Possible Undetected Mpox Infection Among Persons Accessing Homeless Services and Staying in Encampments — San Francisco, California, October–November 2022

Monkeypox (mpox) is a disease caused by an Orthopoxvirus. The 2022 multinational outbreak, which began in May 2022, has spread primarily by close skin-to-skin contact, including through sexual contact. Persons experiencing homelessness have been disproportionately affected by severe mpox (1). However, mpox prevalence and transmission pathways among persons experiencing homelessness are not known, and persons experiencing homelessness have not been specifically recommended to receive mpox vaccine during the 2022 outbreak (2,3). During October 25–November 3, 2022, a CDC field team conducted an orthopoxvirus seroprevalence survey among persons accessing homeless services or staying in encampments, shelters, or permanent supportive housing in San Francisco, California that had noted at least one case of mpox or served populations at risk. During field team visits to 16 unique sites, 209 participants completed a 15-minute survey and provided a blood specimen. Among 80 participants aged <50 years who did not report smallpox or mpox vaccination or previous mpox infection, two (2.5%) had detectable antiorthopoxvirus immunoglobulin (Ig) G antibody. Among 73 participants who did not report mpox vaccination or previous mpox infection and who were tested for IgM, one (1.4%) had detectable antiorthopoxvirus IgM. Together, these results suggest that three possible undetected mpox infections occurred among a sample of persons experiencing homelessness, highlighting the need to ensure that community outreach and prevention interventions, such as vaccination, are accessible to this population.

During July–October 2022, the San Francisco Department of Public Health (SFDPH) identified cases of mpox among persons who were experiencing homelessness (Deborah Borne, MD, San Francisco Department of Public Health, personal communication, October 2022). SFDPH invited a CDC field team to conduct an orthopoxvirus seroprevalence survey among persons experiencing homelessness to better understand the prevalence of disease and possible transmission pathways. Existing collaboration between SFDPH and government- and community-based organizations serving persons experiencing homelessness facilitated site engagement in the project. Homeless service sites were prioritized for inclusion if they had provided services to at least one person with confirmed mpox or served persons at increased risk for mpox, including those identifying as lesbian, gay, bisexual, transgender, or queer; men who have sex with men; and persons who engage in sex work.

Participants provided oral consent to respond to a 15-minute orally administered survey and to provide a serum specimen. Persons aged ≥18 years were eligible to participate. The survey included questions about demographic characteristics and behaviors that could be related to mpox transmission in the context of homelessness, including frequency of sharing objects such as clothing and utensils as well as sexual activity and drug use in the preceding month. Surveys were conducted in person in English or Spanish or through a phone-based language interpreter if another language was requested. A trained phlebotomist performed venipuncture. When venipuncture was unsuccessful or when success was considered unlikely (e.g., because of needle aversion or inadequately visualized or damaged veins), participants were offered use of a microneedle device that passively collected capillary blood from the upper arm (4). Each participant received up to two $25 grocery store gift cards for participating in the survey and blood collection. This activity was reviewed and approved by CDC and conducted consistent with applicable federal law and CDC policy.^†

Blood specimens were centrifuged in serum separator tubes and shipped to CDC for processing. All serum specimens were tested by enzyme-linked immunosorbent assay (ELISA) for presence of antiorthopoxvirus IgG. Those specimens with detectible IgG or from participants who reported higher mpox risk behaviors (sex work, multiple sexual partners, and persons assigned male at birth who identified as gay, bisexual, or transgender), were tested for IgM (5). Possible undetected mpox infection was defined as 1) detectable antiorthopoxvirus IgG (optical density minus cutoff value [OD-COV] ≥0.1) in a participant aged <50 years without reported smallpox or mpox vaccination or 2) detectable antiorthopoxvirus IgM (OD-COV ≥0.1) in a participant without reported mpox vaccination. Possible undetected infections identified by IgG testing were restricted to persons aged <50 years to avoid inclusion of participants who received smallpox vaccine during childhood. Participants who reported unknown vaccination history or previous mpox infection were not included as having possible undetected mpox infections. Descriptive analyses were performed, and outcomes were reported by vaccination history and serologic results.

The CDC field team recruited 284 participants from 16 unique sites (seven shelters, five service centers, two supportive housing locations, and two encampments); 11 (4%) participants were excluded from analysis because they reported living in their own private residence without accessing homeless services during the preceding month. Among the remaining 273 participants, 240 (88%) consented to blood collection, and blood was successfully collected from 209 (77%) (Table 1). Average participant age was 46 years; 69% reported male sex at birth, 59% reported male gender, and 9% were transgender. Most (77%) were heterosexual. The highest proportion of participants was non-Hispanic White (38%), followed by non-Hispanic Black or African American (32%); 70% were non-Hispanic. The most common recruitment sites were shelters (46%), followed by homeless service sites (33%).

