The report Saturday of the first U.S. case of a relatively new mpox strain — a case that appears to be mild and well-contained — underscored concerns expressed at #TropMed24 regarding risks that the strain, known as clade 1B, could cause a new global mpox outbreak.

Rosamund Lewis, the WHO’s technical lead for the global mpox response, said that isolated reports this year of clade 1B mpox occurring outside of the Democratic Republic of Congo (DRC), which is currently battling a rapid surge of infections, are reminiscent of isolated reports from 2017 to 2019 involving a different mpox strain, known as clade 2. The clade 2 cases emerged in countries where the disease had been rarely if ever detected and may have been “seeding” the 2022-2023 global mpox outbreak that was driven by clade 2, she said.

Lewis wondered whether we now are on the verge of a new global outbreak driven by the clade 1B strain. 

“We don’t know and I’m not predicting there will be one,” she said. “But the question is, are we in the same place that we were in five or six years ago, and we’re not doing enough about it?”

The DRC remains at the epicenter of the current global mpox threat. Placide Mbala, an epidemiologist with DRC’s University of Kinshasa, said that last year, the country experienced a huge increase in the number of cases, especially in children under 15 years old. He said the cases have involved two subtypes of the clade1 virus: 1A and 1B. Clade 1B appears to be mostly spread through human-human transmission, either via sexual activities or household contacts, he said. But Mbala worries that “what we are seeing now is just the tip of the iceberg.”

Particularly concerning, he said, is that while mpox is a type of virus associated with a low rate of mutations, since 2022, he has seen more and more mutations, and a virus that is becoming more and more transmissible.

Jean-Jacques Muyembe-Tamfum, director general of the DRC’s National Institute of Biomedical Research,has observed a decades-long shift of mpox in the DRC from a relatively small number of cases caused by contact with animals to a large number of infections caused primarily by human-to-human transmission, he said. While the WHO and Africa CDC have responded to the current challenge by declaring mpox a public health emergency, he said he is not feeling a direct benefit in the DRC. 

For example, Muyembe-Tamfum said that due to a lack of diagnostic tests, only 40% of samples from suspected cases are being tested. He also said the DRC has not received a large quantity of vaccines and will face logistical problems with widely distributing them if and when more become available. 

As for the recently detected clade 1B case in California, Christy Hutson, head of the poxvirus and rabies branch at the U.S. Centers for Disease Control and Prevention (CDC), said that it involves an individual who had traveled to the DRC in early October. Hutson said the person, who has experienced only mild symptoms, is isolating at home and did not report close contacts beyond a household member. 

Hutson noted that over the last year, when 1B cases cropped up in Thailand, Sweden, Germany and India, rapid interventions from health authorities prevented the disease from spreading locally. 

“We’re hoping for a similar picture here,” Hutson said.  “Thanks to the preparedness that we have been doing, we were able to detect this case pretty quickly and put into place the measures that can prevent onward spread, though the case is still being investigated.”