Evaluation of Public Health Contact Tracing for Mpox Among Gay, Bisexual, and Other Men Who Have Sex With Men—10 US Jurisdictions, May 17–July 31, 2022

Abstract

Objectives. To examine the potential impact of contact tracing to identify contacts and prevent mpox transmission among gay, bisexual, and other men who have sex with men (MSM) as the outbreak expanded.

Methods. We assessed contact tracing outcomes from 10 US jurisdictions before and after access to the mpox vaccine was expanded from postexposure prophylaxis for persons with known exposure to include persons at high risk for acquisition (May 17–June 30, 2022, and July 1–31, 2022, respectively).

Results. Overall, 1986 mpox cases were reported in MSM from included jurisdictions (240 before expanded vaccine access; 1746 after expanded vaccine access). Most MSM with mpox were interviewed (95.0% before vaccine expansion and 97.0% after vaccine expansion); the proportion who named at least 1 contact decreased during the 2 time periods (74.6% to 38.9%).

Conclusions. During the period when mpox cases among MSM increased and vaccine access expanded, contact tracing became less efficient at identifying exposed contacts.

Public Health Implications. Contact tracing was more effective at identifying persons exposed to mpox in MSM sexual and social networks when case numbers were low, and it could be used to facilitate vaccine access. (Am J Public Health. 2023;113(7):815–818. https://doi.org/10.2105/AJPH.2023.307301)

Contact tracing is used to interrupt transmission of infectious diseases, including mpox, by identifying exposed persons (contacts) so that they can receive prevention services. The success of contact tracing in preventing transmission largely depends on how many patients’ contacts are identified and reported during interviews with health department staff.

During the 2022 US outbreak, the first case of mpox was confirmed on May 17, 2022.¹ Daily case counts peaked at 631 on August 1, 2022.² Most mpox cases occurred among gay, bisexual, and other men who have sex with men (MSM) and were attributed to sexual or close intimate contact during the 3 weeks before symptom onset.³ JYNNEOS vaccine supply was initially limited, and health departments prioritized access for postexposure prophylaxis for persons aged 18 years or older named as contacts by persons with mpox. On June 28, 2022, access was expanded to include persons aged 18 years or older at high risk for acquiring mpox.⁴ We sought to examine the potential impact of contact tracing among MSM to identify contacts and prevent mpox transmission in 10 US jurisdictions before and after vaccine access expanded.

METHODS

Ten US jurisdictions (Colorado, District of Columbia, Florida, Hawaii, Idaho, Michigan, New York City, North Carolina, South Carolina, Wisconsin) provided aggregated contact tracing data for mpox cases among MSM aged 18 years or older diagnosed during May 17 through July 31, 2022. Jurisdictions provided the numbers of MSM with mpox who were reported, were interviewed, named at least 1 contact with whom they had close physical contact within 21 days after symptom onset, and named at least 1 sexual contact with whom they engaged in sex or other intimate contact within 21 days after symptom onset. Total numbers of named contacts, named sexual contacts, and unnamed contacts (insufficient information was available to initiate follow-up) were also provided.

We stratified data by the period before (May 17–June 30, 2022) and after (July 1–31, 2022) access to the JYNNEOS vaccine was expanded.⁴ We calculated contact indices (the number of contacts divided by the number of interviewed MSM with mpox) for all named contacts, named sexual contacts, and unnamed contacts.

RESULTS

In participating jurisdictions, 240 (12%) mpox cases among MSM were reported before vaccine access expansion and 1746 (88%) were reported after vaccine access expansion. Case investigators interviewed similar proportions of persons during both periods (95.0% and 97.0%, respectively; Figure 1). Case investigators elicited locating information for at least 1 named contact from 179 (74.6%) patients before vaccine access expansion and 679 (38.9%) afterward. Similarly, case investigators elicited at least 1 named sexual contact from 92 (38.3%) patients before vaccine access expansion and 473 (27.1%) after expansion.

FIGURE 1—

Number and Proportion of Mpox Cases Reported and Interviews Conducted Among Gay, Bisexual, and Other Men Who Have Sex With Men in (a) May 17‒June 30, 2022, and (b) July 1‒31, 2022: 10 US Jurisdictions

Note. The proportion’s denominator is the total reported for each time period. The 10 jurisdictions were CO, DC, FL, HI, ID, MI, New York City, NC, SC, and WI.

