England’s population enthusiastically met the re-opening of pubs, restaurants and non-essential retailers this week as hairdressers extended hours to meet the demand and consumers flocked to London’s Oxford Street.

The partial return to normality is the the first step of a balancing act that many hope heralds the end of lockdown stops and restarts for the U.K., which has the highest virus death toll in Europe but has now vaccinated almost half of its population with at least one dose.

Scientists are still cautious. Many say the country is far from reaching the herd immunity it needs to open fully with confidence. University College London researchers who estimated three-quarters of the population now have immunity have faced heavy criticism from peers for their modeling.

Unlike nations like Taiwan and New Zealand that have reopened after bringing new case numbers to about zero, the U.K.’s step comes as thousands of people per day are still testing positive for the virus and the government weighs allowing its population to travel overseas this summer.

That means the vaccines will have to work.

Viruses mutate faster the more they circulate, so reopening could produce variants which may be more resistant to vaccines. The U.K. bet early and heavily on AstraZeneca Plc’s shot, which the company’s research says is 100% effective against severe disease and hospitalization after two doses. Concern over clots doesn’t appear to have dented confidence in the country’s vaccine drive, according to one poll, as it has in mainland Europe.

Boris Johnson’s government plans to lift all restrictions by June 21. With nightclubs open, just 35% of the population will be fully vaccinated by then, according to projections from London-based research firm Airfinity. New vaccines will likely be needed: AstraZeneca is one of several developers preparing a version to fight new variants in time for the fall.