While wealthier nations have stockpiled vaccines for their citizens, many poorer countries are still scrambling to secure doses. A few, like Chad, have yet to receive any.

The World Health Organization says nearly a dozen countries — many of them in Africa — are still waiting to get vaccines. Those last in line on the continent along with Chad are Burkina Faso, Burundi, Eritrea and Tanzania.

As a result, the U.S. government laid claim not only to vast quantities of finished COVID-19 vaccines but also to vaccine components and equipment all along the supply chain, according to a Reuters review of more than a dozen contracts involving some major suppliers.

“Delays and shortages of vaccine supplies are driving African countries to slip further behind the rest of the world in the COVID-19 vaccine rollout and the continent now accounts for only 1% of the vaccines administered worldwide,” WHO warned Thursday.

And in places where there are no vaccines, there’s also the chance that new and concerning variants could emerge, said Gian Gandhi, UNICEF’s COVAX coordinator for Supply Division.

“So we should all be concerned about any lack of coverage anywhere in the world,” Gandhi said, urging higher-income countries to donate doses to the nations that are still waiting.

COVAX, the U.N.-backed program to ship COVID-19 vaccines worldwide, is aimed at helping low- and middle-income countries get access. A few of the countries, though, including Chad, have expressed concerns about receiving the AstraZeneca vaccine through COVAX for fear it might not protect as well against a variant first seen in South Africa.

Chad is expected to get some Pfizer doses next month if it can put in place the cold storage facilities needed to keep that vaccine safe in a country where temperatures soar each day to 43.5 degrees Celsius (110 degrees Fahrenheit).

Some of the last countries also took more time to meet the requirements for receiving doses, including signing indemnity waivers with manufacturers and having distribution plans in place.