The United States has confirmed its first known case of clade 1 mpox, a more severe strain of the virus, in California, according to the US Centers for Disease Control and Prevention (CDC). The case, reported on Saturday, has raised concerns, though the CDC stressed that the risk to the public remains low.
The patient recently travelled from Eastern Africa, where an ongoing outbreak is driven by clade 1. Upon arrival in the US, the person sought medical treatment at a local facility and was later released. The CDC confirmed that the patient has been isolating at home and their symptoms are improving. Authorities are working closely with the state and local health departments to trace potential contacts.
Mpox, previously known as monkeypox, is a viral disease related to smallpox. The virus can spread through close contact, including touching, kissing, or sexual activity, as well as through contaminated materials such as sheets, clothing, and needles. Early symptoms typically include fever, chills, headache, muscle weakness, and exhaustion, followed by a painful or itchy rash that develops into raised lesions which eventually scab over and resolve over several weeks.
The clade 1 strain is associated with more severe illness compared to the more commonly circulated clade II strain, which has been active in the US since the large outbreaks of 2022 and 2023. While the US has mainly been dealing with clade II cases, the more recent outbreaks in Africa have been caused by clade I, known for its higher severity.
The subvariant of clade 1, called clade 1b, has also been causing concern. Travel-related cases of clade Ib have been reported in countries including Germany, India, Kenya, Sweden, Thailand, Zimbabwe, and the United Kingdom. Although some of these countries have experienced locally transmitted cases, the CDC noted that no deaths have been reported in these cases so far.
“The recent travel-associated clade 1 mpox cases outside of Africa have all been attributed to subclade Ib; there have been no deaths associated with these cases and available data for a subset has detailed relatively mild disease courses,” the CDC said.
Historically, clade 1 mpox has caused more severe illness and higher mortality rates compared to clade II. However, the CDC pointed out that recent data shows a shift in the pattern of the disease. "While outbreaks of clade 1 mpox used to have death rates around 3 per cent-11 per cent, more recent outbreaks have had death rates as low as approximately 1 per cent when patients received good medical oversight and supportive clinical care," the agency explained. "Death rates are expected to be much lower in countries with stronger healthcare systems and treatment options, including the United States."
In Africa, mpox cases have been rising, particularly in the Democratic Republic of Congo, Burundi, and Uganda. The World Health Organisation (WHO) is monitoring the situation closely and will convene an emergency committee meeting on November 22 to assess whether the current outbreak still qualifies as a public health emergency of international concern.
The CDC advises people to protect themselves by avoiding contact with individuals showing symptoms and contaminated materials like clothing, bedding, and sex toys. Vaccination is recommended for eligible individuals, who should receive two doses of the mpox vaccine to reduce the risk of infection.
