California has detected the country’s first case of a new mpox strain that is spreading from person to person. The infected individual had recently traveled from Eastern Africa, where multiple countries are battling transmission of this virus.

The individual was treated in a health facility in San Mateo County and is in isolation at home, the California Department of Public Health announced in a statement released Saturday. In a separate statement, the Centers for Disease Control and Prevention said the person is not currently being treated with anti-mpox medication, which may be a signal that the infection is mild.

The version of the virus the person is infected with is known as clade Ib, an offshoot of clade I viruses that traditionally spread to people from an animal source in countries in East Africa where the virus lives in nature. It is different from the clade IIb virus that is responsible for the large international mpox outbreak that began in 2022.

California health authorities said at this time there is no evidence that the infected person had transmitted the virus to anyone else.

Four weeks ago the UK became the fifth non-African nation to report an imported Mpox Clade Ib case in a recent traveler to Africa. Since then 3 house mates of this index case have become infected, and we've seen the both the United States and Canada report imported cases. 

Unlike the milder Clade II Mpox virus, which began its world tour in the spring of 2022, clade Ib is believed to be more virulent, and potentially more easily transmitted. For that reason, it has been designated a high consequence infectious disease (HCID) in the UK

Today the UK's Health Security Agency has announced a second imported clade Ib case, this time in Leeds. 

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The case was detected in Leeds and the individual is now under specialist care at Sheffield Teaching Hospitals NHS Foundation Trust. They had recently returned from Uganda, which is seeing community transmission of Clade Ib mpox. The UKHSA and NHS will not be disclosing any further details about the individual.  

This is the fifth case of Clade Ib mpox confirmed in England in recent weeks. This case has no links to the previous cases identified. All 4 previous cases were from the same household and all have now fully recovered.   

Close contacts of the case are being followed up by UKHSA and partner organisations. Any contacts will be offered testing and vaccination as needed and advised on any necessary further care if they have symptoms or test positive. 

The  clade Ib Mpox virus which emerged last year in the DRC is reportedly more virulent, and potentially more transmissible, than the clade II mpox virus which began its world tour in the spring of 2022.  

A week ago the WHO published a new, updated risk assessment, which puts the risk of national and international spread as HIGH. 

The Mpox virus continues to evolve, and we now have at least 4 clades of Mpox in circulation (I, Ib, II, IIb), with