Governments engaged in a desperate race to roll out vaccines to beat back a winter surge of the coronavirus have turned to a new tactic: freeing up scarce vaccine supplies so they can inoculate more vulnerable people faster with a single shot by delaying the second dose.

The first to act was the U.K., which is dealing with new Covid-19 infections propelled by a fast-spreading variant of the virus. Its plan to expand vaccine supplies by delaying the second dose has been followed elsewhere in Europe and part of Canada. The U.K.’s staterun National Health Service will wait up to three months to deliver the second shot instead of the three to four weeks recommended based on drug-company trials.

The moves come as many countries are struggling to ramp up vaccination programs. France has inoculated only a few thousand people, while Belgium began its mass-vaccination program on Tuesday and the Netherlands on Wednesday. Even in places like the U.K., Germany and the U.S. where more people have been vaccinated, vaccination efforts are falling behind a winter wave of infections that threatens to overwhelm some hospitals.

The approach has prompted pushback by some medical authorities, including the U.S. Food and Drug Administration, which this week called delaying the second dose potentially detrimental to public health. However, other U.S. health officials have said they are looking into options such as tinkering with dosage sizes and other ways to stretch the reach of available vaccines. Elsewhere in Europe, some authorities are leaning toward the British approach.

Denmark on Monday approved a delay of up to six weeks between doses while Germany is also considering stretching out the interval. Belgium has also shifted gears, using all available vaccine for first doses, hoping to administer second doses in three to six weeks.

In Canada, Quebec said in late December that it would delay second doses of the vaccine developed by Pfizer Inc. and Germany’s BioNTech SE to immunize more people with one dose. But Canada’s national public-health agency has resisted requests to consider turning an existing two-dose vaccine by Moderna Inc. into a single-dose shot. Canadian officials said data are too limited.

British medical authorities calculated that giving more at-risk people some immunity with one vaccine dose would save more lives than conferring more complete immunity to half the number with a double dose.