In a recent study posted to the medRxiv* preprint server, researchers illustrated the baseline trajectory of the monkeypox virus (MPXV) epidemic anticipated under the only effect of infection-derived or herd immunity.

Study: Accumulation of immunity in heavy-tailed sexual contact networks shapes monkeypox outbreak sizes. Image Credit: Dotted Yeti/Shutterstock

Background

The researchers firmly imagine that evaluation of the MPXV case trend is incomplete unless studies adjust for inherent saturation effects, similar to the heavy-tailed nature of the sexual partnership distribution amongst men having sex with men (MSM).

From November 2022, MPXV cases began declining across the US (US) and Europe after their rapid initial surge because the outbreak began in April 2022.

The present outbreak was novel because most cases were amongst MSM with no reported exposure to animals or travel history in endemic countries. Although the secondary attack risk (SAR), specifically amongst sexual partners, stays debatable, a big selection of sexual SAR values could have led to sustained outbreaks over MSM sexual contact networks. A heavy-tailed empirical distribution of sexual partners amongst MSM led to sustained human-to-human transmission on this population while not in others.

Many countries identified the present MPXV outbreak and introduced public health interventions, similar to contact tracing and vaccination. Increased awareness amongst high-risk populations also triggered behavioral changes, thus, reducing disease spread. Nonetheless, evidence is overall inadequate to quantify the relative contribution of those responses to the case decline in several countries.

Concerning the study

In the current study, researchers developed a mathematical model of MPXV transmission over the MSM sexual contact network accounting for infection-derived immunity. It modeled the chance of a person being in touch with an infectious sexual partner as directly proportional to the variety of their sexual partners over 14 days. They assumed that upon recovery, infected individuals would develop long-term immunity and maintain their sexual behavior without risk of re-infection. Moreover, the team directly modeled the connection between the cumulative variety of cases per MSM population and the effective reproduction number (R eff).

The researchers compared model outputs with observed MPXV outbreak data. They identified the period during which reported cases likely peaked in European countries, the US, and Canada. They fitted Gompertz curves to the cumulative reported case count over time in each of the included countries and US states to estimate the cumulative variety of MPXV cases per MSM population size by the cumulative incidence proportion at the height of an epidemic (CIPP). Moreover, the team anticipated similar CIPPs across different MSM populations in the event that they shared the identical partnership distribution and SAR.

 The “consensus range” is a set of values inside the CIPPs of no less than 50% of included countries/states. Nearly 70% of nations had their CIPP ranges overlapping at 0.24–0.27%. The consensus range amongst US states was 0.14%–0.65%, and CIPPs of 69% of US states shared 0.21–0.26% in common.

Study findings

The study model replicated MPXV epidemics over an MSM sexual contact network. It showed cases began to say no even before 1% of the MSM population experienced infection despite having an R 0 of above one. The study model suggested that with a plausible SAR in a highly heterogeneous MSM sexual contact network consistent with the observed heavy-tailed sexual partnership distribution amongst MSM, an epidemic rapidly hits the herd immunity threshold and starts to say no. It’d explain the present decline in MPXV cases in lots of countries, with different timing and intensity of interventions.

Moreover, the researchers noted that lots of the observed MPXV epidemics formed a peak when the cumulative variety of cases reached ~0.1–0.7% of the estimated sexually energetic MSM population size. The study model reproduced such patterns, with a SAR between 10 to 30% per sexually-associated contact without accounting for any interventions or behavioral change.

Moreover, the model projected that the declining phase of an epidemic in a heavy-tailed MSM contact network is likely to be gradual, especially if the SAR was high. Thus, whatever the aspects driving peak MPXV incidence, promoting and providing sustainable technique of prevention, particularly vaccination, to those in danger—not only in newly affected countries but in addition in countries where monkeypox has long been endemic—stays crucial to ending the worldwide epidemic. Sustained concerted efforts are needed to beat the consequences of immunity waning or turnover within the MSM population with probably the most partners, which could replenish susceptible individuals, and the epidemic potential.

Conclusions

The study results suggested that early infection of people with the best risks in a heavy-tailed sexual partnership distribution could have been sufficient to cause downward trends in monkeypox epidemics even without effective control measures. The authors reiterated that their findings don’t show the consequences of interventions and behavioral changes in the present MPXV outbreak. They explain CIPPs at each country- and US-state levels, that are of comparable order and substantially lower than the classical herd immunity threshold, even without interventions or behavioral changes.

Nonetheless, this pattern would have remained the identical if included countries and US states exhibited similar interventions or behavioral changes at their epidemic peaks. Moreover, MSM, with the best variety of partners, likely drove the present MPXV outbreak. Nonetheless, more data is required to discriminate the role of interventions and behavioral change from the saturation of infection. Until that’s clarified, attributing the decline in MPXV cases to those aspects alone may overstate their impact. Further studies incorporating these findings on the saturation effect from infection-derived immunity would facilitate a greater understanding of the evolving MPXV epidemiology.

*Essential notice

medRxiv publishes preliminary scientific reports that aren’t peer-reviewed and, due to this fact, shouldn’t be considered conclusive, guide clinical practice/health-related behavior, or treated as established information.