A few weeks ago, doctors in Michigan hospitals began noticing their intensive care units were filling up again with coronavirus patients — something they had hoped the state’s high level of vaccinations would have prevented. Unlike in earlier waves of the virus, the patients who were being admitted were largely young adults, laid low by a disease many of them thought would not cause them serious problems.

The state is now the epicentre of a regional surge in infections which experts warn could become yet another national wave if not contained. Doctors say it is also a warning to the rest of the country not to relax restrictions before a larger section of the population has been vaccinated, especially as new virus variants become dominant.

“If you are in Michigan right now, the next wave has already started,” Dr Walid Gellad, associate professor of medicine at the University of Pittsburgh, said. “Hospitalisations are rising at the same rate they did before and it is very concerning.” Covid-19 infections have been dropping broadly across the US since their January peak as the weather has got warmer, encouraging people outside, and the country’s vaccination programme has accelerated. But public health officials havewarned in recent weeks that there could be another wave of the disease before the country reaches socalled “herd immunity” — the point where so many people have antibodies that the virus cannot easily spread.