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鈥檛 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鈥檛 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鈥檛 mean it can spread via the air, she says. 鈥淭here is no evidence of any cases where there鈥檚 airborne transmission.鈥�
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聽Ib is even more transmissible than IIb, with signs of increased spread outside of households.聽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鈥檛 yet confirmed, says at the University of Cambridge. 鈥淭here isn鈥檛 any hard evidence.鈥� 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鈥檚 spread in wealthier countries.
鈥淭hey were highly effective,鈥� says at the International Society for Infectious Diseases. 鈥淭hey brought down case numbers to really very low levels.鈥�
It remains to be seen whether Ib can be contained by measures such as contact tracing, not least because we don鈥檛 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. 鈥淭here is a potential for that to happen,鈥� 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. 鈥淭here鈥檚 a chance that it can change in a way that we can鈥檛 predict,鈥� he says.
Assuming Ib doesn鈥檛 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鈥檛 limited to a relatively small high-risk group, as with IIb.
鈥淭here aren鈥檛 currently 10 million shots of vaccine available,鈥� 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聽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. 鈥淎nd we鈥檒l go聽through this whole cycle all over again.鈥�
