Prevalence of mpox immunity among the core group and its potential to prevent future large-scale outbreaks

The 2022 mpox outbreak resulted in nearly 98 000 confirmed cases and more than 200 deaths across 116 countries,1WHO2022-2024 Mpox (monkeypox) outbreak: global trends.https://worldhealthorg.shinyapps.io/mpx_global/Date: June 26, 2024Date accessed: July 9, 2024Google Scholar leading to an international public health emergency. With another clade of the virus showing human-to-human transmission and intermittent reports of additional small-scale outbreaks, fears of another larger-scale mpox outbreak exist. However, such fears could be unfounded. We have argued that mpox is predominantly transmitted via sexual contact and thus should be considered a sexually transmitted infection.2Allan-Blitz LT Gandhi M Adamson P Park I Bolan G Klausner JD A position statement on mpox as a sexually transmitted disease.Clin Infect Dis. 2023; 76: 1508-1512Crossref PubMed Scopus (0) Google Scholar For other sexually transmitted infections, a mathematical, clinical-epidemiological, and sociocultural concept of the core group has helped to guide public health interventions and understanding. The core group is a subpopulation at disproportionate risk for acquiring and transmitting infection owing to some sexual behaviours (eg, concurrent partnerships, group sex, and increased number of new sexual partners) compared to the general population.3Thomas JC Tucker MJ The development and use of the concept of a sexually transmitted disease core.J Infect Dis. 1996; 174: S134-S143Crossref PubMed Google Scholar A core group contributes to disease transmission in the general population through secondary sexual networks. As most sexually transmitted infections have a basic reproduction number of <1, these infections cannot spread in the general population without transmission through core groups. The core group theory explains why the mpox outbreak in the summer of 2022 was characterised by limited spread within the general population, and why, after immunity developed through either recovery from infection or vaccination, the rates of infection fell within and never substantially spread beyond the core group. Robust immunity to monkeypox virus is observed after infection or immunisation. A 2024 report indicated that less than 1% of new mpox cases occurred among those who had received two doses of vaccination.4Guagliardo SAJ Kracalik I Carter RJ et al.Monkeypox virus infections after 2 preexposure doses of JYNNEOS vaccine - United States, May 2022–May 2024.MMWR Morb Mortal Wkly Rep. 2024; 73: 460-466Crossref PubMed Scopus (2) Google Scholar However, the exact proportion of the core group with immunity to the monkeypox virus remains unknown. Case counts of sexually transmitted infections tend to underestimate the burden of infection. A modelling study comparing reported cases of Chlamydia trachomatis infection to the estimated true prevalence of infection based on incomplete reporting, diagnostic test characteristics, and level of screening coverage indicated that the prevalence was underestimated by two-fold to six-fold.5Learner ER Powers KA Torrone EA Pence BW Fine JP Miller WC The influence of screening, misclassification, and reporting biases on reported chlamydia case rates among young women in the United States, 2000 through 2017.Sex Transm Dis. 2020; 47: 369-375Crossref PubMed Scopus (3) Google Scholar Mpox, similar to other sexually transmitted infections, is highly stigmatised but also was a disease rarely seen before 2022. Thus, a substantial proportion of individuals with mpox likely went undetected and undiagnosed. Large population-level surveillance data are scarce; however, a few studies have evaluated mpox seroprevalence or vaccination coverage to estimate the prevalence of immunity in different groups. A study in the UK among 84 men (predominantly men who have sex with men) indicated that 73% of the study population was immune to mpox either because of previous infection (22%) or vaccination (51%).6Rosenthal M Possible asymptomatic mpox points to need for increased testing and vaccination.Infectious Disease Special Edition. May 10, 2024; https://www.idse.net/HIV-AIDS-STIs/Article/05-24/Asymptomatic-Mpox-Points-to-Need-for-Increased-Testing-and-Vaccination/73750Date accessed: July 11, 2024Google Scholar In another study among 315 men who have sex with men presenting to sexual health clinics in the Netherlands, a 45% seroprevalence of mpox IgG antibodies was observed.7Shamier MC Zaeck LM Götz HM et al.Scenarios of future mpox outbreaks among men who have sex with men: a modelling study based on cross-sectional seroprevalence data from the Netherlands, 2022.Euro Surveill. 2024; 292300532Crossref PubMed Scopus (1) Google Scholar In a study among 12 jurisdictions in the USA, 22·5% of 917 sexually active men who have sex with men with ages ranging 18–49 years were fully immunised against mpox between 2022 and 2023.8McQuiston JH Braden CR Bowen MD et al.The CDC domestic mpox response - United States, 2022–2023.MMWR Morb Mortal Wkly Rep. 2023; 72: 547-552Crossref PubMed Google Scholar Thus, we expect the prevalence of immunity among the core group to be high and likely high enough to limit the spread of infection. This hypothesis is supported by a 2023 modelling study, which concluded that infection-induced immunity among the core group was the primary driver of reduced mpox virus spread in 2022.9Brand SPC Cavallaro M Cumming F et al.The role of vaccination and public awareness in forecasts of mpox incidence in the United Kingdom.Nat Commun. 2023; 14: 4100Crossref PubMed Scopus (17) Google Scholar Although future large-scale outbreaks are unlikely, vigilance remains essential. When adolescents and younger adults who have never been infected with or immunised against mpox begin to engage in sexual behaviours that might place them in the core group, the prevalence of immunity within the core group will decrease. Additionally, there is a possibility of new variants emerging that could escape existing immunity. Thus, continuing to vaccinate individuals with behaviours that put them at high risk of infection and to monitor for new cases is important. However, within the current landscape, we believe that the core group, and thus the general population, will remain protected against subsequent large-scale mpox outbreaks for the foreseeable future. We declare no competing interests. There was no funding source for this study.