What drives sustained growth of monkeypox cases

Sustained transmission in heterosexual population is unlikely in all scenarios studied

Sustained transmission in heterosexual population is unlikely in all scenarios studied

As of June 15, a total of 1,882 cases of monkeypox have been confirmed in the lab from more than 30 countries around the world. With 1,158 confirmed cases from 22 countries, Europe has reported the highest number of cases to date, according to the European Center for Disease Prevention and Control. The US reported 83 cases in 19 states on June 16.

Although monkeypox is endemic to about a dozen countries in Central and West Africa, the virus is not endemic to humans. “Almost all” monkeypox outbreaks in these countries are due to the virus jumping across the species barrier from animals to humans.

Persistent human-to-human transmission in the endemic countries is “rare,” write Dr. Christian Happi of the African Center of Excellence for Genomics of Infectious Diseases (ACEGID), Redeemer’s University, Nigeria and others in a report placed in Virology Research. org.

Outbreaks in Africa’s endemic countries have witnessed only a “limited” proportion of cases spreading among humans, with sustained transmission of only up to seven generations being observed. And the baseline reproduction number (R0) for monkeypox in the endemic countries was less than one, even among populations that have not received smallpox vaccination. If the base reproduction number is less than one, that would mean that “outbreaks would disappear if there were no continued animal introductions,” Dr. Akira Endo of the London School of Hygiene & Tropical Medicine, London.

In contrast, the current outbreak in the US, Europe, the Middle East and Australia has shown a clear trend of continued spread among people, particularly among men who have sex with men (MSM). Before this outbreak, there were about 100 cases of monkeypox outside of Africa, with the 2003 US outbreak being the largest with more than 70 people infected with the virus. But all 70 cases were due to exposure to imported animals with no human-to-human transmission reported at the time. In the past, outside of Africa, the virus had spread to just one health worker and two household contacts, a far cry from the ongoing human-to-human transmission being reported.

Modeling exercise

To explain the vastly different pattern of distribution to a large number of people within a month of the first case being reported in the UK on May 7, Dr. Endo and others developed a mathematical model based on data on sexual partnerships in the UK to explain the epidemiology of the current outbreak. The results of the modeling exercise have been published in medRxiv preprint server; preprints have yet to be peer-reviewed.

Increased imports of the virus, undetected spread in the community, virus evolution and increased susceptibility to infection with monkeypox virus due to the cessation of smallpox vaccination several decades ago could be possible reasons for the current outbreak outside Africa. The researchers point out that most of these factors are “not strongly supported by external (albeit circumstantial) evidence” nor can they explain the human spread, primarily among men who have sex with men (MSM) and not widespread in the general population. population in the community.

The driver

The researchers found that a “small proportion of individuals with a disproportionate number of partners” must be responsible for the ongoing human-to-human transmission in men who have sex with men.

This is the largest outbreak, mainly among men who have sex with men. In Nigeria, a higher prevalence and lesions in the genital area have been identified in a recent outbreak.

“Our model suggests that the MSM population may have always been at risk for an ongoing outbreak. The reason we haven’t seen an outbreak among MSM before could be that the virus hadn’t reached this network, given the number of cases in total,” Dr. And do.

The researchers explain this in more detail in the preprint when they say that even in the past, the monkeypox virus has always had “significant transmission potential in the MSM” community, but the small number of imported cases outside Africa likely prevented the virus from spreading. spread to this country. network results in large-scale transmission.

The researchers also modeled outbreaks that are sexually associated in both MSM and non-MSM populations. They considered different assumptions for the risk of transmission between sexual partners in both groups.

Unlike a small fraction of MSM individuals who have a large number of partners, increasing the spread of the virus to many people in the MSM community, the researchers found continued spread in the non-MSM population “unlikely” because of the smaller chance of a few individuals having multiple sex partners. But “an additional 10-10,000 cases can be observed if a significant number of infections are introduced into the non-MSM sex network,” they write.

Sowing the outbreak

They note that a “very small number of sexually related transmissions” in the MSM community is enough to trigger a major outbreak. In contrast, a large number of non-sexually-associated imported cases are needed for the virus to gain a foothold in the MSM community and then spread to a large number of people via sexual-associated spread. And this may be why the virus failed to reach the MSM community through imported non-sexual cases in the past. They note that better surveillance in the endemic countries might be needed if the current outbreak outside Africa in the MSM community was the result of a few sexually related cases imported from the endemic countries.

“Without adequate interventions, we may see continued growth in MSM. The Monkeypox virus can infect anyone; the non-MSM population will also be affected if we don’t contain it. Tailored and non-stigmatizing messaging/support for those most at risk would be key to getting as soon as possible,” tweeted Dr. And do. Targeted messaging is necessary for prevention and early detection of cases among MSM who have multiple partners, the authors write.

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