A total of 207 participants were included in serologic analysis, after exclusion of two participants (1%) who reported both previous mpox infection and mpox vaccination (Table 2). Among these participants, 82 (40%) reported previous vaccination against smallpox, mpox, or both, including 50 (24%) who reported only smallpox vaccination, 22 (11%) who reported only mpox vaccination, and 10 (5%) who reported both. Neither smallpox nor mpox vaccination was reported by 117 (60%) participants; vaccination status of eight (4%) was unknown. Among the 32 participants who reported any mpox vaccination, 26 (81%) reported receiving 1 dose, and six (19%) reported receiving 2 doses.

Among 50 participants who reported only smallpox vaccination, antiorthopoxvirus IgG was detected in 19 (38%) and IgM in one (3%). Among 22 participants who reported only mpox vaccination, antiorthopoxvirus IgG was detected in 12 (55%) and IgM in four (21%). Among 80 participants aged <50 years who did not report smallpox or mpox vaccination, two (3%) had detectable antiorthopoxvirus IgG. Among 68 participants without reported mpox vaccination who were tested for antiorthopoxvirus IgM, one (1.5%) had detectable antibodies. These results yielded a total of three possible undetected mpox infections.

None of these three participants reported sexual contact during the preceding month, being transgender, being a gay or bisexual man, or having a rash at the time of the survey (Table 3). Among the three participants with possible undetected mpox infection, two reported that during the previous month they had shared unwashed utensils, spent time around and touched someone with a rash, and shared smoking devices.

Acknowledgments

Specimen Receiving Team, San Francisco Public Health Laboratory; participants described in this report and the community-based organizations or agencies that provided services for them; CDC Foundation; Lina Castro, Darlene Daevu, Thomas Knoble, Godfred Masinde, Mary Mercer, Frank Strona, Andi Tenner, Mpox Response Team, San Francisco Department of Public Health; San Francisco Department of Homelessness and Supportive Housing; Whole Person Integrated Care Street Medicine and Shelter Health teams, San Francisco Department of Public Health; Darpun Sachdev, California Department of Public Health; Ninnie Wynn, CDC Laboratory and Testing Task Force.

Corresponding authors: Emily Mosites, lwx7@cdc.gov; Grace Marx, lwx1@cdc.gov.

¹CDC Mpox Emergency Response Team; ²Epidemic Intelligence Service, CDC; ³San Francisco Department of Public Health, San Francisco, California.

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* Survey responses were indicated as unknown if the participant declined to answer the question or answered that they did not know.
^† The specific value for any cell with fewer than five participants is suppressed in accordance with data standards for the San Francisco Department of Public Health and are indicated within the cell as <5.0.
^§ Among the 19 participants who identified as transgender women, 11 indicated that they were sexually oriented toward men (heterosexual), seven indicated that they were bisexual, gay or lesbian, or pansexual, and one declined to answer.

Abbreviations: IgG = immunoglobulin G; IgM = immunoglobulin M.
* Categories are not mutually exclusive.
^† Optical density minus cutoff value ≥0.1.
^§ Percentage of those with self-reported vaccination history.
^¶ Percentage of participants who received antiorthopoxvirus IgM testing.

Abbreviations: IgG = immunoglobulin G; IgM = immunoglobulin M.
* Most surveys (198; 94.7%) were conducted in English, 10 (4.8%) in Spanish, and one (0.5%) in Mandarin.
^† Possible undetected mpox infection was defined as detectable antiorthopoxvirus IgG antibody in a specimen from a person who did not report smallpox or mpox vaccination and was born after 1972 (aged <50 years), or in a specimen from a person with detectable antiorthopoxvirus IgM antibody who did not report mpox vaccination.
^§ Survey responses are marked as unknown if the participant declined to answer the question or answered that they did not know.
^¶ Based on response to the question, “In the last month have you shared unwashed personal care items (razors, toothbrushes, hairbrushes, etc.)?”
** Based on response to the question, “In the last month did you shoot up or inject any drugs – by shooting up, we mean using drugs with a needle, either by hitting a vein, skin popping, or muscling?”
^†† Based on response to the question, “Have you shared bubbles/vapes/bongs/pipes/other smoking devices in the last month?”
^§§ Based on responses to the following questions: 1) “Do you consider yourself to be a sex worker?” and 2) “In the last month did you have sex for money, drugs, food, housing, or other life needs?” Participants were classified as engaging in sex work if they responded affirmatively to either question.