But many African governments pursued swift, severe lockdowns that — while financially ruinous, especially for their poorest citizens — slowed the rate of infection. Some deployed networks of community health workers. The Africa C.D.C., the W.H.O. and other agencies helped expand testing and moved in protective gear, medical equipment and pharmaceuticals.

The reported toll of the pandemic on the continent — 2.6 million cases and 61,000 deaths, according to the Africa C.D.C. — is lower than what the United States alone currently experiences in three weeks.

But that accounting is almost certainly incomplete. Evidence is growing that many cases were missed, according to an analysis of new studies, visits to nearly a dozen medical institutions and interviews with more than 100 public health officials, scientists, government leaders and medical providers on the continent.

“It is possible and very likely that the rate of exposure is much more than what has been reported,” Dr. Nkengasong said in an interview.