The CDC expects that samples from four other people in the United States will test positive for monkeypox virus, which would bring the U.S. total to five cases amid a widening outbreak of the disease in countries where it is not endemic.

Jennifer McQuiston, DVM, MS, deputy director of the CDC’s Division of High Consequence Pathogens and Pathology, said in a telebriefing Monday that four U.S. men have tested positive for orthopoxvirus, a genus of viruses that includes monkeypox virus.

States can test samples for orthopoxviruses but confirmatory testing for monkeypox virus must be done at the CDC, McQuiston explained. She said the CDC presumes that all four men — two in Utah and one each in New York City and Florida — have monkeypox.

“It’s likely that there are going to be additional cases reported in the United States,” McQuiston said.

The CDC last week confirmed a case of monkeypox in a Massachusetts man. The evolving outbreak has also included cases in Canada, Australia and numerous countries in Europe, most of them mild.

Monkeypox is rare outside of a handful of African countries where it is endemic, including Nigeria and the Democratic Republic of the Congo. There were two reported cases in the U.S. last year — both linked to travel from Nigeria, where an outbreak that has been ongoing for years has shown evidence of human-to-human transmission.

McQuiston said the CDC was able to quickly sequence a sample from the Massachusetts case and that it closely matched sequencing from Portugal.

A vaccine that protects against both monkeypox and smallpox — Bavarian Nordic’s Jynneos — was approved by the FDA in 2019 for adults and is kept in the U.S. Strategic National Stockpile.