Chicago's Mpox Outbreak Reaches 40

The U.S. Centers for Disease Control and Prevention (CDC) today announced the mpox outbreak in Chicago, Illinois, has reached 40 individuals.

As of June 23, 2023, the CDC's Morbidity and Mortality Weekly Report confirmed that mpox vaccine breakthrough cases were reported to the Chicago Department of Public Health.

The observed proportion of cases among persons who had received JYNNEOS® (MVA-BN) smallpox/mpox or ACAM2000 smallpox vaccines in this cluster was unusual.

Mpox breakthrough cases averaged eight months since the last vaccination.

During March 18–June 12, 2023, 40 laboratory-confirmed mpox cases were identified in Chicago, including 22 (55%), five (13%), and 13 (33%), respectively, among patients who had received two doses of JYNNEOS or one dose of ACAM2000 vaccine, those who had received one vaccine dose of JYNNEOS vaccine, and those who had not received any vaccines for mpox.

All cases occurred among persons assigned male sex at birth; 37 (93%) identified as male and 28 (70%) as gay.

The median age was 33 (IQR = 23–49).

Eleven (28%) patients were living with HIV, 10 of whom had received two doses of JYNNEOS or one dose of ACAM2000 vaccine and whose HIV was well-controlled (CD4 count >200 cells/mm3 and viral load <200 viral copies/mL).

Three (8%) patients experienced concurrent sexually transmitted infections during mpox diagnosis.

Of six live births, two neonates developed lesions within one week after their mothers became symptomatic.

This investigation is ongoing. However, no similar clusters are being seen elsewhere in the U.S.

Preliminary sequencing indicates the virus is the same B.1 variant of Clade IIB, the predominant variant of the 2022–2023 outbreak.

The CDC wrote that this increase in mpox cases before significant summer events scheduled nationwide and in Chicago raised concerns about possible future case increases.

Although the cause of this cluster has not yet been determined, leading hypotheses include a potentially high number of sexual exposures in a network with many vaccinated persons, decreased vaccine effectiveness due to waning of humoral immunity, or vaccine mishandling or administration errors.

Mpox outbreak news and other sexually transmitted disease vaccine news are posted by Precision Vaccinations.