The new mpox variant may seem less deadly, but it is spreading quickly. Complacency would be a grave mistake | Debora MacKenzie

ILast week, the World Health Organization (WHO) declared a “public health emergency of international concern” for mpox for the second time in two years. A new version of the virus formerly known as monkeypox has spreads mainly by heterosexual transmission and is showing up in places as far away as Kenya and Sweden. More will follow. You might be thinking, didn’t that disappear two years ago? Is this more deadly? Is it easy to catch?

There’s been some confusion in what you may have read, and that’s not surprising. Mpox has morphed into four different diseases in recent years, and official sources often confuse them. There’s the original clade I in Central Africa and clade II in West Africa, which we’ve known about since the 1970s. Then there’s a recently evolved, sexually transmitted “b” version of each. Clade Ib is causing the most commotion right now. And it could very well cause a pandemic.

Pandemic is a scary word. But it really just means an epidemic that goes global and affects many people. Two years ago, the sexually transmitted clade IIb did just that, and it has now infected nearly 100,000 people in 116 countries, dead 208 of them. Now the question is whether clade Ib could do the same—and, if so, whether it would be worse. Fortunately, so far it has not been as deadly as some feared.

Mpox is a virus that occurs in small mammals like rodents in Africa, and is closely related to smallpox. People who get it, often by eating an infected animal, develop pustules and fever, similar to smallpox but less deadly. Until recently, people could pass the virus to other people by extensive skin contact, and those people could pass it on, but the infection would disappear after two or three such passes. So it seemed relatively harmless, especially in places where no animals were carrying it, and it got little official attention.

But mpox wasn’t incompetent—it was just held back by competition. Until 1980, most adults had survived smallpox or had been vaccinated, so they were immune to mpox. When a human-to-human transmission chain reached them, it stopped. Then we eradicated smallpox and stopped vaccinating against it. By 2010, there were mpox cases in the Democratic Republic of Congo (DRC) had increased 20-fold, and almost all of it in people born after 1980 who were not immune. Scientists warned that the number of cases would continue to rise: with fewer and fewer people in the population immune, any infection could spread further.

More cases meant more opportunities for mpox viruses to adapt, become more efficient at transmitting, and outcompete other viruses to the next human. In 2016, a clade II virus in Nigeria did just that, and began spreading sexually, primarily between men. Without the need for animal carriers, it was able to spread globally. Nigerian doctors noticed a nasty new diseasebut no official alarms were raised until a case emerged in London in 2022, and the WHO declared a state of emergency. Scientists figured out what had happened by analyzing the insidious mutations that a virus acquires only in humans.

Doctors found that vaccinating and tracing contacts among men with the most partners, who were in fact superspreaders, was enough to contain outbreaks. Rich countries used the smallpox vaccine they had stockpiled against bioterrorism. Poor countries got none, but people also became immune to infection. Cases fell sharply by January 2023although the virus has not yet been eradicated and will probably continue to exist for some time.

Now let’s compare clade I, which is raging in the DRC and has gone from less than 6,000 known cases in 2022 to more than 16,000 so far this year. Experts fear that war and rising poverty are driving more people to eat rodents. But because so few people in the DRC are old enough to have been vaccinated against smallpox, the virus could spread to more people before they reach a dead end. Chains of infection in most of the DRC still begin in animals, but are now stay with people longer.

Late last year, the southeastern DRC, which had been free of mpox, began finding the virus among female sex workers. Clade Ib, a new variant of clade I, has been spreading among humans since September 2023. Its number of cases is now skyrocketing faster than the old clade I virus in the rest of the country, and invade new countrieswhich led to the WHO sounding the alarm last week.

Fortunately, it is not as bad as some feared. Clade I has killed 5% of the people infected with it in the DRC, mostly children. But data shows that clade Ib only 0.6% deathsExperts don’t know whether this virus can spread in heterosexual contact networks in the same way that clade IIb did in men, but we could soon find out.

Frustratingly, we could have tackled mpox in Africa before it got to this point. Experts have called for vaccination to contain outbreaks. But WHO rules make it difficult for poor countries to obtain expensive smallpox vaccines outside a official state of emergency. The DRC still has no supplies: the 50,000 doses the US donated in May won’t arrive for another two months. But the WHO’s declaration of emergency means that could be accelerated as more donations are announced.

If we had bought vaccines earlier for countries with mpox, manufacturers might have prepared to make more. As it stands, the only major manufacturer, Bavarian-Nordic in Denmark, will be busy replacing the 5 million stockpile doses the U.S. used in 2022 through the end of the year. It can only make 10 million by the end of 2025By then, the world could be clamoring for more vaccines, and rich countries would almost certainly be first in line again.

There is hope. BioNtech, the German company that pioneered mRNA vaccines for Covid, has a promising one for mpox that could be faster and cheaper to make than the current vaccine. It is also building a vaccine factory in RwandaBut the vaccine has not yet passed large-scale human trials.

Meanwhile, as another obscure virus explodes, potentially causing a pandemic that we will all have to deal with, we still haven’t learned the great lesson of Covid: a disease anywhere can be a threat everywhere. We can’t protect our own people without working better with others. Maybe this time we’ll learn that—before another virus becomes a global problem.