“If virus genomes from this outbreak are very similar to earlier ones, we’d feel more confident that there hasn’t been some evolution-driven jump in transmissibility,” says Jo Walker, a researcher at the Yale School of Public Health.
It seems more likely that this outbreak has stemmed from a flare in cases within parts of Africa, combined with a spike in air travel following the end of pandemic restrictions, and waning immunity against orthopoxviruses—the viral family that contains monkeypox, cowpox, smallpox, and others—across large swathes of the planet. Jamie Lloyd-Smith, a University of California, Los Angeles professor who has been studying monkeypox for more than a decade, says immunity against this family of viruses has been declining in humans ever since smallpox was eradicated in 1980.
“Eradicating smallpox stands as one of the greatest public health accomplishments of all time,” he says. “But a natural consequence of eradicating the one orthopoxvirus that circulated widely among humans, and then stopping the vaccination program that led to eradication, is that generations of people have no immune experience with any orthopoxvirus. There is no question that this makes life easier for monkeypox. It’s like a big pile of fuel that has never seen a spark.”
But this still doesn’t answer the prevailing question of why now? Studies show that monkeypox has been on the rise for several years in Nigeria, the Democratic Republic of the Congo (DRC), and other African nations, but the precise tipping point that has led to a much wider outbreak today remains a mystery. Scientists hope that further genetic sequencing and contact tracing will shed more light on exactly when and how the outbreak began.
“The two trends of declining immunity and growing air travel don’t explain why this is happening now and not a few years ago, or a few years in the future,” says Walker. “Maybe a random series of mutations in the last few months set this all off. Or maybe our luck ran out, and it took until now for a monkeypox case to make its way to a major population center, get on an airplane, and kickstart an outbreak without hitting a dead end.”
There is also the enigma of how exactly the virus is being passed from person to person. The UKHSA has suggested that the transmission routes could be sexual, based on data showing that a proportion of the confirmed cases are men who have sex with men, presenting with painful genital ulcers. However this has never been described before with monkeypox, and other scientists have cautioned against drawing such conclusions at this early stage of the outbreak, due to the potential stigma of characterizing an expanding outbreak as being fueled by sexual promiscuity.
“I don’t like to speculate because I do not think it is helpful and may fuel misinformation,” says Boghuma Titanji, a researcher at Emory University who has studied past monkeypox outbreaks. “Sexual transmission is always a possible mode of spread for any pathogen which spreads by close physical contact.” However, as new cases emerge and viruses are isolated and sequenced, it will become more possible to piece together transmission networks, Titanji says, and it may give us additional clues as to exactly how the virus spreads.