First US Case of Clade I Mpox Reported – CDC Monitoring & Response

California health officials have confirmed the first case of clade I mpox in the United States, a development that has prompted increased vigilance but is not currently considered a high risk to the general public. The case, identified in an individual who recently traveled from Eastern Africa, marks the first instance of this particular strain of mpox detected within the country. While clade II mpox has been circulating at low levels since the 2022 global outbreak, this new case signals the emergence of a different variant, prompting a focused response from both state and federal health agencies.

Clade I vs. Clade II: Understanding the Different Strains

Mpox, formerly known as monkeypox, is caused by a virus with two main clades: clade I and clade II. Both can be spread, treated and prevented in similar ways, but they differ in their geographic distribution and, historically, in the severity of illness they cause. The recent outbreak in Central and Eastern Africa involves clade I, while the 2022 global outbreak was driven by clade IIb. It’s important to note that determining the specific clade doesn’t change how a person presents with symptoms – the rash and other indicators are similar regardless of the strain. However, understanding the clade is crucial for tracking the virus’s spread and assessing potential risks.

Historically, clade I mpox has been associated with more severe illness and higher fatality rates than clade II. However, recent data from the current clade I outbreak suggest that infections may not be as clinically severe as in past outbreaks, particularly when patients receive prompt medical care. Death rates in recent outbreaks have been as low as approximately 1%, a significant decrease from the 3%-11% seen in historical outbreaks. This is likely due to improved healthcare access and supportive clinical care.

The California Case and Public Health Response

The individual diagnosed with clade I mpox in California was treated shortly after returning from Eastern Africa and has since been isolating at home, with symptoms improving. Specimens have been sent to the Centers for Disease Control and Prevention (CDC) for further analysis to characterize the virus and understand its genetic makeup. The CDC is also working with California health officials to identify and monitor potential contacts of the individual.

Health officials emphasize that casual contact, such as might occur during travel, is unlikely to pose a significant risk of transmission. The CDC has not changed its existing guidance on mpox prevention, which includes avoiding close contact with individuals exhibiting symptoms, avoiding contact with contaminated materials, and getting vaccinated if eligible. You can find more information on mpox prevention at the CDC website.

Wastewater Surveillance and Early Detection

The CDC is actively increasing its capacity for early mpox detection through wastewater testing. This surveillance method allows for the identification of the virus in communities even before individuals begin to exhibit symptoms. Data from wastewater samples are combined with other surveillance data to determine if further testing or interventions are needed. This proactive approach aims to provide an early warning system for mpox activity and spread.

Broader Trends: Clade I and Clade II Activity

While this marks the first confirmed case of clade I mpox in the U.S., there have been travel-associated cases reported in several other countries, including Germany, India, Kenya, Sweden, Thailand, Zimbabwe, and the United Kingdom. Since November 2024, the U.S. Has reported 11 cases of clade I mpox, all linked to recent travel to Central and Eastern Africa or contact with travelers from those regions. Notably, three cases reported in October 2025 had no recent travel history, but were linked to a U.S. Case from August 2025 following travel to an outbreak area.

Clade II mpox continues to circulate at low levels globally, with a slight uptick in U.S. Cases during the summer of 2025, some of which were linked to outbreaks in Sierra Leone, Liberia, and other West African countries. The CDC continues to monitor both clades and operate with international partners to prevent further spread. More information on the current situation can be found on the CDC’s monkeypox situation summary page.

What Does This Mean for Risk?

The CDC maintains that the risk posed by the clade I mpox outbreak to most people in the United States remains low. Simulations conducted by the CDC suggest that close-contact transmissions are unlikely to result in a large number of cases. Experiences in other countries, such as Sweden, Thailand, Germany, and India, where travel-associated cases did not lead to widespread transmission, further support this assessment. However, ongoing surveillance and vigilance are crucial to ensure a rapid response should the situation change.

Looking Ahead: Continued Surveillance and International Collaboration

The CDC is continuing its work in Central and Eastern Africa to help stop mpox transmission at the source. This includes providing laboratory training, diagnostic testing supplies, support for surveillance, and assistance with infection prevention and control. The agency is also collaborating with state and local public health partners to monitor for cases, identify contacts, and provide guidance to healthcare providers.

Given the CDC’s experience responding to the 2022 global clade II mpox outbreak, existing public health systems and structures are already adjusted to respond to any potential outbreak of clade I mpox in the United States. The CDC has also issued guidance for travelers to countries in Central and Eastern Africa experiencing outbreaks. Clinicians are encouraged to request expedited clade-specific testing for suspected cases with a travel history to affected regions.

For the most up-to-date information on mpox, please visit the CDC’s mpox webpage.