US officials find 2 monkeypox strains

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Genetic analysis of recent monkeypox cases suggests there are two distinct strains in the U.S., health officials said Friday, raising the possibility that the virus has been circulating undetected for some time.

Many of the U.S. cases were caused by the same strain as recent cases in Europe, but a few samples show a different strain, federal health officials said. Each strain had been seen in U.S. cases last year, before the recent international outbreak was identified.

Monkeypox on a child in Nigeria (Credits: Nigeria Centre for Disease Control via WHO)

"The strain of the monkeypox virus affecting patients in this outbreak is the West African clade and that is less severe than other known clades [such as] the Congo Basin clade, meaning that in historical outbreaks in Africa it has led to fewer deaths." said Jennifer McQuiston, deputy director for the CDC’s Division of High Consequence Pathogens and Pathology. "Analysis from many more patients will be needed to determine how long monkeypox has been circulating in the U.S. and elsewhere."

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While the strain is believed to be a less severe one, McQuiston stressed that it should not be minimized as the virus can still result in pain due to its characteristic rash as well as severe scarring once the lesions have healed.

This 2003 electron microscope image made available by the Centers for Disease Control and Prevention shows mature, oval-shaped monkeypox virions, left, and spherical immature virions, right, obtained from a sample of human skin associated with the 2003 prairie dog outbreak. (Cynthia S. Goldsmith, Russell Regner/CDC via AP)

Monkeypox typically begins with a flu-like illness and swelling of the lymph nodes, followed by a rash on the face and body. The disease is endemic in parts of Africa, where people have been infected through bites from rodents or small animals. It does not usually spread easily among people.

Last month, cases began emerging in Europe and the United States. Many — but not all — of those who contracted the virus had traveled internationally, and health officials in a growing number of countries are investigating.

"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," McQuiston told reporters Friday.

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Most monkeypox cases in the U.S. have been found among men who have sex with men, however, the CDC confirmed that one case has been found in a woman who had traveled to West Africa and had a heterosexual lifestyle. 

A vile of the smallpox vaccine that is being provided for the military is held up by a health worker. (Reuters)

The number of identified monkeypox cases in the U.S. have doubled in the past week to at least 20 cases in 11 states. The Center of Disease Control and Prevention (CDC) says that the risk to the public remains low, and the strains are believed to be a less severe version of the disease.  

As of Friday, the U.S. had identified at least 20 cases in 11 states. No monkeypox-related deaths have yet been reported.

Hundreds of other cases have been found in other countries, many apparently tied to sexual activity at two recent raves in Europe.

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The CDC shared that past data from Africa suggests that the smallpox vaccine is at least 85% effective in preventing monkeypox. The federal government has delivered about 1,200 doses of smallpox vaccines. The two licensed smallpox vaccines in the United States are Jynneos and the older ACAM2000 and JYNNEOSTM (also known as Imvamune or Imvanex).

The Associate Press contributed to this report.