Dr. Andersen and other virus watchers still see variants as a potential source of trouble in the months to come — particularly one that has battered Brazil and is growing rapidly in 17 U.S. states. But they are also taking stock of the past few months to better understand how the nation dodged the variant threat. Experts point to a combination of factors — masks, social distancing and other restrictions, and perhaps a seasonal wane of infections — that bought crucial time for tens of millions of Americans to get vaccinated. They also credit a good dose of serendipity, as B.1.1.7, unlike some of its competitors, is powerless against the vaccines. “I think we got lucky, to be honest,” said Nathan Grubaugh, an epidemiologist at Yale University. “We’re being rescued by the vaccine.”