Investigating the effects of herd immunity on monkeypox outbreak size

Investigating the effects of herd immunity on monkeypox outbreak size

In a recently published study medRxiv * Preprint server, the researchers illustrated the initial course of the monkeypox virus (MPXV) epidemic, which is expected under the sole effect of infection-derived or herd immunity.

Lernen: Die Akkumulation von Immunität in stark beschwänzten sexuellen Kontaktnetzwerken prägt die Größe des Affenpockenausbruchs. Bildnachweis: Dotted Yeti/Shutterstock

background

Researchers strongly believe that MPXV case trend analysis is incomplete unless studies account for inherent saturation effects, such as: B. the strong distribution of sexual partnerships among men who have sex with men (MSM).

As of November 2022, MPXV cases in the United States (US) and Europe were declining after rapidly increasing since the outbreak began in April 2022.

The current outbreak was novel in that most cases occurred among MSM, with no reported exposure to animals or history of travel to endemic countries. Although secondary attack risk (SAR), particularly among sexual partners, remains controversial, a wide range of sexual SAR levels may have led to persistent outbreaks across MSM sexual contact networks. A strong empirical distribution of sexual partners among MSM resulted in sustained person-to-person transmission in this population, while this was not the case in others.

Many countries have recognized the current MPXV outbreak and implemented public health interventions such as contact tracing and vaccination. Increased awareness among high-risk groups also triggered behavioral changes, thereby reducing the spread of disease. However, overall evidence is insufficient to quantify the relative contribution of these responses to case declines in different countries.

About studying

In the present study, researchers developed a mathematical model of MPXV transmission through the sexual contact network of MSM that takes into account infection-induced immunity. It models the risk of a person coming into contact with an infectious sexual partner as being directly proportional to the number of their sexual partners over 14 days. They assumed that once infected people recovered, they would develop long-term immunity and maintain sexual behavior without the risk of reinfection. In addition, the team directly modeled the relationship between the cumulative number of cases per MSM population and the effective reproduction number (R eff).

The researchers compared the model results with observed MPXV outbreak data. They identified the period when reported cases likely peaked in European countries, the United States and Canada. They fitted Gompertz curves to the cumulative reported number of cases over time in each of the included countries and US states to estimate the cumulative number of MPXV cases per MSM population size by the cumulative incidence proportion at the peak of an epidemic (CIPP). Furthermore, the team expected identical CIPPs across different MSM populations if they shared the same partnership distribution and SAR.

The “consensus range” is a range of values ​​within the CIPPs of at least 50% of the countries/states included. For almost 70% of countries, the CIPP ranges overlapped by 0.24-0.27%. The consensus range among US states was 0.14%-0.65%, and the CIPPs of 69% of US states had 0.21-0.26% in common.

Study results

The study model replicated MPXV epidemics across an MSM sexual contact network. Cases were found to be decreasing even before 1% of the MSM population became infected, despite an R 0 above one. The study model suggested that with a plausible SAR in a highly heterogeneous MSM sexual contact network consistent with the observed distribution of sexual partnerships among MSM, an epidemic quickly reaches the herd immunity threshold and begins to decline. This could explain the current decline in MPXV cases in many countries with different timing and intensity of interventions.

Additionally, researchers found that many of the observed MPXV epidemics peaked when the cumulative number of cases reached ~0.1-0.7% of the estimated size of the sexually active MSM population. The study model reproduced such patterns with a SAR between 10 and 30% per sexually associated contact, without taking into account interventions or behavioral changes.

Furthermore, the model projected that the declining phase of an epidemic could be gradual in a strong-tailed contact network of MSM, particularly when the SAR was high. Therefore, regardless of the factors causing the peak incidence of MPXV, promoting and providing sustained prevention measures, particularly vaccinations, to vulnerable individuals – not only in newly affected countries but also in countries where monkeypox has long been endemic – remains critical to ending the world epidemic. Continued concerted efforts are needed to overcome the impact of waning immunity or turnover in the most partnered MSM population, which could replenish vulnerable individuals, and the epidemic potential.

Conclusions

The study results suggest that early infection of those at highest risk in a distribution of heavy-tailed sexual partnerships may have been sufficient to cause downward trends in monkeypox epidemics even without effective control measures. The authors reiterated that their results do not show the impact of interventions and behavioral changes in the current MPXV outbreak. They explain CIPPs at both the national and US state levels that are of similar magnitude and substantially lower than the classic herd immunity threshold, even in the absence of interventions or behavioral changes.

However, this pattern would have remained the same if the included countries and US states had demonstrated similar interventions or behavioral changes at their peak of the epidemic. Furthermore, MSM with the highest number of partners is likely to have driven the current MPXV outbreak. However, more data are needed to differentiate the role of interventions and behavior change from infection saturation. Until this is clarified, attributing the decline in MPXV cases to these factors alone may overstate their impact. Further studies incorporating these findings on the saturation effect of infection-induced immunity would facilitate a better understanding of the evolving MPXV epidemiology.

*Important NOTE

medRxiv publishes preliminary scientific reports that have not been peer-reviewed and therefore should not be considered conclusive, guide clinical practice/health-related behavior, or treated as established information.

Reference:

• Hiroaki Murayama, Carl AB Pearson, Sam Abbott, Fuminari Miura, Sung-mok Jung, Elizabeth Fearon, Sebastian Funk, Akira Endo. (2022). Die Akkumulation von Immunität in stark beschwänzten sexuellen Kontaktnetzwerken prägt die Größe des Affenpockenausbruchs. medRxiv. doi: https://doi.org/10.1101/2022.11.14.22282286 https://www.medrxiv.org/content/10.1101/2022.11.14.22282286v1

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