A global outbreak of monkeypox has caused 87,000 cases and 116 deaths, with 90 of these in the Americas. The virus has been surprising experts since the first warnings were issued in May last year, when infections were detected outside the endemic zones of central and eastern Africa in London, Madrid, and Lisbon. The virus had previously demonstrated to be relatively inefficient at transmitting from person to person, but a surge of diagnoses from last spring caused concerns that the epidemic could spread further. The World Health Organization declared a public health emergency in July last year. However, diagnoses began to fall steadily after this decision, with current cases being much lower than the nearly 1,000 new cases reported daily last summer.
Belgian researchers have now discovered an explanation that has been supported by data: the “herd immunity effect,” caused by “network immunity.” The study found that “people located in the center of sexual networks were the first infected, which allowed them to achieve natural immunity that generated a ‘network immunity’ that stopped the epidemic” by protecting people in more peripheral positions in the contact networks, explains one of the authors, Christophe Van Dijck of the Institute of Tropical Medicine in Antwerp. This explains why “in most countries, the decline in cases had already begun before a significant part of the population at higher risk had been vaccinated” in an environment of a severe shortage of doses that has not yet been resolved.
According to the WHO, 96.4% of those infected since May are young men, almost all of whom attend events or parties where sexual activity plays a central role. The virus has found an effective way to spread through close contact and the exchange of fluids that occur in these situations. There have also been other forms of transmission in the household and social spheres, which have affected women and children, but in these cases, the virus has behaved as expected, with relatively few transmissions and short, easy-to-interrupt chains of transmission. The international, closely interconnected network of the gay community was what facilitated the rapid expansion of the virus, according to the study. Cities with a vibrant nightlife and international festivals became nodes in this network, where the virus was able to propagate inside and spread to other nodes through travel.