On Friday, June 3, the Centers for Disease Control and Prevention announced that genetic testing identified two strains of monkeypox in the US, according to the Associated Press. Most US cases involved the strain linked to the recent outbreak first identified in European countries, according to Medscape, but the presence of a second variant suggests the virus may have been spreading unnoticed for some time.  

“I think it’s certainly possible that there could have been monkeypox cases in the United States that went under the radar previously, but not to any great degree,” Jennifer McQuiston of the CDC told reporters, according to the AP. 

Monkeypox symptoms, which can include fevers, headaches, rashes, and lesions, may have been misdiagnosed as other illnesses, making it difficult to track when cases first emerged in the US, the AP reports. However, McQuiston says that data from many more patients will be required to estimate how long the virus has been circulating in the US. 

As of June 2, the ongoing outbreak has involved 780 confirmed cases of monkeypox in 27 countries outside of West or Central Africa, where the virus is endemic, the World Health Organization reports. So far, the CDC has confirmed 21 US cases, according to Medscape, and the agency has released details on the first 17 of those. Fourteen of the first 17 people with confirmed cases had travelled internationally, and 16 self-identified as men who have sex with other men, reports Medscape, though officials note that anyone can be infected. According to The New York Times, three US cases involve people with compromised immune systems, and the AP reports that one case is a heterosexual woman. 

In an effort to slow the spread, the CDC has provided 1,200 orthopoxvirus vaccine doses to people exposed to the monkeypox, and it continues to track and isolate cases, according to Medscape.

However, Angela Rasmussen, a virologist at the University of Saskatchewan, says that the new evidence that monkeypox has been circulating under the radar could indicate the outbreak will be more difficult to contain than previously thought, according to the AP. “We don’t really have a good sense of how many cases there are out there,” she says.

Rasmussen adds it’s possible that, even if the current outbreak is contained in humans, it could pass to and spread within the rodent population.