^aNamed contact refers to a close contact in the 21 days after symptom onset for whom the interviewed patient provided sufficient contact information for the health department to initiate a contact tracing investigation.

^bNamed sexual contact refers to a named contact with whom the interviewed patient engaged in sex or close intimate contact.

Case investigators obtained locating information for 754 named contacts before vaccine access expansion (107 [14%] were sexual contacts) and 1378 named contacts after access expansion, including 317 (23%) sexual contacts. The named contact index decreased from 3.31 before vaccine access expansion (when case counts were low) to 0.81 after expansion. The named sexual contact index was less than 1 during both periods and decreased from 0.47 before vaccine access expansion to 0.19 after expansion. MSM with mpox reported 341 (42.1%) unnamed contacts during interviews before vaccine access expansion and 469 (57.9%) afterward. The unnamed contact index decreased from 1.50 before vaccine access expansion to 0.28 afterward.

DISCUSSION

Although health department staff interviewed most mpox patients among MSM, identifying their contacts became more challenging as the number of mpox cases and investigator workload increased and vaccine access expanded beyond named contacts. In this analysis, more than half of MSM with mpox did not provide locating information for any contacts. Without such information, health departments cannot notify contacts of exposure, limiting the ability to reduce transmission. Additional public health strategies beyond contact tracing, such as vaccination and behavior change recommendations,⁵ might be needed to reduce mpox transmission among MSM.

Contact tracing for other sexually transmitted diseases often has low reporting rates of sexual partners.^6–10 An estimated 80% of the partners of syphilis patients are either unreported or not found because health departments lack sufficient locating information.⁶ In our evaluation, we did not assess reasons why mpox patients did not name contacts. For sexually transmitted diseases (and as might be the case with mpox), patients may be unable to name sexual partners because encounters were anonymous or facilitated by dating Web sites and apps.^8,9 Patients may prefer to notify partners themselves, desire privacy, or believe there is little value in participating in public health‒sponsored contact tracing.^9,10

Several jurisdictions anecdotally described patients providing locating information for persons from their social and sexual networks to facilitate access to the vaccine even after access was expanded. However, the observed reduction in the reporting of named contacts after expanded vaccine availability suggests this practice was infrequent. The proportion of MSM patients naming contacts might have been artificially inflated early in the outbreak to facilitate vaccine access, contributing to an apparent decrease in naming contacts once vaccine access expanded. Reductions in the proportion of MSM with mpox who named a nonsexual or sexual contact after vaccine expansion resulted in fewer than 1 named contact per interviewed mpox patient. During periods of increased transmission, contact indices less than 1 suggest that contact tracing alone will be insufficient to reduce incidence within a community.

The Centers for Disease Control and Prevention developed messaging for MSM about ways to modify sexual behaviors to prevent mpox acquisition. A survey of MSM conducted during August 2022 found that nearly half of respondents had reduced their number of sexual partners since learning about the mpox outbreak.¹¹ The decrease in both the named sexual contact and unnamed contact indices in our analysis might be partially related to reductions in the number of sexual partners among MSM. The incubation period for mpox may also contribute to the observed decrease in contacts; MSM receiving a diagnosis in early July were likely exposed in June (possibly at Gay Pride Month events) and could have fewer contacts at the time of diagnosis.

The decrease in the proportion of patients who named at least 1 contact suggests that, as mpox cases began to surge, MSM may have limited their interactions with all contacts because of increased awareness of symptoms or that the interview process might have been less successful at eliciting locating information about all contacts, not just sexual contacts. Many staff conducting interviews for mpox worked in general communicable disease programs and might have lacked expertise in discussing sexual behavior.

These findings might not be generalizable to other jurisdictions or time periods. The amount and type of locating information needed to classify a contact as named and the definition used to identify MSM was determined locally and varied among jurisdictions, possibly limiting comparability.

PUBLIC HEALTH IMPLICATIONS

Contact tracing for mpox is challenging because contacts may be unreported or reported without sufficient locating information. Expansion of vaccine access from only named contacts to all persons at high risk of acquisition, increases in caseload, and behavior modification may have contributed to a decreased proportion of MSM with mpox naming contacts. Contact tracing is a resource-intensive strategy that may have benefited some, but likely did not reach most exposed persons to offer prevention services, limiting its impact. Continued promotion of other public health strategies, such as behavior change recommendations and vaccination, in addition to contact tracing, may be more effective in reducing mpox transmission among MSM.