Africa’s C.D.C. director urges wealthy nations to forego Covid vaccine boosters and donate them instead.

The decision by some rich nations to offer booster shots will hinder coronavirus vaccine access for low-income countries, the director of the Africa Centers for Disease Control and Prevention said on Thursday, arguing there is no conclusive evidence healthy people who are not immunocompromised need an extra shot.

In wealthy countries — including Germany, France, Israel and the United States — there has been growing momentum to offer additional doses to certain vulnerable populations, including older citizens, and to the general public.

“The problem we have with the third doses is that we have not seen enough science behind them,” the director, Dr. John Nkengasong, said in an online news conference with journalists on Thursday. “It is really still confusing to me as to why we are moving toward a vast recommendation for a booster dose.”

By offering booster shots, he added, “we will surely be gambling.”

The World Health Organization has warned that booster shots could divert vaccine supplies from countries with largely unvaccinated populations. On Wednesday, the agency asked wealthy countries to hold off on administering booster shots for healthy patients until at least the end of the year as a way of enabling every country to vaccinate at least 40 percent of their populations.

W.H.O. officials have tried to distinguish between booster shots that increase immunity in already vaccinated populations, and additional doses that may be needed by the immunocompromised to develop immunity in the first place. Officials are not opposed to additional doses for the immunocompromised.

Despite the flurry of booster programs in wealthier nations, the science of whether they are needed is not yet clear.

Some studies suggest that the protection that the vaccines provide against infection and mild disease may be waning. But they remain highly effective at preventing the worst outcomes, including severe disease and death, and scientists have said that a blanket recommendation for boosters is premature.

Experts generally agree, however, that a third shot is warranted for people with compromised immune systems, who may not have mounted a strong immune response to the initial doses. Several countries, including the United States, are now offering additional shots to this vulnerable group.

Dr. Nkengasong’s comments came as the W.H.O.’s Africa director, Dr. Matshidiso Moeti, said on Thursday that the continent will receive fewer Covid-19 vaccine doses than expected for the rest of the year from Covax, the global immunization program.

On Wednesday, Covax slashed its forecast for doses available in 2021 by roughly a quarter, another setback for an effort that has been hampered by production problems, export bans and vaccine hoarding by wealthy nations. Dr. Moeti said the fewer doses were “in part because of the prioritization of bilateral deals over international solidarity.”

Around 3 percent of Africa’s population, or 39 million of 1.3 billion residents, has been fully vaccinated, and 72 percent of all doses received have been administered, according to the W.H.O.

The African continent is coming off a severe third wave of the pandemic, driven largely by the Delta variant.

The continent has so far reported 7.9 million cases and over 200,000 deaths from the virus as of Thursday, according to the Africa C.D.C.

Dr. Nkengasong said wealthy nations should first come through on their commitments to donate hundreds of millions of doses, so as to help end the acute phase of the pandemic.

Dr. Moeti said those donated doses were not only the clearest pathway out of the pandemic but would help alleviate already strained health care systems. The Democratic Republic of Congo, for example, is dealing with a meningitis outbreak and the risk of the resurgence of deadly diseases like measles.

“If producing countries and companies prioritize vaccine equity, this pandemic can be over quickly,” Dr. Moeti said.

Emily Anthes contributed reporting.