GENEVA (AP) — A U.N.-backed program to deploy COVID-19 vaccines to the neediest people worldwide, especially in poor countries, announced plans Wednesday for an initial distribution of some 100 million doses by the end of March and more than 200 million more by the end of June.

The COVAX Facility, which is seeking the fair distribution of vaccines at a time of short supplies, said that nearly all of the doses expected for the first phase will come from British-Swedish drugmaker AstraZeneca and its partner, the Serum Institute of India.

Dr. Seth Berkley, the CEO of GAVI, the Vaccine Alliance, said COVAX plans for the initial distribution of 336 million doses of the vaccine, which AstraZeneca developed with Oxford University, through June to dozens of countries.

GAVI expects that nearly one-third of those doses — more than 100 million — will start being delivered to targeted countries by the end of March, officials said.

Another 1.2 million doses of the vaccine from U.S.-based Pfizer and German partner BioNTech are expected to be shared by 18 countries during the first quarter of the year.

The AstraZeneca vaccine rollout will be contingent on the World Health Organization authorizing the shot for emergency use, which is expected to happen this month. The Pfizer-BioNTech vaccine already has received such approval, but supplying it to poorer nations is a challenge because the vaccine requires storage at extremely cold temperatures.

Such rollouts are contingent on regulatory approvals and countries’ readiness to receive the vaccines.

Some 190 countries and territories that are participating in COVAX have been awaiting details of the rollout. The participants include “self-financing” upper- and middle-income countries that have put up money and 93 lower-income countries which are expected to benefit.

WHO officials have consistently said the way to beat the pandemic is to make sure that everyone is safe from it — not just those in wealthier countries that launched vaccination drives in December.

COVAX has faced challenges as rich countries have scooped up vaccine supplies, sometimes at premium prices, and undercut WHO’s goal of equitable vaccine distribution. Program leaders have faced issues trying to strike deals with pharmaceutical manufacturers, and only a fraction of the 2 billion doses that have been secured for COVAX involve firm deals.

The 18 countries set to receive the Pfizer vaccine are Bhutan, Bolivia, Bosnia, Cabo Verde, Colombia, El Salvador, Georgia, Maldives, Moldova, Mongolia, Peru, Philippines, South Korea, Rwanda, South Africa, Tunisia, and Ukraine, as well as the Palestinian territories.

COVAX has raised over $6 billion of the $8 billion needed to cover procurement and distribution to the poorest countries, including a $4 billion commitment from the U.S. Congress in December. The European Union and its members have committed about $800 million and Britain has added more than $700 million, GAVI officials said.

GAVI the Vaccine Alliance is leading COVAX along with the Coalition for Epidemic Preparedness Innovation and WHO.