Scientists say the risk to the public remains low.
This week, the UK Health Safety Agency announced that it had now identified three more cases who lived in the same household as the first patient.
They too are now being treated in a hospital in London.
“Mpox is highly contagious in households with close contact, so it is not unexpected to see new cases within the same household,” said Susan Hopkins, chief medical adviser to the UK’s Health Safety Agency.
The new mpox variant was first discovered earlier this year in eastern Congo.
Scientists believe it causes milder symptoms that are harder to notice, making it easier to spread because people may not know they are infected.
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Britain recorded more than 3,000 cases of the second type of measles during the 2022 outbreak, which affected more than 100 countries.
The new mpox variant has also caused outbreaks in Burundi, Kenya, Rwanda and Uganda.
Individual cases among travelers were also recorded in Sweden, India, Germany and Thailand.
To date, there have been approximately 43,000 suspected cases of measles in Africa, including more than 1,000 deaths, mostly in the Congo.