Exploring the effect of herd immunity on monkeypox outbreak sizes

In a recent study published in medRxiv* preprint server, the researchers elucidated the primary course of the predicted monkeypox virus (MPXV) epidemic under the sole influence of derived infection or herd immunity.

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


Researchers firmly believe that trend analysis of MPXV status is incomplete unless studies adjust for inherent saturation effects, such as the tail-heavy nature of the distribution of sexual partnerships among men who have sex with men (MSM).

As of November 2022, cases of MPXV have begun to decline across the United States (US) and Europe after their rapid initial rise since the outbreak began in April 2022.

The current outbreak was novel because most cases were among men who have sex with men and had no reported exposure to animals or a history of travel in the endemic countries. Although the risk of secondary attack (SAR), specifically among sexual partners, is still debated, a wide range of sexual SAR values ​​can lead to sustained outbreaks across MSM sexual contact networks. The heavy-tailed experimental distribution of sexual partners among MSM resulted in persistent human-to-human transmission in this population while it did not in others.

Several countries have identified current MPXV outbreaks and introduced public health interventions, such as Contact tracing and vaccination. Increasing awareness among the population at high risk has also led to behavioral changes, thus reducing the spread of the disease. However, the evidence overall is insufficient to determine the relative contribution of these responses to the decline in status in different countries.

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In this study, the researchers developed a mathematical model of MPXV transmission through the MSM sexual contact network to calculate infection-derived immunity. It modeled an individual’s risk of contact with an infectious sexual partner as directly proportional to the number of sexual partners they had over a 14-day period. They hypothesized that upon recovery, infected individuals would develop long-term immunity and maintain sexual behavior without the risk of re-infection. Furthermore, the team directly modeled the relationship between the cumulative number of cases per MSM group and the effective reproduction number (R eff).

The researchers compared the model’s output to the observed MPXV outbreak data. They determined the period when reported cases were likely to have peaked in European countries, the United States and Canada. They fitted Gompertz curves to the cumulative number of cases reported over time in both the included countries and the United States to estimate the cumulative number of MPXV cases per population size that has sex with men by the cumulative incidence ratio at the peak of the epidemic (CIPP). Moreover, the team predicted identical CIPPs across the different MSM groups if they had the same distribution of PSA and SAR.

A “consensus range” is a range of values ​​within the CIPPs for at least 50% of the countries/countries included. Almost 70% of countries have CIPP ranges overlapping at 0.24-0.27%. The consensus range among US states was 0.14%-0.65%, and the CIPPs 69% of US states share 0.21-0.26%.


The study model replicated MPXV epidemics across the MSM sexual contact network. It showed that cases began to decline even before 1% of the MSM population experienced infection despite an R of 0 above one. The study model suggested that with a SAR in a highly heterogeneous MSM sexual contact network consistent with the observed heavy-tailed sexual partnership distribution among MSM, the epidemic rapidly reaches the herd immunity threshold and begins to decline. It may explain the current decline in MPXV cases in many countries, with varying timing and intensity of interventions.

In addition, the researchers note that many of the observed MPXV epidemics formed a peak when the cumulative number of cases reached ~0.1–0.7% of the size of the sexually active MSM population. The study model reproduced such patterns, with SARs ranging from 10 to 30% per sex-related contact without accounting for any interventions or behavioral change.

Moreover, the model predicted that the recessive phase of the epidemic in a heavy-tailed MSM network might be gradual, especially if the SAR was high. Thus, regardless of the factors that lead to the peak incidence of MPXV, promotion and provision of sustainable prophylaxis, especially vaccination, for those at risk – not only in newly affected countries but also in countries where monkeypox has long been endemic – remains critical to ending global disease. epidemic. Ongoing concerted efforts are needed to overcome the effects of diminished or circulating immunity in the MSM community, which may rejuvenate susceptible individuals, and the potential for an epidemic.


The results of the study indicated that early infection of individuals with the highest risk in the heavy-tailed sexual partnership distribution may be sufficient to cause downward trends in monkeypox epidemics even without effective control measures. The authors reiterate that their findings do not show the effects of interventions and behavioral changes in the current MPXV outbreak. They explain CIPPs at both the state and US levels, which have a similar order and are well below the classical herd immunity threshold, even without interventions or behavioral changes.

However, this pattern would have remained the same if the included countries and the United States had shown similar interventions or behavioral changes at the peak of the epidemic. Moreover, it is likely that MSM, with the largest number of partners, drove the current MPXV outbreak. However, more data are needed to differentiate the role of interventions and behavioral change from infection saturation. Until this is clarified, attributing the decrease in MPXV cases to these factors alone may overestimate their effect. Further studies integrating these findings on the saturation effect of infection-derived immunity would facilitate a better understanding of the evolving epidemiology of MPXV.

*Important note

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, directing clinical practice/health-related behaviour, or treated as hard information.

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