Why is mpox being transmitted mainly between children?

One of the big differences between the mpox outbreak spreading across Africa and the 2022 outbreak in Europe is how it is being transmitted. While transmission two years ago was confined almost exclusively to sexual intercourse, now the victims are mainly children, who account for 70% of cases. It seems that something has changed in the way in which this new variant (Clade 1b) is transmitted, and the extent to which this has changed is still to be clarified. But perhaps it is not changes in the virus we should be looking at so much as changes in the environment.

More and more voices are questioning assumptions made about this new variant of monkeypox (renamed mpox): that it is more dangerous, that it is more transmissible, and that it generates more serious disease. All this is possible, but it is yet to be confirmed, something that will require more precise information than that coming from a context as complicated as that of the Democratic Republic of the Congo (DRC), a place with very few diagnostic and clinical resources in the midst of a military conflict, with terrible poverty and overcrowding.

Last week, it was the European Centre for Disease Control (ECDC) that warned of the unknowns in the face of the growing number of warnings from experts about the greater virulence and lethality of the new variant: “Although in the past it has been reported that the morbidity and case fatality rate of Clade 1 are higher than those of Clade 2, the current preliminary data from Africa do not show greater clinical severity in confirmed cases,” the ECDC stated, something which has been echoed by virologist José Antonio López Guerrero in an interview in EL PAÍS.

Case fatality is calculated by dividing the number of cases by the number of deaths. The less accurate the diagnosis, the more biased — and higher — that figure will be. “There is under-diagnosis in all diseases, even here [in Europe] where there is much better health coverage, so lethality is an unknown quantity right now,” says Adrián H. Aginagalde, coordinator of the Academy of Medical Sciences of Bilbao’s Public Health Section.

What is not in doubt is that transmission is behaving differently. But this should not be surprising either, according to Aginagalde: “Poxviruses (such as smallpox, and now mpox) have always spread more among children than adults, and more among women than men, because it is usually women who take care of children. This is not new, and it has already happened with mpox in Africa,” he says.

There are many infections that have a higher rate of transmission in children, because children’s way of interacting involves much closer physical contact: they play, fight and touch each other more. In pathogens whose spread requires close proximity or very direct contact, it is not uncommon for them to be the most affected. The context has changed with respect to 2022, in which it was transmitted mainly during sexual encounters, with the largest number of cases occurring among bisexual people and gays.

Aginagalde points out that poxviruses have great “flexibility” when it comes to adapting to the population, so that changes in the sociodemographic profile of those affected “do not necessarily indicate, although neither do they exclude” variations in the transmission of the virus produced by changes in their genetics.

In principle, the transmission route is still considered to be direct and continuous contact. That clearly exists in the case of sexual relations, but it also happens when touching the secretions of an infected person, touching their clothing or even talking in close proximity for a period of time, with the potential for saliva droplets to be exchanged. These are the possible transmission routes cited by the World Health Organization (WHO). The more contact and the longer the contact, the greater the likelihood of transmission.

“It’s not the new Covid”

This is very different from the airborne aerosol transmission that occurred with SARS-CoV-2. And it is one of the reasons why the evolution of the epidemic is expected to be slower and different in other ways. “Mpox is not the new Covid,” said Hans Kluge, director general of the WHO in Europe, on August 20.

Rafael Toledo, professor of immunoparasitology at the University of Valencia, says that there are many differences between mpox and Covid. “The most obvious is its size. The mpox virus is twice the size of SARS-CoV-2. That makes it virtually impossible for it to mutate in a way that would allow it to stay in the air for long,” he says. “If you are on a bus next to someone for 20 minutes, you might get infected through their breath, or by contact, but it is not going to infect the driver.”

According to Toledo, if there was airborne transmission via aerosols, there would be a much greater age span in the cases. Although a very different phenomena, Toledo compares the transmission of mpox to that of head lice: “We could say that they are transmitted through the air, because they jump from one person to another. But there has to be proximity. And it is usually children who are most affected, because they have more of this type of contact. But if we change the context — say, for example, there’s an outbreak in a war zone — the affected population may be crowded together.”

According to Toledo, the most frequent transmission is still fluid exchange, and he believes it is very difficult to assess how this new variant might behave in Europe. The ECDC has described the risk among the European population as “low,” and this is something Toledo agrees with. “It is a different scenario, a different disease, with a much simpler traceability, in which migration has little influence, because it has an incubation period of two weeks, and anyone coming from the DRC to Spain does not make the trip in four hours. It takes a long time, and in that case the disease has already manifested itself, because it is very visible,” he adds.

The previous outbreak of mpox in Europe was controlled precisely by enhancing the traceability of cases, providing information to at-risk people, and facilitating vaccination. From the 7,521 infected persons that Spain recorded in 2022, the number of cases fell to 264 this year, none of them, as far as is known, suffering from the new variant.

Experts from the WHO, the ECDC and the Spanish Ministry of Health are starting to be more vigilant. In Spain, the Public Health Commission will issue recommendations on August 21, after the meeting held on August 20 by the Alert Committee, in which measures such as border control and mass vaccination were ruled out to align with international organizations. The idea is to put the focus on the DRC and neighboring countries, where the new variant of the virus has started to be detected for the first time.

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