Mpox Among the Vaccinated: A Chicago Cluster, Spring 2023

After two months in which Chicago had recorded a single case of mpox, 13 were reported to the Chicago Department of Public Health between April 17 and May 5, 2023. The cluster stood out twice over. Across the United States, cases were then turning up alone or in clusters of fewer than three. And more than 69 percent of the people in this one had been fully vaccinated — with two doses of JYNNEOS or one of ACAM2000.

Some infections in vaccinated people are expected, given what is known about how well JYNNEOS works. But a share this large was unusual, and with big summer events coming up in Chicago and across the country, it raised fears of more cases to come.

The response

On May 9, 2023, the health department issued a health alert urging clinicians to watch for mpox and encouraging vaccination for people at risk. With CDC, it opened an investigation to:

1. establish how far the cluster reached and what caused it — by looking at what patients had in common, how the JYNNEOS vaccine had been kept cold along its supply chain, the whole genomes of virus from patient samples, and patients' immune responses after infection;
2. find the risk factors for exposure that prevention should target.

Who got sick

From March 18 to June 12, 2023, Chicago had 40 laboratory-confirmed cases:

What they found

• Vaccinated patients recovered without hospital care. A preliminary review of their records found self-limited illness, managed as outpatients. Patients with one dose or none more often had lesions on the genital mucosa (43% against 6%) and the eye (29% against none).
• Fully vaccinated patients reported more partners: a median of three in the three weeks before symptoms began.
• The two hospitalized patients were unvaccinated and had advanced HIV, with fewer than 200 CD4 cells per cubic millimeter.
• The virus had not changed. Preliminary sequencing from one unvaccinated and three fully vaccinated patients matched the B.1 variant of clade IIb, the predominant variant of the 2022–2023 outbreak. There were very few point mutations compared with published genomes, and none predicted to change the virus's proteins or make it more harmful.

Why it might have happened

The cause was not yet known. The leading hypotheses were a high number of sexual exposures within a network where many people were vaccinated; vaccine protection fading as antibody levels waned; or mistakes in handling or giving the vaccine.

What the authors advised

• Health departments should report mpox patients' vaccination status to CDC, so similar clusters among vaccinated people are spotted quickly.
• Anyone with a known or suspected exposure should isolate and get tested if symptoms appear — even if vaccinated.
• Temporary changes, such as fewer new or multiple partners and avoiding settings with anonymous sex with multiple partners, can also help prevent mpox.
• Everyone eligible, especially people with advanced HIV or other conditions that weaken the immune system, should get both doses of JYNNEOS.
• More research is needed on how long JYNNEOS protection lasts.

At the time, vaccination was recommended for gay, bisexual and other men who have sex with men, and for transgender, nonbinary and gender-diverse people, who in the previous six months had a new sexually transmitted infection or more than one sex partner; and for anyone who had a known or suspected exposure, a partner diagnosed with mpox in the previous two weeks, or sex in the previous six months at a commercial sex venue, at a large commercial event or where the virus was spreading, or in exchange for money or other items.

Sources

Based on "Notes from the Field: Emergence of an Mpox Cluster Primarily Affecting Persons Previously Vaccinated Against Mpox — Chicago, Illinois, March 18–June 12, 2023," by Emily A. G. Faherty and colleagues (Chicago Department of Public Health and CDC), Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.