US President Joe Biden will announce on Tuesday the donation of 1 million mpox vaccine doses and at least $500 million to African countries to support their response to the outbreak, according to a senior administration official.

Biden will make the announcement in New York, where he is attending the United Nations General Assembly, and call on other countries to follow suit, the person told Reuters.

In August, the World Health Organization declared mpox a global public health emergency for the second time in two years, following an outbreak of the viral infection in Democratic Republic of Congo that has spread to neighboring countries and beyond, including India, raising alarm.

"It wouldn't be a huge surprise if there were a case in the United States, either," the official said, noting that recent cuts in pandemic support by Congress may hamper a domestic response.

"We're really squeezing right now to be able to respond effectively in this moment. And I think anticipating potentially additional cuts in the future will make it more and more difficult for us to have a robust response to these types of outbreaks."

Mpox can spread through close contact. Usually mild, it is fatal in rare cases. It causes flu-like symptoms and pus-filled lesions on the body.

The outbreak in Congo began with the spread of an endemic strain, known as clade I. But a new variant, clade Ib, appears to spread more easily through routine close contact, including sexual contact.

The expectation is that the U.S.-donated doses will be of the Bavarian Nordic vaccine known as Jynneos in the United States, and many will come from a stockpile in the United States. The 1 million doses is in addition to the 60,000 previously donated shots.

The U.S. official said the administration expects Gavi, a public-private alliance which co-funds vaccine purchases for low-income countries, to distribute the donated vaccines.

"We feel really strongly that Gavi is the multilateral institution that is best positioned to procure and equitably distribute vaccines," the official said.

Last week, Gavi said it will buy 500,000 doses of Bavarian Nordic's vaccine, its first purchase to help battle the outbreak.

The Biden administration is also pushing for more vaccines to be manufactured in low and middle-income countries, and is working with Brazil to identify how the Group of 20 club of nations can support the mpox response.

There is not currently a manufacturer in Africa that is able to make the complex drug substance needed to make the vaccine, the U.S. official said.