drug Administration’s center that oversees vaccines, told The Wall Street Journal.

The FDA late last year approved a two-dose regimen for the Moderna and Pfizer vaccines and more recently approved use of a one-dose regimen for a vaccine from Johnson & Johnson.

While some scientists and lawmakers have called for shifting to a one-dose regimen for all the vaccines, senior government scientists at the FDA and the National Institutes of Health said such a shift isn’t warranted, adding the evidence used to approve the Pfizer and Moderna vaccines was based on two doses.

These scientists said one dose may offer short-term protection, but the longer-term protection is a question mark.

Asked for comment Saturday, Moderna in a statement said that “in ongoing clinical studies and assessment of real-world evidence, we do and will look at lots of secondary matters, including potentially this question” of a one-dose regimen.

Pfizer didn’t respond to a request for comment. The company has previously said it doesn’t have data for the single-dose approach.