The Pan American Health Organization has struck a deal with the Chinese manufacturer Sinovac to buy millions of Covid-19 vaccines for countries in Latin America and the Caribbean as part of an effort to make more shots available in a region where access has been highly unequal.

The agency, part of the World Health Organization, is negotiating with two other manufacturers and expecting to announce new deals soon, Dr. Jarbas Barbosa, its assistant director, said at a news conference on Wednesday.

Sinovac has offered to sell 8.5 million doses this year and an additional 80 million next year, he said. Countries in the region that want the vaccine will have to buy it from the health organization.

“This is a purchase — it isn’t a donation,” Dr. Barbosa said, noting that the Inter-American Development Bank was offering loans to countries that need them.

The direct purchases begin at a time when, on average, only 35 percent of people in Latin America and the Caribbean have been fully vaccinated against Covid-19, said Dr. Carissa F. Etienne, the agency’s director.

And that coverage has been unequal. While some countries, including Chile and Uruguay, have fully vaccinated over 70 percent of their populations, she said, others have yet to reach the 20 percent mark. Countries on the lower end of inoculation rates include the Bahamas, Grenada, Guatemala, Haiti, Honduras, Jamaica, Nicaragua, St. Lucia, St. Vincent and the Grenadines, and Venezuela. Haiti is a particularly dire case, she said, with less than 1 percent of the population fully vaccinated.

The health organization, which is also working to expand vaccine manufacturing in the region, announced last week that a facility in Brazil and another in Argentina would receive technical support to begin production of messenger RNA vaccines, the type used in the Pfizer-BioNTech and Moderna shots.

But for the time being, Dr. Etienne said, “vaccine donations remain the fastest way to support countries in our region.”