Enlarge / A negative stain electron micrograph of an mpox virus virion in human vesicular fluid. (credit: Getty | BSIP)

The World Health Organization on Wednesday declared an international health emergency over a large and rapidly expanding outbreak of mpox that is spilling out of the Democratic Republic of the Congo.

It is the second time in about two years that mpox's spread has spurred the WHO to declare a public health emergency of international concern (PHEIC), the highest level of alarm for the United Nations health agency. In July 2022, the WHO declared a PHEIC after mpox cases had spread across the globe, with the epicenter of the outbreak in Europe, primarily in men who have sex with men. The outbreak was caused by clade II mpox viruses, which, between the two mpox clades that exist, is the relatively mild one, causing far fewer deaths. As awareness, precautions, and vaccination increased, the outbreak subsided and was declared over in May 2023.

Unlike the 2022–2023 outbreak, the current mpox outbreak is driven by the clade I virus, the more dangerous version that causes more severe disease and more deaths. Also, while the clade II virus in the previous outbreak unexpectedly spread via sexual contact in adults, this clade I outbreak is spreading in more classic contact patterns, mostly through skin contact of household members and health care workers. A large proportion of those infected have been children.