In the decades since smallpox vaccination ended, the proportion of people protected against monkeypox has fallen substantially, allowing the virus to spread more easily from animals into humans and from person to person, fuelling the risk of a major outbreak.

“This outbreak was really waiting to happen,” said Dr Romulus Breban, a researcher at the Pasteur Institute in Paris. Mathematical modelling by Breban and his team in 2020 found that in the Democratic Republic of the Congo, for example, immunity to monkeypox fell from 85% in the early 1980s to 60% in 2012. With declining immunity, monkeypox posed “an ever-increasing threat for health security”, he wrote at the time. In 2020 the DRC had more than 4,000 suspected cases and at least 171 deaths.

“Our immunity level is almost zero,” Breban said. “People aged 50 and above are likely to be immune but the rest of us not, so we are very, very susceptible.” He believes the outbreak can be contained and said it was an opportunity to propose vaccination campaigns in countries where the virus is endemic.