‘Network immunity’ may have helped mpox outbreak fade
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Key takeaways:
- The number of recent sexual partners of people with mpox declined over the course of the outbreak.
- Data suggest the outbreak faded because of infection-induced immunity among at-risk populations.
“Network immunity” may have helped halt the global mpox outbreak, according to a study presented at the European Congress of Clinical Microbiology & Infectious Diseases.
The theory, which researchers based on data collected by the Institute of Tropical Medicine Antwerp in 2022, suggests that a combination of behavioral change among populations at risk for mpox — for the most part, gay, bisexual and other men who have sex with men (MSM) — and, more significantly, infection-induced immunity among a core of these populations generated a network immunity that halted the outbreak.
“The decline in the number of partners reported by individuals diagnosed with mpox toward the end of the epidemic suggests a change in behavior of the population at risk,” Christophe Van Dijck, MD, PhD, a post-doctoral researcher in internal medicine at the Institute of Tropical Medicine Antwerp, said in a press release. “Therefore, we propose an alternative hypothesis.”
The 2022 mpox outbreak, which may have been exacerbated by waning smallpox immunity, probably smoldered for some time before officials recognized the growing number of cases in May.
Ninety-eight percent of people with mpox were reported to be gay or bisexual men, and sexual activity was suspected to cause 95% of mpox infections, according to findings published in July 2022 in The New England Journal of Medicine.
Cases declined significantly by the fall. Globally, more than 85,000 people were infected in the outbreak, Van Dijk and colleagues noted in their abstract, including more than 30,000 people in the United States, according to the CDC. Cases were reported in 110 countries.
As cases started to peak, vaccination programs were launched around the world, aimed at gay, bisexual and other MSM.
“Insufficient knowledge of the disease, as well as asymptomatic and pre-symptomatic transmission may have enhanced disease spreading,” Van Dijck said. “However, in most countries, including Belgium, the decline in mpox cases had already started before a substantial proportion of the population at risk had been vaccinated.”
Van Dijck and colleagues conducted two separate data analyses for the study presented at the conference, both at the Institute of Tropical Medicine Antwerp in 2022 on two separate questionnaires — one among patients with mpox upon their diagnosis and the second among men who were followed up for HIV PrEP.
The first analysis included 155 mpox cases, 95.5% of which occurred among MSM. The median number of sexual partners in the previous 3 weeks was two (range 0 to 50), but declined 0.86 partners per week (95% CI, 0.849-0.889) as the outbreak continued.
In the second analysis, among 1,322 men using PrEP, 99.6% were MSM and 55.9% regularly visited the clinic. Among these patients, the median number of partners in the previous 3 months was five (range 0 to 90). The researchers split these participants into two groups: a core-group defined as those with a history of syphilis and a noncore group without history of syphilis.
Core-group PrEP users reported 1.31 more partners per week during the outbreak than non-core-group members (95% CI, 1.04-1.63), and over time, the number of partners in both groups increased by 1.008 partners per week (95% CI, 1.006-1.009).
Although the numbers of partners of patients with mpox declined overall during the outbreak, suggesting behavioral changes in the at-risk population, Van Dijck and colleagues said the increase in partners among the PrEP-using population suggests that core members of the sexual network were infected first, with noncore and peripheral members being infected later.
The suggestion is that infection-induced immunity in the core population generated a “network immunity” among the core, noncore and other populations that halted the outbreak, the researchers said.
“We are currently working on serological and modeling studies to establish whether this hypothesis is true,” Van Dijck said. “In the meantime, we need to be aware that future mpox outbreaks may occur if the ‘network immunity’ is disturbed — for example, waning immunity of infected or vaccinated persons or when previously uninfected, peripheral members of the sexual network become more sexually active.”
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Van Dijck C, et al. Shift in sexual risk profile of mpox cases suggests “network immunity” contributed to epidemic decline. Presented at: European Congress of Clinical Microbiology & Infectious Diseases; April 15-18, 2023; Copenhagen, Denmark.
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