Possible Undetected Mpox Infection Among Persons Accessing Homeless Services and Staying in Encampments

San Francisco, California, October-November 2022

Caroline J. Waddell, PhD; Thomas D. Filardo, MD; Namrata Prasad, PhD; Gerald J. Pellegrini Jr.; Neela Persad, MPH; William C. Carson; Terese Navarra; Michael B. Townsend, PhD; Panayampalli S. Satheshkumar, PhD; David Lowe; Deborah Borne, MD; Julia Janssen, MD; Nnenna Okoye, MD; Anamaría Bejarano, MPH; Grace E. Marx, MD; Emily Mosites, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2023;72(9):227-231.

Discussion

Among 207 persons who were experiencing homelessness or accessing homeless services in San Francisco and voluntarily participated in an antiorthopoxvirus seroprevalence survey during a large multinational mpox outbreak, three possible undetected mpox infections were detected. Mpox infections might be undetected because of subclinical, atypical, or mild disease or because of barriers to seeking or accessing health care systems, which could have occurred among the participants in this survey. None of the participants with possible undetected mpox infections reported sexual contact during the preceding month, although some reported sharing utensils and smoking devices and spending time around or touching someone with a rash. However, the timing of mpox exposure among these three persons is not known and could have preceded the survey period. The transmission route for the three possible undetected mpox infections could not be determined; additional studies are needed to identify mpox transmission pathways among persons experiencing homelessness. In the current outbreak, mpox has primarily spread through sexual activity but can be spread through touching contaminated objects and through close personal contact outside of sexual activity, although this risk is considered to be lower.^[6]

Previous mpox vaccination was reported by 16% of survey participants. However, only 54% of participants reporting mpox vaccination had detectable antiorthopoxvirus IgG, and 21% had detectable IgM. A 2-dose mpox vaccination series is recommended to optimize immunity.^[7] Most participants who received mpox vaccine reported receiving a single dose, which might in part explain the lower seroprevalence.^[2,7] SFDPH and community partners collaborated to conduct pop-up vaccination events in San Francisco during September–December 2022 for persons experiencing homelessness; this active outreach approach can be further leveraged to better facilitate complete vaccination for eligible persons. A single mpox vaccine dose might still offer some protection against severe mpox-associated illness and hospitalization, for which persons experiencing homelessness^[1,7,8] might be at higher risk.

The findings in this report are subject to at least seven limitations. First, participants were recruited as a convenience sample at prioritized locations; therefore, the findings are not generalizable to the entire population of persons experiencing homelessness or accessing homeless services in San Francisco. Second, the sample size was small because of time and resource limitations as well as logistical challenges associated with collecting blood samples from persons in encampments and at homeless service sites.^[9] Third, because of the small numbers of participants with potential undetected infection, statistical analyses could not be conducted to further refine possible transmission routes. Fourth, the survey ascertained frequency of behaviors during the preceding month to improve recall; however, it was not possible to determine whether behaviors that would increase mpox transmission risk were present outside that time frame. Fifth, the orthopoxvirus ELISA does not detect antibodies specific to Monkeypox virus; therefore, previous or acute mpox infection among the participants could not be distinguished from other orthopoxvirus infections or previous vaccination. Sixth, the survey relied on self-reported vaccination history and behaviors, which can be subject to recall and social desirability biases. Finally, it is possible for antibody testing to produce false-positive results. Efforts to improve specificity of possible mpox infection included restricting IgG results to persons aged <50 years to avoid inclusion of participants who received childhood smallpox vaccine, asking about previous military service to avoid inclusion of participants who received smallpox vaccination in the service, and using a higher OD-COV in ELISA tests to reduce risk for false-positive results.^[5,10] However, participants close to the cutoff age of 50 years could have received childhood smallpox vaccine, as was the case of one participant without reported vaccination history who had detectable antiorthopoxvirus IgG.

These findings suggest that undetected mpox infections might have occurred among a small percentage of persons experiencing homelessness in San Francisco. It is still unknown whether unique mpox transmission pathways exist for persons experiencing homelessness. However, given that known and possible undetected mpox transmission occurred, and that severe mpox disease among persons experiencing homelessness is possible,^[1] accessible prevention measures are needed. Prioritization and inclusion in public health response planning, focused outreach, and on-site vaccination events can ensure that prevention measures reach persons experiencing homelessness.

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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.

Table 1. Characteristics of persons experiencing homelessness or accessing homeless services who participated in an mpox seroprevalence survey (N = 209) — San Francisco, California, October–November 2022
Characteristic	No. (%)
Mean age, yrs (range)	46 (19–83)
Sex at birth
Female	61 (29.2)
Male	145 (69.4)
Intersex	<5.0^†
Unknown*	<5.0^†
Gender
Cisgender man	124 (59.3)
Cisgender woman	63 (30.1)
Nonbinary	<5.0^†
Transgender man	0 (—)
Transgender woman^§	19 (9.1)
Unknown*	0 (—)
Sexual orientation
Bisexual	22 (10.5)
Gay or lesbian	14 (6.7)
Heterosexual	160 (76.6)
Pansexual	<5.0^†
Unknown*	<5.0^†
Other	8 (3.8)
Race
American Indian or Alaska Native	10 (4.8)
Asian	9 (4.3)
Black or African American	66 (31.6)
Native Hawaiian or other Pacific Islander	<5.0^†
White	80 (38.3)
Multiracial or other	38 (18.2)
Unknown*	5 (2.4)
Hispanic origin
Hispanic or Latino	59 (28.2)
Non-Hispanic	147 (70.3)
Unknown*	3 (1.4)
Type of recruitment site
Encampment	16 (7.7)
Permanent supportive housing	30 (14.3)
Service center	68 (32.5)
Shelter	95 (45.5)

*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.

