The additional 36,000 doses, from Danish drugmaker Bavarian Nordic, should arrive in the near future, CNBC reported Monday, citing the U.S. Department of Health and Human Services. The company is holding more than 1 million doses for the U.S. and can provide more than 16 million additional vaccines if and when the federal government requests them.

Nordic announced last month that it had contracted with the U.S. to manufacture a freeze-dried version of the Jynneos vaccine, with the option to convert the country’s existing vaccine stash into approximately 13 million freeze-dried doses. The contract is worth up to $299 million. The first doses won’t be available until 2023.

The vaccine is already being sent to those at high risk for exposure, such as health care workers, and more than 1,000 doses have been distributed, Dr. Raj Panjabi, the head of the White House’s pandemic efforts, told reporters on a June 3 media call.

The U.S. has 20 confirmed cases of monkeypox, with an additional case in an American diagnosed and treated elsewhere, federal health officials said last week. As of June 2, nearly 800 confirmed cases had been reported to the World Health Organization in 27 countries.

Multiple options to treat poxviruses

Smallpox vaccine Jynneos is approved to treat monkeypox in adults because of evidence that animal-transmitted pox viruses like monkeypox and rabbitpox “typically cross-react and provide protection against other pox viruses,”  Dr. Alexandra Brugler Yonts, an infectious disease specialist at Children’s National Hospital in Washington, D.C., told Fortune last month. She assisted in the FDA’s review of the vaccine.

Other smallpox vaccines were used to prevent monkeypox transmission in outbreaks before the development of Jynneos, though they weren’t approved for such a purpose, she added.

The U.S. also has more than 100 million doses of an older generation smallpox vaccine that can be used to treat monkeypox called ACAM2000, CNBC reported, citing HHS officials. That vaccine, however, uses a live virus that replicates and can have serious side effects, including stillbirth and life-threatening infection. It’s considered unsafe for women who are pregnant or breastfeeding, people with weakened immune systems, and those with heart disease.

Antivirals are available too, including tecovirimat, also known as TPOXX, an oral antiviral approved for smallpox in adults and children. In the European Union it’s approved for the treatment of monkeypox and cowpox, “and would likely be allowed to be given off-label in the case of a monkeypox case in the U.S.,” Brugler Yonts said.

There’s also cidofovir, an antiviral used for other viruses like adenovirus, with demonstrated success against monkeypox that could also be used in humans, she added.

A changing pathogen

At least two genetically distinct variants of monkeypox have been discovered in the U.S. since a large global outbreak was detected last month, federal health officials said Friday.

Both cases share common ancestors with strains that have developed in Nigeria since 2017 and are similar to those seen in a 2021 case imported to the U.S., officials said, adding that not all of the 20 U.S. cases identified as of Friday have been genetically sequenced yet.

Of the 10 U.S. cases sequenced by the CDC so far, “most are closely related to the cases in Europe, but three are unrelated and represent separate importations from Africa or the Middle East,” Eric Toner, senior scientist and health security expert with the Johns Hopkins Bloomberg School of Public Health, told Fortune on Friday.

The news of the newly identified strains comes amid reports of transmission patterns atypical for the disease. More than 800 cases have been identified globally since last month, but such extensive spread is not normal for the virus. The pathogen is usually found in rural African areas where people have close contact with infected rats and squirrels, and is typically transmitted from human to human through close contact or large respiratory particles. It’s usually difficult to catch, with household transmission rates around 10% or lower.

Symptoms of the new variants also appear to differ from classic cases—at least in some instances—with recent reports of lesions more subtle than usual and some cases involving just one lesion, U.S. health officials acknowledged Friday.

Monkeypox, a rare disease related to smallpox and cowpox, was first identified in 1958 among colonies of monkeys kept for research, according to the CDC. African rodents and nonhuman primates like monkeys may harbor the disease and infect people. The first human case was recorded in 1970 in the Democratic Republic of Congo, where the majority of infections now occur. It has since been reported in humans in other African countries including Cameroon, Central African Republic, and Ivory Coast.

Prior to this year’s outbreak in Europe, cases had been reported in the U.S. as well as Israel, Singapore, and the United Kingdom, according to the CDC. Last year two cases were identified in the U.S., one in November and one in July. Both involved travelers who had just returned from Nigeria. Human monkeypox was first reported outside of Africa in the U.S. in 2003, when 47 confirmed and probable cases were reported. All cases had contact with pet prairie dogs that had been housed near small mammals—including rope squirrels, tree squirrels, and African giant-pouched rats—imported from Ghana.

Symptoms are similar to but milder than those of smallpox, according to the CDC. Initial symptoms usually include fever, headache, muscle aches, and exhaustion. Within one to three days, patients develop a rash, usually starting on the face and then spreading to other parts of the body. Lesions progress through various stages before scabbing. The illness usually lasts two to four weeks. The typical incubation period is seven to 14 days but can range from five to 21 days.