The world is highly vulnerable to a deadly pandemic caused by a group of viruses related to smallpox, a top global health expert has warned. Smallpox is estimated to have killed up to 500 million people in the 20th century before it was finally eradicated in 1980. It killed around three in 10 people infected and left many more with side effects such as permanent scarring or blindness. A global vaccination campaign led to its eradication, with the last naturally occurring case recorded in Somalia in 1977.

However, humanity’s huge success in defeating smallpox may have left it vulnerable to related viruses. Dr Raina MacIntyre, a professor of global diosecurity at the University of New South Wales in Australia, said: “Before smallpox was eliminated, people were frequently exposed to the virus and there were mass vaccination campaigns that resulted in a baseline of protection to orthopoxviruses more broadly. But the population of the world, at the moment, are just sitting ducks for any orthopox virus emergence, because we don’t have immunity.”

Dr Jonas Albarnaz, a senior virologist at the Pirbright Institute, told The Telegraph: “Public health experts predicted before eradication that other orthopoxviruses could ‘occupy’ the niche vacated by variola virus [which caused smallpox], with mpox a prime candidate.”

Related viruses in the orthopoxvirus family include mpox, which is currently causing an outbreak in Madagascar.

More than 260 suspected and 94 confirmed cases had been recorded by the country’s Ministry of Public Health as of January 19.

Previously known as monkeypox, mpox is an infection that causes a rash and flu-like symptoms.

A surge in mpox cases in the Democratic Republic of the Congo and neighbouring African countries also led to the declaration of a Public Health Emergency of International Concern in 2024.

Pandemics can be triggered when a virus mutates to become more easily transmissible — something that is more likely to happen when it is spreading uncontrolled.

A new hybrid strain of mpox was reported in the UK last year, which was a mix of the clade I and clade II strains.

Dr Boghuma Titanji, assistant professor of medicine at Emory University in Georgia, US, said the discovery of a recombinant strain was “precisely what experts in the field feared would happen if the virus continued to spread globally without a decisive response to stop it”.

She added: “Orthopoxviruses [like mpox] are well known for their ability to exchange portions of their genome and recombine to generate new variants. This is a core mechanism of their evolution.”