The Centers for Disease Control and Prevention’s (CDC) issued a Health Alert Network (HAN) Health Advisory to information the public that a case of clade I mpox was confirmed by the California Department of Public Health (CDPH) on November 15. This is the first case in the United States. The person had traveled to areas where clade I transmission has been ongoing.¹

In terms of the person’s condition, the individual is receiving care in the US and is isolating from others. The patient, who has no underlying health conditions, has not had any severe manifestations of disease, and symptoms are improving, according to the CDC’s statement. The federal agency said it is working closely with the local and state health authorities to investigate the circumstances surrounding this case and to prevent spread of the virus. As of November 18, no additional clade I mpox cases have been reported in the country.1

Distinguishing Between Clade I and Clade II

According to the CDC, mpox has 2 distinct genetic clades: clade I (with subclades Ia and Ib), which has been endemic to some countries in Central Africa, and clade II (with subclades IIa and IIb) is historically endemic to some countries in West Africa.¹

Clade I mpox has previously been observed to be more transmissible and to cause a higher proportion of severe infections than clade II MPXV.²

In a previous interview with Contagion, Carlos del Rio, MD, FIDSA, executive associate dean, Emory School of Medicine & Grady Health System, distinguished professor, Department of Medicine, Division of Infectious Diseases, Emory University School of Medicine, discussed the differences between the 2 clades.

“The current mpox clade I outbreak in the DRC has already infected over 14,000 persons (mostly women and children under 15) and has caused over 500 deaths. The outbreak has now spread to other countries in Africa and there is the possibility of spread beyond the continent,” said del Rio. “Clade I is more virulent and has a higher mortality than clade II (which caused the 2022 outbreak mostly among MSM [men who have sex with men]). The available JYNNEOS vaccine is effective in the prevention of mpox clade I but the challenge is not having enough vaccine available in countries that need it most.”²

There have been outbreaks of clade II in the US, as previously reported by Contagion.

The Clade 1 Statistics

From January 1 through November 15, 2024, about 12,000 confirmed cases of clade I mpox and at least 47 deaths have been reported in Central and Eastern African countries. These countries include Burundi, Central African Republic, Democratic Republic of the Congo, Republic of the Congo, Rwanda, and Uganda. Data from affected countries indicate that a large proportion of clade I mpox cases among adults were associated with heterosexual contact. Transmission to close contacts within households, including to children, also has been reported.¹

Back in August, the Africa Centers for Disease Control and Prevention (Africa CDC) has officially declared the ongoing mpox outbreak a Public Health Emergency of Continental Security (PHECS)—the first such declaration by the agency since its inception in 2017.²

References

1. First Case of Clade I Mpox Diagnosed in the United States. CDC. November 18, 2024. Accessed November 20, 2024.
https://emergency.cdc.gov/han/2024/han00519.asp

2. Contagion Editorial Team. Africa CDC Director General: Mpox Outbreak a Public Health Emergency of Continental Security. Contagion. August 14, 2024.Accessed November 20, 2024.