Table 2. Self-reported smallpox and mpox vaccination history and antiorthopoxvirus serology results among persons experiencing homelessness or accessing homeless services, who participated in an mpox seroprevalence survey — San Francisco, California, October–November 2022
Self-reported vaccination history*	No. (row %)
Self-reported vaccination history*	All participants	IgG-positive result^†,§	Received IgM test^§	IgM-positive result^†,¶
All participants	207 (100.0)	48 (23.2)	102 (48.8)	5 (4.9)
Any smallpox or mpox vaccination	82 (39.6)	38 (46.3)	60 (73.2)	5 (8.3)
Only smallpox vaccination	50 (23.9)	19 (38.0)	31 (62.0)	1 (3.2)
Only mpox vaccination	22 (11.5)	12 (54.5)	19 (86.3)	4 (21.1)
Both smallpox and mpox vaccination	10 (4.8)	7 (70.0)	10 (100.0)	0 (—)
Unknown vaccination history	8 (3.8)	2 (25.0)	5 (62.5)	0 (—)
No smallpox or mpox vaccination	117 (56.0)	10 (8.5)	37 (31.6)	0 (—)
No mpox vaccination	167 (80.7)	27 (16.1)	68 (40.4)	1 (1.5)
Age >50 yrs, no smallpox vaccination	37 (17.7)	8 (21.6)	17 (45.9)	0 (—)
Age ≤50 yrs, no smallpox or mpox vaccination	80 (38.6)	2 (2.5)	24 (30.0)	0 (—)

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.

Table 3. Behaviors reported during the preceding month, by persons experiencing homelessness or accessing homeless services, who participated in an mpox seroprevalence survey* — San Francisco, California, October–November 2022
Behavior and frequency	No. (%)
Behavior and frequency	All participants (N = 209)	Possible undetected mpox infection^† (n = 3)
Shared unwashed cups, spoons, or forks
Never	152 (72.7)	1 (33.3)
A few times	34 (16.3)	2 (66.7)
Nearly every day	8 (3.8)	0 (—)
Every day	15 (7.2)	0 (—)
Unknown^§	0 (—)	0 (—)
Shared unwashed clothing or bedding
Never	164 (78.5)	3 (100.0)
A few times	22 (10.5)	0 (—)
Nearly every day	7 (3.3)	0 (—)
Every day	16 (7.7)	0 (—)
Unknown^§	0 (—)	0 (—)
Shared unwashed personal care items^¶
Never	185 (88.5)	3 (100.0)
A few times	15 (7.2)	0 (—)
Nearly every day	3 (1.4)	0 (—)
Every day	6 (2.9)	0 (—)
Unknown^§	0 (—)	0 (—)
Spent time around someone with a rash, lesion, or sore
Never	175 (83.7)	1 (33.3)
A few times	22 (10.5)	2 (66.7)
Nearly every day	5 (2.4)	0 (—)
Every day	7 (3.3)	0 (—)
Unknown^§	0 (—)	0 (—)
Touched someone with a rash, lesion, or sore
Never	195 (93.3)	1 (33.3)
A few times	10 (4.8)	2 (66.7)
Nearly every day	1 (<1.0)	0 (—)
Every day	2 (1.0)	0 (—)
Unknown^§	1 (<1.0)	0 (—)
Had a rash, lesion, or sore at the time of survey
No	181 (86.6)	3 (100.0)
Yes	28 (13.4)	0 (—)
Unknown^§	0 (—)	0 (—)
Injected drugs**
Never	160 (76.6)	3 (100.0)
A few times	16 (7.7)	0 (—)
Nearly every day	10 (4.8)	0 (—)
Every day	20 (9.6)	0 (—)
Unknown^§	3 (1.4)	0 (—)
Shared smoking devices^††
Never	84 (40.2)	1 (33.3)
A few times	60 (28.7)	1 (33.3)
Nearly every day	16 (7.7)	1 (33.3)
Every day	47 (22.5)	0 (—)
Unknown^§	2 (1.0)	0 (—)
Had vaginal, anal, or oral sex
No	111 (53.1)	3 (100.0)
Yes	91 (43.5)	0 (—)
Unknown^§	7 (3.3)	0 (—)
Engaged in sex work^§§
No	187 (89.5)	3 (100.0)
Yes	14 (6.7)	0 (—)
Unknown^§	8 (3.8)	0 (—)
Where slept most often
Friend or family's house	9 (4.3)	1 (33.3)
Hotel or motel	2 (1.0)	0 (—)
Private residence	12 (5.7)	0 (—)
Permanent supportive housing	37 (17.7)	0 (—)
Shelter or navigation center	99 (47.4)	2 (66.7)
Unsheltered	46 (22.0)	0 (—)
Other	2 (1.0)	0 (—)
Unknown^§	2 (1.0)	0 (—)

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.

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Possible Undetected Mpox Infection Among Persons Accessing Homeless Services and Staying in Encampments

San Francisco, California, October-November 2022

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