The second threat is of a possible new variant that is resistant to the immunity conferred by existing vaccines and past infections, which could trigger a new cycle of restrictions and require a new round of vaccinations.

If such a variant emerges, manufacturers think vaccines can be updated relatively quickly. Nonetheless, James Stock, an economist at Harvard University who has studied the virus and the economy, warned of a “scenario of this thing sticking around for a much longer time frame.’’

“All the economic adaptations have been viewed as temporary: restaurants at 25% capacity and so forth,” he said. “If this were to become chronic there would have to be really massive reorganizations” of the U.S. economy.