The U.S. is to ship nearly 10 million shots to Nigeria and South Africa.

The United States is ramping up vaccine deliveries to Africa as a third wave of the pandemic continues to accelerate across the continent.

On Wednesday, Washington was scheduled to ship almost 10 million Covid-19 vaccines to two of Africa’s most populous nations, with 5.6 million Pfizer doses going to South Africa and four million Moderna doses to Nigeria. The deliveries are part of a pledge President Biden made in June to share 80 million doses globally — with about 25 million doses expected to arrive in 49 African states.

Over the past two weeks, Covax, the global vaccine partnership, has, in collaboration with the African Union, delivered millions of Johnson & Johnson doses from the United States to countries including Cameroon, the Central African Republic, Djibouti, Gambia, Lesotho, Niger, Tunisia, and Zambia. The latest shipments to Nigeria and South Africa brings the total number of vaccines donated to 16.4 million doses.

The donations from the United States come as Africa continues to lag behind the rest of the world in Covid vaccination. Only about 21 million of the continent’s 1.3 billion people have been fully vaccinated, according to the World Health Organization, with 77 percent of all the doses received already administered. At current rates, almost two-thirds of African countries will not reach a W.H.O. target of vaccinating 10 percent of their populations by the end of September.

Vaccine availability in Africa has been hampered because wealthy nations have bought excess doses, and was set back further with India’s decision in March to cut back on vaccine exports, particularly the supplies from the Serum Institute of India that Covax had been relying on. Because of those issues, the African continent will most likely not be able to meet the slightly longer-term goal of vaccinating 20 percent of the population by the end of 2022.

The continent is experiencing vaccine shortages even as the severe third wave overwhelms health care systems and pushes countries to institute lockdowns and extend overnight curfews. The current surge in cases has been attributed to a lack of inoculation; loose compliance with public health measures, such as mask wearing and social distancing; and the spread of more contagious variants. More than 20 African countries have seen cases rise by more than 20 percent for at least two weeks, according to the W.H.O., with the Delta variant reported in 26 countries.

The W.H.O. has said that political crises in several countries threaten to undermine efforts to vaccinate people and curb the virus. That includes Ethiopia, where the conflict in Tigray is set to intensify, and Eswatini, where deadly antigovernment protests broke out this month. In South Africa, the looting and killings that followed the imprisonment of former President Jacob Zuma have had a negative impact on vaccination efforts in the eastern province of KwaZulu-Natal, Dr. Matshidiso Moeti, the health organization’s Africa director, has said.

Both the W.H.O. and the Africa Centers for Disease Control and Prevention have said that vaccine deliveries will continue to ramp up. Besides the United States, millions of doses from the European Union are expected to arrive in the coming weeks. And Britain said it would start delivering nine million doses of the Oxford-AstraZeneca vaccine this week to countries including Kenya. China also raised its vaccine pledges to Africa this month, sending Sinovac doses to nations including Tanzania and Uganda. Covax has said that it will deliver over 500 million vaccines to Africa by the end of the year.

Vaccines are now reaching countries like Tanzania, which previously made no effort to secure doses and whose former president played down the pandemic and pronounced that God helped eliminate the virus.

On Saturday, Tanzania received over one million Johnson & Johnson doses from the United States. On Wednesday morning, President Samia Suluhu Hassan received her Covid-19 vaccine, beginning the country’s vaccination campaign.

During the ceremony, Ms. Hassan assured the public of the safety of the shots and urged those vaccinated to continue following public health measures.

“I have agreed to be vaccinated today,” Ms. Hassan said, “just as my body has been vaccinated a lot since childhood.”