A little over a year ago, when the COVID-19 pandemic was still a public health emergency, a little-known virus came seemingly out of nowhere to add to the world's worries – a virus known at the time as monkeypox.

Monkeypox – since renamed mpox following criticisms that the name held misleading and racist connotations – went from sickening only a few dozen people a year to quickly infecting over 87,000 people around the globe, mostly in countries with no previous history of infection.

That led the World Health Organization to declare the mpox outbreak a Public Health Emergency of International Concern on July 23, 2022. But following a peak in late summer of 2022, cases dropped dramatically and just ten months later, the public health emergency was declared over.

So was the mpox outbreak simply a passing threat? Or is there reason to believe another global outbreak like last year's could happen? Experts say mpox has not gone away, but how much of a threat to human health it poses remains an open question.

Cases went up and then down — why?

Much of the world may not have heard of mpox until last year, but it has been around for over 50 years. The disease was first found in monkeys – hence the original name monkeypox – though they're not thought to be frequent carriers of the virus.

For decades, cases of mpox were rare, occurring mostly in children who likely came in contact with an infected animal, presumably a rodent or another small mammal. Then something changed: "In 2017 we [started having] yearly outbreaks of mpox in Nigeria," says Dr. Dimie Ogoina, the Nigerian doctor who warned the world about mpox years before the outbreak of 2022.

Beginning in 2017, hundreds of new cases of mpox were reported in Nigeria. Instead of children, many of the mpox patients in Nigeria were middle-class adult men living in cities. They also were presenting with warts and blisters around their genitals instead of their face and extremities, which led Ogoina to suspect that mpox was spreading via sexual contact.

In May 2022, this virus that had been confined to Africa began to appear in other places. Mpox was reported in the United Kingdom – and proceeded rapidly to infect people in more than 100 countries.

"We were in a place where we had an unprecedented outbreak," says Dr. Demetre Daskalakis, deputy coordinator for the White House National Mpox Response Team. In the U.S, "over 30,000 cases were reported last year."

By the end of 2022, Nigeria reported over 800 confirmed cases, which is "far more than the cases that were reported in the previous five years," Ogoina says. He also notes this is likely an undercount given the issues Nigeria has with disease surveillance.

The populations affected by the disease made it clear that mpox was spreading via sexual contact. Roughly 98% of cases were found in sexually active gay and bisexual men and other men who have sex with men.

Unequal access to vaccines

Fortunately, an FDA-approved vaccine effective against mpox already existed: Jynneos, a two-dose vaccine designed to prevent infection from the closely related smallpox virus.

But the world wasn't prepared to get that vaccine to those who needed it most. "A year ago, we didn't have an adequate supply of vaccine on the ground in the United States and we didn't have commercial testing for mpox," Daskalakis says. "Where we are now is a completely different place."

Messaging and vaccination campaigns targeting vulnerable groups in the U.S. began last summer. Initially, not every American who wanted the vaccine could get it, but eventually the supply improved and so did vaccination rates – which ultimately helped drive down case rates, according to the CDC.

"Where we are today is that we really have an epidemic curve that demonstrates what I think is a public health success," Daskalakis says. "We went from a peak of over 400-plus cases on average per day to somewhere between zero and two cases per day. We went from not having enough vaccines to having plenty."

But that success story in the U.S. does not explain why cases also declined in the Global South, where vaccines have been scarce. "To the best of my knowledge an mpox vaccine has never been used in Nigeria," Ogoina says. "There was a promise from the U.S. government to donate a few thousand vaccines to the country. I'm not sure if those vaccines have arrived."

According to reports in early April from Mojisola Adeyeye, director general of the Nigerian National Agency for Food and Drug Administration and Control, 50,000 doses of vaccine were on their way to Nigeria from the U.S. According to WHO and Africa CDC, multiple countries in Africa were set to receive vaccines earlier in the year or will receive them soon thanks to donations from other countries and vaccine manufacturers.

How many vaccines have actually been delivered is still unclear.

An outbreak resolved or a 'false decline'?

Even though case counts are down in Nigeria and other African countries, Ogoina believes that many cases are simply going unreported. "I think this is a false decline," he says. "I believe that in Nigeria and many other African countries we have continuous transmission. We're just seeing the tip of the iceberg."

Dr. Amesh Adalja, an infectious disease expert and senior scholar at Johns Hopkins University Center for Health Security, agrees that cases are undercounted across the Global South, though he says the situation has improved over the last year.

"The diagnostic capacity for mpox has increased everywhere and you have much more attention on it than before," he says. "So even though mpox testing is inadequate in many parts of the Global South, it's probably better now than it's ever been."

WHO also shares those concerns. In a webinar hosted by WHO June 28, its experts acknowledged that "a local outbreak is not yet over" in Africa and that surveillance in the region is still inadequate. But those are long-term problems that WHO is working to solve and do not represent a public health emergency, they note.

However, the end of the public health emergency has Ogoina concerned that global attention to mpox will decline, leaving African countries to deal with this disease on their own — and perhaps having an impact of the accuracy of case counts. "When there is increased global or national awareness of [mpox], it's like cases just come from nowhere. Then when that awareness declines, cases stop coming," he says.

Adalja is much more optimistic that mpox will still get the sustained attention it deserves. "There is still much more awareness of mpox now in 2023, even without a public health emergency declared," he says.

Experts divided over likelihood of another outbreak

The 2022 mpox outbreak died down across the globe, despite dramatically different public health responses among countries. The fact that we don't fully understand what drove the global decline has both Ogoina and Daskalakis worried.

"It's like we're telling the story over and over again" that an outbreak is not something to worry about, Ogoina says, recalling the warnings about mpox he gave years ago that went unheeded.

"Based on the modeling, we are at risk for [another] outbreak [in the U.S.] to the magnitude equal to or larger than the peak that we saw in 2022," Daskalakis says.

Adalja takes some comfort that widespread immunity will help tamp down any future outbreak. "I don't think that the virus can reach the same magnitude that it once had," he says. "And that's due to a saturation of high risk individuals [who are now immune through vaccination or past infection] and the awareness in sexual networks about mpox's association with sexual behavior."

That doesn't mean mpox will just go away, Adalja says, particularly in the Global South. "Because the animal reservoir is in those endemic countries [such as Nigeria], there's always going to be some mpox transmission occurring."

So what action can be taken to help reduce the possibility and severity of future mpox outbreaks? The steps will probably look very different in the U.S. than in countries like Nigeria.

In the U.S., Daskalakis says, a huge part of the effort remains focused on vaccines and education about good sexual health strategies. "We need to vaccinate more people and finish vaccinating the people that started. The more immunity that there is in the community, the less likely it is that we will have any outbreaks."

In Nigeria and much of the Global South, vaccination and education are critically important, but Ogoina thinks that surveillance remains a big issue. "We will need to step up and be more deliberate about surveillance in order to know the true picture of mpox. I've always said that we're walking blindly in Nigeria and most parts of Africa."