The latest virus to make the news is the Clade Ib strain of mpox, it looks like a person was infected overseas but infected two members of their household in the UK. There seems to be a lot of confusion as to how it spreads, The Guardian, the NHS, and the USA’s CDC all have different ideas.
So I looked for reviews in the literature, there is a recent review, from January of this year by Kuehn and coworkers. I was struck by this from the review:
In situations in which investigators could identify a single route of human-to-human transmission resulting in infection, 2366/4309 (54.9%) cases were transmitted via direct physical sexual contact, 6/4309 (0.1%) cases were transmitted via direct physical non-sexual contact, and 2/4309 (0.0%) cases via fomites.
OK, so it seems clear that having sex with someone infected with mpox is asking to become infected. But of the 1,943 remaining cases, they only claim to know how transmission occurred in 8 of them. And their claim that 6 were “transmitted via direct physical non-sexual contact” is based on their references 31 – Doshi and coworkers – and 121 – Yadev and coworkers – and oddly neither paper mentions transmission via direct physical non-sexual contact at any point that I can find. For example, Table 2 of Doshi and coworkers, lists how what they think the exposure was and it is mainly but not exclusively household contacts, i.e., lived with someone infected. This is mostly as a family member such a son or sister, not a partner.
So how did these 1,943 people become infected? Well absence of evidence is not evidence of absence so in some cases the transmission may occur during sexual contact, but there is at least limited evidence of transmission within a household.
How did this occur? Was this airborne transmission? In other words, is mpox mostly transmitted between sex partners, with less frequent transmission via a similar mechanism to COVID and flu, albeit much less infectious than COVID and flu?
This is a hard question to answer, as disease transmission is complex. As far as I know there are no known superspreading events for mpox. This suggests (but does not prove) that there is at least one difference between mpox and COVID-19: for COVID-19 it seems likely that some people breathe out huge quantities of virus, much more than do others. Possibly because the virus is (mostly?) being aerosolised (i.e., becoming airborne) from skin lesions, not coming from the respiratory tract as SARS-CoV-2 does. Perhaps the amount of airborne virus being shed by someone with mpox is always low? Something like an infectious dose an hour or so?
This would explain the absence of superspreader events while also being consistent with transmission amongst people who live with, but don’t have sex with, an infected person. But this is just a hypothesis. A hypothesis we don’t have the data to test.
Fortunately for us mpox is much less infectious than COVID-19. It would have to evolve to become much more infectious, to cause a pandemic. But this is just luck. The NHS guidance talks about infection via “the coughs or sneezes of a person with mpox when they’re close to you”. There is no evidence that I can see that you have to be close to someone to become infected, and this is strongly reminiscent of the beliefs of some, especially early in the pandemic, that you had to be close to someone to catch COVID-19. This assumption was not based on evidence, and when the evidence came along it was proved wrong.
The UK’s COVID-19 enquiry is ongoing. Hopefully we will learn some lessons from the pandemic, before a disease more infectious that mpox comes along. We haven’t learnt much so far.
Chance & Necessity 