Breakthrough mpox cases linked to risky sexual behaviors
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Key takeaways:
- Among 49 mpox cases assessed, 28 were among patients who received two doses of the Jynneos vaccine.
- Patients reported group sex, anonymous sex and attending a large event in the 3 weeks before symptom onset.
An investigation of mpox cases during a 2023 outbreak revealed that cases were likely due to frequent behaviors associated with mpox transmission despite most patients being vaccinated with a relatively highly effective vaccine.
Although many of these cases were among people vaccinated against mpox, none of these patients were hospitalized after their breakthrough infection, researchers found.
“The Chicago Department of Public Health received an unusually high number of mpox reports during April 17-May 5, 2023,” Emily A. Groene Faherty, PhD, Epidemic Intelligence Service Officer for the CDC and the Chicago Department of Public Health, told Healio. “This cluster triggered substantial media attention and raised concerns about the duration of protection of the two-dose Jynneos vaccine series.”
The Jynneos vaccine, which is now commercially available in the U.S. and accessible at many facilities that perform routine vaccinations such as pharmacies and physician offices, has been shown to be up to 86% effective at preventing mpox in people who received two doses, whereas even a single dose was shown to reduce hospitalization risk and disease severity.
“We launched a multifaceted investigation to assess outbreak scope, differences between vaccinated and unvaccinated patients, and hypotheses for mpox virus infection postvaccination,” Faherty said.
As part of the investigation, the researchers interviewed patients and reviewed medical records to assess demographic, behavioral and clinical characteristics, and mpox vaccine status. They also evaluated serum antibody levels after infection and compared patient viral genomes with mpox virus infection sequences in available databases.
Between March 18, 2023, and June 27, 2023, the researchers identified 49 mpox cases — 46 confirmed and three probable. Most cases were among cisgender males (88%) who reported men who have sex with men contact (86%).
According to the study, 25 (51%) patients had a sexually transmitted infection (STI) 12 months or less before their mpox diagnosis, whereas four (8%) had a concurrent STI at mpox diagnosis. Additionally, seven (16%) patients reported group sex, 14 (33%) reported anonymous sex and 11 (23%) reported attending a large event, such as a concert or festival, in the 3 weeks before symptom onset.
Among all of the patients identified, 28 (57%) were among fully vaccinated patients who received the two-dose series of the Jynneos vaccine either subcutaneously (57%), intradermally (7%) or via heterologous administration (36%). These fully vaccinated patients had more sexual partners on average compared with patients who were not fully vaccinated (three vs. one).
The researchers obtained isolates from 42 (86%) of patient specimens for whole-genome sequencing — 37 of which had adequate coverage for analysis, including 14 from fully vaccinated patients and 23 from patients who were not fully vaccinated.
Through whole-genome sequencing, the researchers determined that 36 sequences fell within a single phylogenetic clade — mpox B.1 sublineage B.1.20, finding that no genetic differences were observed between mpox virus sequences from vaccinated and unvaccinated patients. Among all 36 B.1.20 isolates, 18 were identical, 16 were closely related and two were less closely related.
“Infections after full vaccination may occur in communities with behaviors associated with mpox transmission, such as having multiple sex partners in 3 weeks before symptom onset or having one-time partnerships, even when vaccination coverage is high,” Faherty said. “However, confidence in the Jynneos vaccine should be strengthened through clinicians communicating the benefits of the two-dose vaccination series and emphasizing that the global outbreak is ongoing.”
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