ALL EYES are on Denmark, which subjects a large share of its coronavirus cases to genomic surveillance. That window into the pandemic shows the virus variant B.1.1.7, which is far more transmissible, has moved rapidly through the Danish population, as it did in Britain. A new study, though preliminary, suggests the same is happening in the United States; the number of people infected with it is doubling about every 10 days. Every effort must be made to use all known countermeasures, including face masks, distancing, good hygiene and shutdowns. But only vaccines can save the day — if administered in time.

The variant, first seen in Britain, is significantly more transmissible than the older variants. In the United States, if unimpeded, it could drive daily new case counts — now declining to about 107,000 — back to the January peaks of more than 200,000. Hospitalizations and deaths would rise, too. In Denmark, genomic surveillance shows the new variant had a reproduction number of 1.07 — spreading fast, while the old variant was 0.78, indicating decline. One Danish expert told Kai Kupferschmidt of Science magazine, “This is the calm before the storm.”

The new variant has shown a tendency to keep spreading even during lockdowns. Denmark had already closed schools and restaurants, but rules were tightened by cutting the number of people allowed to gather from 10 to five, among other things. In the United States, stricter lockdowns may be politically difficult, given pent-up fatigue and frustration.

One approach to speed up vaccination would be to temporarily delay the planned second dose for the two vaccines and use those doses to get as many people first shots as possible before the variants hit. The Food and Drug Administration’s emergency use authorization is based on a second dose of the Pfizer vaccine in 21 days and Moderna’s in 28 days . For now, no compromise should be made on giving the second dose. But the government should study as quickly as possible whether second doses could be administered later without loss of efficacy. In the meantime, the new variant s , including those first identified in Brazil and South Africa as well as B.1.1.7, only increase the urgency of the vaccine rollout now underway.