Mpox Outbreak — Los Angeles County, California, May 4–August 17, 2023

Epidemiologic Investigation and Findings

During May 4–August 17, 2023, a total of 56 laboratory-confirmed mpox cases occurred in Los Angeles County (LAC), based on illness onset date or laboratory specimen collection date (if onset date was missing) (Figure). In contrast, during the 3 months preceding May 4, 2023, only seven mpox cases were reported in LAC. In addition to requirements for laboratory reporting of all mpox tests, health care providers must report all mpox or orthopoxvirus infections and information on illness characteristics to the LAC Department of Public Health (LACDPH). LAC residents with laboratory-confirmed mpox were contacted for interview by a public health disease investigator to obtain information on demographic, epidemiologic, and clinical characteristics. Clinical information was obtained from a combination of self-report from interviews and the medical provider report from the patients’ provider. Among the 56 patients, 32 (57%) were unvaccinated, eight (14%) were partially vaccinated, and 16 (29%) were fully vaccinated.* All 56 cases occurred in persons who were assigned male sex at birth and who identified as male (Table). Overall, 45 (80%) mpox patients identified as gay or bisexual. The median patient age was 35 years (IQR = 26–42 years). Overall, 21 patients (38%) were non-Hispanic White (White) men, 18 (32%) were Hispanic or Latino (Hispanic), 13 (23%) were non-Hispanic Black or African American (Black), and four (7%) identified as another race. More than one half of patients (57%; 32) lived in the Los Angeles metropolitan area. Among 55 interviewed patients, 48 (87%) reported sexual contact in the 3 weeks preceding symptom onset. No common social events were reported. Two pairs of patients were epidemiologically linked (i.e., a patient disclosed sexual contact with another patient in the 3 weeks preceding symptom onset). Forty-two (76%) interviewed patients did not report any travel outside of LAC in the 3 weeks before symptom onset, suggesting local mpox transmission. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.^†

Demographic and Other Characteristics by Vaccination Status

Demographic and other patient characteristics were assessed by vaccination status (fully vaccinated with JYNNEOS vaccine, partially vaccinated, or unvaccinated). The median age of fully vaccinated patients was 37 years (IQR = 31–46 years), of partially vaccinated patients was 35 years (IQR = 25–45 years), and of unvaccinated patients was 30 years (IQR = 26–38 years). Black persons accounted for 23% of all cases; however, no Black patients were fully vaccinated. Likewise, whereas Hispanic persons accounted for approximately one third of all patients, only three of 18 were fully vaccinated at the time of infection. In contrast, among White patients, who accounted for 38% of all patients, 57% were fully vaccinated. Seventeen (30%) patients were living with HIV, five of whom were fully vaccinated. Three patients living with HIV had CD4 counts <350 cells/mm³; none of these patients was fully vaccinated. Among HIV-negative patients, 100% of those who were fully vaccinated were receiving HIV preexposure prophylaxis (PrEP) at time of interview, compared with 48% who were unvaccinated. Fully vaccinated patients reported more sex partners in the 3 weeks preceding symptom onset (median = three) than did those who were unvaccinated (one) or partially vaccinated (two).

Previous Mpox Diagnosis

Two unvaccinated patients had previously received a diagnosis of mpox in 2022, 10 months and 12 months, respectively, before their 2023 infection. Review of clinical information confirmed that both patients had complete resolution of their previous infections before new symptom onset. Both secondary infections occurred in Black persons aged 35–45 years. One patient was living with HIV with a CD4 count <350 cells/mm³. Signs and symptoms in both patients with secondary infection were mild, and complete resolution was noted within the 3-week follow-up period.

Mpox Severity

Data for fully assessing clinical severity according to the mpox severity scoring system (1) are incomplete; however, patient interviews indicated that most cases were mild. Compared with patients who were fully vaccinated with JYNNEOS vaccine, a larger proportion of those who were unvaccinated reported signs or symptoms of fever (47% versus 31%) and chills (34% versus 19%); other symptoms were similar irrespective of vaccination status. Eighteen (32%) patients, including seven who were fully vaccinated, received the antiviral drug tecovirimat to treat mpox symptoms. One HIV-negative patient with no immunocompromising conditions and who had not received any JYNNEOS vaccine was hospitalized for pain management and infectious disease evaluation while awaiting mpox laboratory test results.

Mpox Vaccination History

Among the 16 fully vaccinated patients, the median interval from receipt of the second JYNNEOS vaccine dose to mpox symptom onset was 10 months (IQR = 9–11 months). Among fully vaccinated patients, six had received 2 subcutaneous vaccine doses and 10 had received 1 subcutaneous and 1 intradermal dose. Among the eight patients who had received 1 vaccine dose, five had received a subcutaneous dose and three had received an intradermal dose.

Laboratory Investigation

Whole-genome sequencing followed by genomic analyses (2,3) were performed on outbreak specimens obtained from 45 patients (14 fully vaccinated, six partially vaccinated, and 25 unvaccinated) as part of LAC’s Monkeypox virus (MPXV) genomic surveillance program; specimens from three patients were sequenced by the California Department of Public Health. Phylogenetic analysis (4) (data set tag 2023–08–01T12:00:00Z) determined that 32 (71.1%) cases involved MPXV belonging to the B.1.20 lineage of clade IIb, which is currently the dominant lineage identified through surveillance in the United States. Twelve (26.7%) cases involved MPXV assigned a lineage of B.1 that formed a monophyletic group defined by four mutations relative to the B.1 reference genome (G70002A, G143951A, C148604T, and G154188A), and might represent an emerging sublineage. One case (2.2%) associated with travel to China involved MPXV belonging to the C.1 lineage, which is prevalent in East Asia. Recently, a tecovirimat-resistant MPXV variant was identified in LAC (5); however, mutations associated with tecovirimat resistance were not detected in any outbreak specimens.