Younger Americans should return to workplaces, schools, shops and restaurants, while older Americans would remain cloistered from the virus as it spreads, receiving such services as grocery deliveries and medical care.

Eventually so many younger Americans will have been exposed, and presumably will have developed some immunity, that the virus will not be able to maintain its hold on the communities, the declaration contends.

But it does not offer details on how the strategy would work in practice. Dr. Anthony Fauci, the government’s top infectious disease expert, has dismissed the declaration as unscientific, dangerous and “total nonsense.” Others have called it unethical, particularly for multigenerational families and communities of color.

Alarmed and angry, 80 experts on Wednesday published a manifesto of their own, the John Snow Memorandum (named after a legendary epidemiologist), saying that the declaration’s approach would endanger Americans who have underlying conditions that put them at high risk from severe Covid-19 — at least one-third of U.S. citizens, by most estimates — and result in perhaps a half-million deaths.