Since then, however, that average has fallen more precipitously in Sweden—by 99 percent since April 16, compared to 65 percent in the United States since April 21. The seven-day average of newly confirmed cases also has dropped sharply in Sweden, by nearly 80 percent since late June.

In the United States during the same period, daily new cases initially rose, an ascent that started a month and a half after states began lifting their lockdowns. The seven-day average peaked in late July and has since fallen by 46 percent.

Achieving herd immunity, which protects people in high-risk groups by making it less likely that they will encounter carriers, was never an official goal of Sweden’s policy. But recent trends are consistent with the hypothesis that Sweden has achieved some measure of herd immunity through a combination of exposure to the COVID-19 virus, T-cell response fostered by prior exposure to other coronaviruses, and greater natural resistance among the remaining uninfected population.

In the United States, meanwhile, lockdowns, despite the huge costs they entailed, have not had any obvious payoff in terms of fewer COVID-19 deaths, although they may have changed the timing of those deaths.