A new mpox variant called clade Ib that emerged in the Democratic Republic of the Congo last year has now spread to several neighbouring countries. Sweden and Thailand have also each detected a case in individuals who travelled from this region. So, how serious a threat is it?
It is clear that mpox clade Ib is nowhere near as transmissible as respiratory viruses such as SARS-CoV-2, and thus it almost certainly won’t cause a pandemic on the scale of covid-19. However, initial evidence suggests that clade Ib is significantly better at spreading from person to person than other mpox variants, such as clade IIb, .
Mpox, formerly known as monkeypox, normally infects rats and squirrels. There are two main types of mpox, clade I and clade II, with the former circulating in Central Africa and the latter in West Africa. These viruses occasionally spill into people, but the resulting outbreaks have usually been small and limited to households, because the virus isn’t good at spreading among people. Most infections involve close contact, including, but not limited to, sexual contact, says at the Liverpool School of Tropical Medicine in the UK.
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People can also be infected by touching contaminated objects, such as bedding, but this is rarer. , but this doesn’t mean it can spread via the air, she says. “There is no evidence of any cases where there’s airborne transmission.”
The ongoing outbreak of clade IIb, a type of mpox clade II, is the biggest and by far the most widespread, but it has been limited almost entirely to men who have sex with men. In Europe, . Genetic studies revealed that IIb had been circulating in people for several years prior to its detection.
Now, Ib cases are surging. Reports state that Ib is even more transmissible than IIb, with signs of increased spread outside of households. However, because of limited testing, it is not clear how many of the 20,000 reported cases in all affected African countries so far this year are due to IIb.
Whether it really is more transmissible isn’t yet confirmed, says at the University of Cambridge. “There isn’t any hard evidence.” The picture will become clearer as any Ib outbreaks outside of endemic regions are studied.
In 2022, the expectation was that standard measures such as contact tracing would halt the spread of IIb, which turned out not to be the case. One reason for this may be that people infected with IIb can become infectious several days before showing symptoms or even without showing any symptoms at all. But offering vaccines to those at high risk has greatly reduced the virus’s spread in wealthier countries.
“They were highly effective,” says at the International Society for Infectious Diseases. “They brought down case numbers to really very low levels.”
It remains to be seen whether Ib can be contained by measures such as contact tracing, not least because we don’t know if it can spread before people show symptoms. It is possible that countries with effective healthcare systems will manage to contain outbreaks, but other nations may not. “There is a potential for that to happen,” says Ferguson.
If Ib does spread widely in some countries, there is also a risk of mpox becoming established in animal populations outside of West and Central Africa, which could lead to future outbreaks starting in non-endemic areas.
On the plus side, mpox evolves more slowly than viruses such as SARS-CoV-2. It is therefore thought to be very unlikely that it could evolve to evade existing vaccines. But Ferguson stresses that such conclusions are based on probability. “There’s a chance that it can change in a way that we can’t predict,” he says.
Assuming Ib doesn’t change much, vaccination campaigns should be able to bring any wider outbreaks under control. But vaccine availability is an issue and could become even more of a problem if Ib isn’t limited to a relatively small high-risk group, as with IIb.
“There aren’t currently 10 million shots of vaccine available,” says Ferguson. That is the number estimated to be needed to halt the clade Ib outbreak just in Central Africa. Manufacturers are ramping up production but it will take time.
Because of limited testing and not all cases being reported, there is a lot of uncertainly about the fatality rate of Ib, with one report that the case fatality rate is as low as 0.6 per cent. In countries with better healthcare systems, it could be even lower, as with IIb.
Tambyah thinks Ib will behave similarly to IIb, but it is unlikely to spread to the same extent if countries act quickly, given that they have dealt with mpox before and people may have some immunity. But if nothing is done to stop mpox outbreaks in West and Central Africa, more variants will emerge, he says. “And we’ll go through this whole cycle all over again.”
