
A study today in Morbidity and Mortality Weekly Report looked at a recent cluster of 40 mpox cases in Chicago and suggests that vaccine protection may wane over time, but vaccinated patients do well recovering at home, and vaccination protects against severe mpox.
The cluster, identified from March 18 to June 12, 2023, was notable because in the previous 2 months the Chicago Department of Health received notice of only 1 mpox case. And 55% of case-patients in the cluster reported having completed the two-dose schedule of Jynneos mpox vaccine prior to their illness or had received one dose of ACAM2000, a smallpox vaccine that also prevents mpox.
The 40 laboratory-confirmed cases included 22 (55%), 5 (13%), and 13 (33%) patients who had received, respectively, either 2 doses of Jynneos or 1 dose of ACAM2000 vaccine, 1 Jynneos dose, or no vaccine.
Most of the case-patients reported being vaccinated against the virus in July or August 2022; the median interval from receipt of the second Jynneos vaccine dose to mpox diagnosis was 8.4 months.
Vaccine protection against more severe illness
Patients who received only one dose of Jynneos or no vaccines had a higher prevalence of lesions affecting the genital (43% versus 6%) or ocular (29% versus none) mucosa, the authors said.
"Preliminary medical record review indicates that vaccinated patients experienced self-limited illness, managed in outpatient settings," the authors wrote. None of the patients who had been vaccinated required hospitalization, compared to two unvaccinated patients who were admitted for treatment.
Although the cause of this cluster has not yet been determined, leading hypotheses include a potential high number of sexual exposures.
"Although the cause of this cluster has not yet been determined, leading hypotheses include a potential 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," the authors concluded.