Getting as many people fully vaccinated as possible is the best defense against the evolution of Covid-19 variants, Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said Monday at a White House coronavirus briefing.

Because vaccine demand is greater than supply, the question has arisen in some quarters about whether the government should focus its efforts on providing the first of two necessary vaccine doses to as many people as possible, “with less emphasis on the second dose.”

Fauci said that the first dose of both available vaccines, from Pfizer and Moderna, provides some protection from the coronavirus, but the second shot, administered 21 to 28 days later, boosts the protection level tenfold higher.

That added level is important not only for the potency of the protection, Fauci said, but also because it provides a “greater breadth of response” covering for currently circulating virus variants as well.

“The other theoretical issue that could be problematic with regard to only a single dose is that if you get a sub-optimum response, the way viruses respond to pressure, you could actually be inadvertently selecting for more mutants by a suboptimal response,” he said.

“So for that reason, we have continued to go by the fact that we feel the optimum approach would be to continue with getting as many people on their first dose as possible but also making sure that people, on-time, get their second dose.”

Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said that 699 variant cases have been confirmed across 34 states as of Sunday, and 690 of those cases are the B.1.1.7 variant first detected in the United Kingdom.

Fauci said that variant could be the dominant version present in the US by the end of March, and that the Moderna and Pfizer vaccines are “quite effective against the 1.1.7 lineage.” He added that those vaccines are not as effective against a variant first identified in South Africa, “but hopefully we will get the virus under much better control by the time that there is any indication that that might become dominant.”

Continued proliferation of variants “is a threat that could reverse the recent positive trends we are seeing,” Walensky said, adding that it remains important to wear masks, stay six feet away from other people, and avoid poorly ventilated spaces.

“I recognize that the pandemic has taken an enormous toll on all of us,” she said. “But if we all work together and take these prevention steps, we can finally turn the tide.”

In Massachusetts, Monday marked the first day that businesses, including stores, restaurants and fitness centers, were allowed to operate at 40 percent capacity, after being capped at 25 percent since Dec. 26. The state has a mask order in place, and social distancing and other specific public health measures are required across an array of sectors. Bars, indoor performance venues and indoor recreation facilities like roller rinks remain closed.