Vaccines seen as the main hope for stopping a now-global outbreak of a new, deadlier strain may not even work, scientists have warned.
Professor Marion Koopmans, director of the Centre for Pandemics and Disasters at Erasmus Medical Centre in the Netherlands, said experts simply do not know how the new clade 1b mpox virus would react to current vaccines.
“The honest answer is that we don’t know yet,” he said.
Current mpox vaccines were used during the 2022 global outbreak against the mildest strain of the virus, known as clade 2.
But they still need to be tested against the more potent version that has spread across Africa and has now been detected in Europe and Asia.
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And experts aren’t even sure exactly how much protection they provided against the 2022 outbreak.
The vaccine is, in fact, the vaccine given to prevent smallpox, a close relative of mpox.
Because of the similarity between the two viruses, experts believed it would be effective, which is known as cross-protection.
However, because it was implemented during a live outbreak, it was difficult to determine exactly what benefit it provided, says Professor Koopermans.
During the 2022 outbreak, gay and bisexual men (the group primarily affected by mpox) are thought to have taken other steps to reduce their risk, such as limiting the number of new sexual partners.
“There is some evidence of clinical efficacy during vaccinations that were given during an evolving outbreak, when there are also other things that people do that reduce transmission,” Professor Koopmans told reporters.
‘It is not so easy to say whether the vaccine offers complete protection.
‘The hope is that for clade 1b there will also be sufficient cross-protection, but that is an area in urgent need of study.’
Professor Dimie Ogoina, an infectious disease expert at Niger Delta University, who also spoke at the event, added that another factor that could pose a challenge to the potential rollout of the mpox vaccine in Africa is who gets it first.
“Any vaccination strategy must be based on knowledge of the epidemiology of the disease in the region,” he said.
“I’m not sure we fully understand the transmission dynamics and risk factors for mpox in many parts of Africa.”
Like Professor Koopmans, he also highlighted uncertainties about the efficacy of current mpox vaccines.
“The vaccine efficacy studies that have been done were done in the Global North for clade 2b and among gay and bisexual men,” he said.
‘It cannot be guaranteed that the efficacy was based solely on the vaccine.
‘Some studies have shown that behavioural change was responsible for the decline in mpox cases in parts of Europe and America, while vaccines also helped.’
Professor Ogoina added that there were also uncertainties, such as how long the vaccines offered protection and how effective they were in children who appear to be particularly at risk in the new outbreak.
“We have not replicated these studies in children and that is a big challenge especially in the DRC (Democratic Republic of Congo), where most of the significantly affected people are children,” he said.
“We should use a risk/benefit approach, especially in an outbreak context, when deciding whether to vaccinate children.”
The smallpox vaccine is known to help prevent MPOX because the two viruses are closely related. But experts said there was not yet enough evidence to suggest a vaccine would be effective against the new clade 1b strain.
But Professor Placide Mbala Kingebeni, an epidemiology expert at the Centre de Recherche Clinique at the National Institute for Biomedical Research in the Democratic Republic of Congo, the country worst hit by the current outbreak, said vaccines remained “the best tool we have”.
“Even though we don’t have all the data on efficacy, it’s something we should do,” he said.
The new strain of mpox, formerly known as monkeypox, is much more lethal than the mild strain that spread to more than a dozen countries, including the United Kingdom, in 2022.
Engineered with the mpox clade 1b, it kills about one in 20 adults it infects, but the mortality rate rises to one in 10 in children.
The virus is spread mainly through skin-to-skin contact, which may include sexual intercourse, or through direct care, for example from mother to child.
Clade 1b has swept through Central Africa, killing hundreds of people since the outbreak began.
In recent weeks alone, cases of the new strain have been detected in Sweden and Thailand, meaning it has now reached both Europe and Asia.
Although there are no confirmed cases in the UK yet, experts suspect the new variant is already in Britain as it can take more than two weeks for symptoms such as classic skin lesions to develop.
But experts have said the mortality rates of central African clade 1b are unlikely to be replicated in developed countries because of better access to higher quality healthcare.
The UK Health Security Agency (UKHSA) has already warned that it is “already making plans” for cases of the new strain in the UK.
The news comes after the World Health Organization said it needs more than £66 million ($87 million) over six months to stop the current mpox outbreak.
Mpox normally cIt causes characteristic lumpy lesions, as well as fever, aches and fatigue.
In a small number of cases, it can enter the blood and lungs, as well as other parts of the body such as the brain, causing it to become potentially fatal.
