Vincent Munster, who leads the team that conducted that study, said a vaccine that could mitigate the severity of the Covid-19 pandemic would still be a significant contribution in a world struggling to co-exist with a dangerous new virus.

“If we push the disease from pneumonia to a common cold, then I think that’s a huge step forward,” said Munster, chief of the virus ecology unit at the National Institute of Allergy and Infectious Diseases’ Rocky Mountain Laboratories in Hamilton, Mont.

The rush to develop vaccines means that ideal solutions may be out of reach in the immediate term; Munster said he anticipates seeing second-generation vaccines that could be more protective. Other scientists, though, are cautious about how much the world can expect from vaccines against this pathogen.

Michael Mina, an infectious diseases epidemiologist at Harvard’s T.H. Chan School of Public Health, thinks achieving sterilizing immunity with a vaccine will not be possible for Covid-19. Experience with human coronaviruses — and with multiple pathogens that cause colds — shows immunity that develops after infection with respiratory tract infections is not lifelong. In some cases, the duration is measured in months, not years.