Prevalence of Undiagnosed Monkeypox Virus Infections during Global Mpox Outbreak, United States, June-September 2022

Abstract

Since May 2022, mpox has been identified in 108 countries without endemic disease; most cases have been in gay, bisexual, or other men who have sex with men. To determine number of missed cases, we conducted 2 studies during June-September 2022: a prospective serologic survey detecting orthopoxvirus antibodies among men who have sex with men in San Francisco, California, and a retrospective monkeypox virus PCR testing of swab specimens submitted for other infectious disease testing among all patients across the United States. The serosurvey of 225 participants (median age 34 years) detected 18 (8.0%) who were orthopoxvirus IgG positive and 3 (1.3%) who were also orthopoxvirus IgM positive. The retrospective PCR study of 1,196 patients (median age 30 years; 54.8% male) detected 67 (5.6%) specimens positive for monkeypox virus. There are likely few undiagnosed cases of mpox in regions where sexual healthcare is accessible and patient and clinician awareness about mpox is increased.

Keywords: IgM; Monkeypox virus; Mpox; PCR; United States; orthopoxvirus; undiagnosed; viruses.

Figure 1

Total numbers and percentages of positive results for specimens tested by monkeypox virus–specific PCR under different code categories from the International Classification of Diseases, 10th Revision, Clinical Modification, United States, June–September 2022. *Mpox add-on is defined as having an initial negative result from other testing, after which a provider requested testing for mpox.

Figure 2

Weekly positive detection of monkeypox virus by PCR testing and US Centers for Disease Control and Prevention case detection (https://www.cdc.gov/ecr/index.html), July 24–September 2, 2022. Results are from public health and select commercial laboratories and HealthTrackRx testing and US case detections. The greater number of laboratory positives than US case detections most likely results from testing of multiple samples from a single patient.