How monkeypox is (and isn’t) transmitted

When monkeypox cases were detected this spring outside of countries where the disease is typically found, many experts felt confident that the outbreak, like previous monkeypox outbreaks, could be contained. Unlike COVID-19, monkeypox was a known virus with vaccines and treatments available. Now, less than four months after the first case was reported in the U.K., cases have swelled to over 45,000 worldwide. A third of those cases were reported in the U.S., largely due to a slow early response and delayed vaccine distribution. As we work to contain the outbreak, misinformation continues to circulate about monkeypox, how it spreads, and who is at risk. 

What is monkeypox?

Monkeypox is a viral disease that belongs to the same family as smallpox. The disease, which is characterized by a blister-like rash, was first recorded in humans in 1970 and is endemic to several countries in West and Central Africa. Monkeypox—which some are now calling mpox or mpx to minimize stigma—is divided into two groups, or clades: the Central African clade (Clade I) and the West African clade (Clade II), with the latter being the less severe and more transmissible illness. The monkeypox variants causing the current outbreak mutated from Clade II and are collectively called Clade IIb. 

Two smallpox vaccines have been approved to prevent monkeypox. Despite their availability, these vaccines have never been made accessible to the countries historically most affected by monkeypox. Even today, as wealthy countries worldwide ramp up monkeypox vaccination campaigns, not a single dose has been administered in Africa.

Monkeypox can be transmitted in multiple ways

Until recent years, monkeypox primarily spread through contact with infected animals, rarely spreading between humans. The disease has spread to nonendemic countries on several occasions in the past, usually through travel or contact with imported animals infected with monkeypox. The U.S. successfully contained a monkeypox outbreak in 2003 that was linked to infected pet prairie dogs.

In 2017, a large monkeypox outbreak in Nigeria was driven by human transmission for the first time. A disease that was once confined to rural areas where people were more likely to come in contact with infected animals began to appear in cities. A study published in 2019 found that the outbreak primarily affected adult men, many of whom likely contracted the virus through sexual contact. This outbreak was the first indication that the virus had shifted to a new mode of transmission. The possibility of this more transmissible monkeypox moving beyond countries where the virus has historically circulated was clear, but the evidence went largely ignored until the current global outbreak.

Newer monkeypox variants spread mostly through direct skin contact

The ongoing monkeypox outbreak is following a pattern similar to what emerged in Nigeria five years ago: The virus is primarily spreading through direct skin-to-skin contact with a monkeypox rash, particularly sexual or intimate contact. The Clade IIb variants causing the current outbreak, which are far more transmissible than past variants and are more easily spread through human contact, are believed to have mutated from the variant that caused the Nigerian outbreak. 

Most of the monkeypox cases have been in men who have sex with men and their sexual networks, but monkeypox does not spread exclusively through sexual contact, nor does it only affect men who have sex with men. Anyone can contract monkeypox, even if some populations are at higher risk than others. Monkeypox can spread through contact with clothing or other surfaces used by a person with monkeypox and through bodily fluids, including saliva, although these transmission paths are less common than direct skin contact. There is currently no evidence of airborne transmission, although the possibility of this mode of transmission cannot be ruled out.

Widespread monkeypox transmission was not inevitable

Because global health efforts failed to address monkeypox when it was affecting low-income and lower-middle-income countries in West and Central Africa, the virus continued to circulate and mutate, becoming more transmissible and developing a new mode of transmission. The global monkeypox outbreak, like the COVID-19 pandemic and recent polio outbreaks, is the latest reminder that infectious diseases do not respect borders. Insufficient response to infectious disease outbreaks affecting the Global South and marginalized groups is not only a failure of public health policy: It also enables the wider spread of diseases that could be contained or even eliminated.