Covax will retain an option to purchase another 466 million doses in 2022, according to Moderna and Gavi, one of the organizations behind the vaccine aid program.

The announcement comes as the pandemic accelerates in developing countries such as India and Brazil, which have been reporting record numbers of deaths amid a surge of coronavirus infections that has overwhelmed their healthcare systems.

While countries such as the U.S., Britain and Israel are coming closer to vaccinating large majorities of their populations, developing nations are struggling to get their rollouts off the ground, with vaccine manufacturers focused on richer markets that have preordered the bulk of their supply.

On current trends, emerging economies will have only vaccinated 30% of their populations by the end of the year, the investment bank

UBS

said last month.

“We recognize that many countries have limited resources to access Covid-19 vaccines,” said Moderna Chief Executive

Stéphane Bancel.

“We support Covax’s mission to ensure broad, affordable and equitable access to Covid-19 vaccines,” he said.

Until last week, Covax had only provided about 50 million doses out of its goal of delivering two billion doses by the end of this year, enough to vaccinate around 20% of the receiving countries’ populations with a two-shot regimen. Covax said earlier this year that it expects to ship 145 million vaccines by the end of May.

Gavi said last month that it had agreements to get 340 million doses of the Oxford-

AstraZeneca

PLC vaccine and about 1.2 million doses of shots made by

Pfizer Inc.

and BioNTech SE.

The expected supply of 34 million doses of Moderna’s vaccine later in 2021 would help Covax achieve its goal for the year, but would make up a relatively small portion of the total. Moderna’s contribution could be more significant in 2022, when Covax has an option for the additional 466 million doses.

Moderna, like Pfizer-BioNTech, supplies a vaccine based on what is known as messenger RNA technology.

“We are very pleased to sign this new agreement with Moderna, giving Covax Facility participants access to yet another highly efficacious vaccine,” said Gavi’s CEO, Dr.

Seth Berkley.

The WHO issued last week an emergency-use authorization for Moderna’s Covid-19 vaccine for people aged 18 and older.

—Peter Loftus contributed to this article.

Write to Bojan Pancevski at bojan.pancevski@wsj.com