Pandemic is a scary word. But all it really means is an epidemic that goes global and affects a lot of people. Two years ago, the sexually transmitted clade IIb did just that, and it has now infected almost 100,000 people in 116 countries, killing 208 of them. Now the question is whether clade Ib might do the same – and, if it does, whether it will be worse. Thankfully, it hasn’t so far been as deadly as some feared.

Mpox is a virus found in small mammals such as rodents in Africa, and is closely related to smallpox. People who get it, often by eating an infected animal, develop pustules and fever, like smallpox but less deadly. Until recently, they could pass the virus to other people through extensive skin contact, and those people might pass it on again, but the infection petered out after two or three such passages. So it seemed fairly unthreatening, especially in places where no animals carried it, and it received little official attention.

But mpox wasn’t inept — it was just being held back by the competition. Until 1980, most adults had survived smallpox or been vaccinated, so they were also immune to mpox. When a human-to-human chain of transmission reached them, it stopped. Then we eradicated smallpox and stopped vaccinating against it. By 2010, mpox cases in the Democratic Republic of the Congo (DRC) had increased 20-fold, and nearly all were in people born after 1980, who weren’t immune. Scientists warned cases would keep rising: as fewer and fewer people in the population were immune, each infection could spread farther.

More cases meant more opportunities for mpox viruses to adapt, transmit more efficiently and beat other viruses to the next human. In 2016, a clade II virus in Nigeria did precisely that, and started spreading sexually, mostly between men. With no need for animal carriers, it was able to go global. Nigerian doctors noticed a nasty new disease, but no official alarms were sounded until a case turned up in London in 2022, and the WHO declared an emergency. Scientists worked out what had happened by analysing telltale mutations that a virus only gets in humans.

Doctors found that vaccinating and contact-tracing the men with the most partners, who were effectively super-spreaders, was enough to limit outbreaks. Rich countries used smallpox vaccine stockpiled against bioterrorism. Poor countries got none, but people also became immune through infection. Cases fell steeply by January 2023, although the virus hasn’t been wiped out and it’s probably here to stay.

Now let’s compare clade I, which is raging in the DRC, jumping from fewer than 6,000 known cases in 2022 to more than 16,000 so far this year. Experts fear war and increasing poverty are forcing more people to eat rodents. But with so few people in the DRC now old enough to have been vaccinated against smallpox, the virus can spread to more people before hitting a dead end. Chains of infection in most of the DRC still start with animals, but are now spending longer in humans.

Late last year, south-eastern DRC, which had been mpox-free, started finding the virus among female sex workers. Clade Ib, a new variant of clade I had been spreading in people since September 2023. Cases of it are now shooting up faster than the old clade I virus in the rest of the country, and invading new countries, triggering last week’s alarm from the WHO.

Thankfully, it isn’t as nasty as some feared. Clade I has been killing 5% of people infected with it in the DRC, especially children. But data shows clade Ib is killing only about 0.6%. Experts don’t know if this virus can spread in heterosexual contact networks in the same way that clade IIb did in men, but we could know soon.

Frustratingly, though, we could have attacked mpox in Africa before it came to this. Experts have called for vaccination to limit outbreaks. But WHO rules make it hard for poor countries to buy expensive smallpox vaccine outside an official emergency. The DRC still has no supplies: the 50,000 doses that the US donated in May won’t arrive for another two months. But the WHO emergency declaration means that could be accelerated, while more donations are being announced.

If we had been buying vaccines earlier for countries with mpox, manufacturers might have tooled up to make more. As it is, the only big manufacturer, Bavarian-Nordic in Denmark, will be busy replacing the 5m stockpile doses the US used in 2022 until the end of the year. It can make only 10m by the end of 2025. By then, the world could be clamouring for more vaccines, and rich countries would almost certainly be first in line, again.

There is hope. BioNtech, the German firm 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 plant in Rwanda. But its vaccine hasn’t yet cleared large-scale human trials.

Meanwhile, as another obscure virus explodes, potentially causing a pandemic that will confront us all, we still haven’t learned the big lesson of covid: a disease anywhere can be a threat everywhere. We can’t protect our own populations without working better with others. Maybe this time we will learn that – before yet another virus becomes a global problem.

Debora MacKenzie is a science journalist and author of Stopping the Next Pandemic: How Covid-19 Can Help Us Save Humanity

The Guardian