Scientists are speculating whether the waning smallpox vaccine protection could be behind the recent Monkeypox outbreak that has seen more than 200 confirmed or suspected cases in over a dozen countries.

Owing to the mass immunisation campaign, naturally occurring smallpox was wiped out worldwide by 1980. The smallpox vaccine also offered the bonus of strong protection against Monkeypox.

"Waning of immunity from smallpox vaccination may be contributing to the increasing outbreaks of Monkeypox," Prof. Raina MacIntyre, from the University of New South Wales, was quoted as saying to the Medical Journal of Australia. "It has been more than 40-50 years since mass vaccination ceased," she said.

A study published in the journal PLOS Neglected Tropical Diseases, in February, warned Monkeypox cases were rising as a result of the cessation of widespread smallpox vaccination, as the virus was declared eradicated by the World Health Organisation (WHO).

According to Dr Romulus Breban, a researcher at the Pasteur Institute in Paris, the "outbreak was really waiting to happen", the Guardian reported.

"Our immunity level is almost zero. People aged 50 and above are likely to be immune but the rest of us are not, so we are very, very susceptible," he was quoted as saying.

He, however, believes the outbreak can be contained and said it was an opportunity to propose vaccination campaigns in countries where the virus is endemic. Further, the surge in cases has also raised questions about whether the monkeypox virus has evolved into a more transmissible form.

So far scientists have found no evidence for this being the case, but researchers are studying the DNA to see if mutations in the virus may have changed its behaviour. Genetic studies so far suggest the virus matches strains that reached the UK, Singapore and Israel in 2018 and 2019.

The WHO also believes there is no evidence that the Monkeypox virus has mutated. Viruses in this group "tend not to mutate and they tend to be fairly stable," Dr Rosamund Lewis, who runs the WHO's smallpox research, said recently.

Meanwhile, several countries are planning to stock up on the existing smallpox vaccines that are reportedly 85 per cent effective against the monkeypox virus. The UK has secured a supply of tecovirimat and is offering a smallpox vaccine to close contacts of people diagnosed with monkeypox to reduce the risk of symptomatic infection and severe illness.

Sources suggest an extra 20,000 doses of the vaccine have been ordered to add to the UK's stock of 5,000, Guardian reported. The US is also preparing to give monkeypox vaccines to close contacts of people infected and to deploy treatments, with five cases now either confirmed or probable and the number likely to rise. EU health authorities have also advised member states to prepare a monkeypox vaccine plan.

However, the WHO said it does not believe the monkeypox outbreak outside of Africa requires mass vaccinations as measures like good hygiene and safe sexual behaviour will help control its spread.