But the vaccinations of older people do not explain rising hospitalizations among people younger than 60, including those in their 20s and 30s. Public health experts say the outbreak — driven by the B.1.1.7 variant of the virus, which is more contagious and more severe — is spreading rapidly in younger age groups. And across the state, doctors and nurses are increasingly reporting a concerning trend: Younger patients are coming in more often with serious cases of Covid-19. “I am putting more patients in their 20s and 30s and 40s on oxygen and on life support than at any other time in this pandemic,” said Dr. Erin Brennan, an emergency room physician in Detroit... With steep reductions in cases in much of the country, and with fewer than half as many people hospitalized nationwide as at the winter peak, some health experts said it was conceivable that more younger people were being hospitalized because some hospitals had lowered their standards for admission. “That’s one explanation that you have to think about,” said Dr. Amesh Adalja, an infectious-disease expert and hospital doctor in the Pittsburgh area, who said he sent borderline cases to be treated at home when hospitals were full. “I have a lower threshold to admit when I’m not worried about hospital capacity.”