Chicago mpox outbreak raises alarm over summer spread

A cluster of mpox cases in the Chicago area has spurred concerns of a resurgence this summer, mere months after the end of the public health emergency for the outbreak that began last year.

The U.S. response to the mpox outbreak in 2022 was heralded as a public health success despite some early stumbles. By the beginning of this year, enough progress had been made that the White House allowed the public health emergency to expire.

Officials from the Centers for Disease Control and Prevention (CDC) said last week, however, there is a “substantial risk” of a resurgence this summer. This warning came days after the World Health Organization declared mpox to no longer be a global health emergency.

A worry is that investigators have found more than half of those infected in the Chicago cluster had received some degree of vaccination.

Nallely Mora, research assistant professor at Loyola University Chicago’s Parkinson School of Health Sciences and Public Health, believes the cluster may be the result of people being negligent of getting a full schedule of vaccines.

She noted that while rates of mpox cases became extremely low, ranging from one to three confirmed cases per day in recent months, the virus never went away entirely.

“We know the rates of people willing to get a complete vaccine schedule is not great,” Mora said. “Some of them, they may have their first dose and then maybe they forget to get to the point of the 28 days, when they need to get the second dose to acquire full immunity, and it’s just not happening.”

The discovery in Chicago has led experts to call for more inquiries into the efficacy and duration of the smallpox vaccines used during last year’s outbreak.

Because there is no immunization specifically geared toward mpox, the smallpox vaccines ACAM2000 and the newer Jynneos were deployed because the two viruses are closely related.

Jynneos, administered in two doses 28 days apart, became the most commonly administered shot because it is generally thought to have fewer side effects.

During the early parts of the mpox outbreak, many health departments’ supplies of Jynneos were stretched thin, with some resorting to administering all the doses they had on hand before knowing when they would have enough supplies to provide the second dose.

Due to these circumstances, many people fell through the cracks, not following up to get a second dose.

The difference between one dose and two doses of Jynneos is potentially vast. CDC officials noted during a press briefing that the estimated vaccine effectiveness from one dose of Jynneos ranged from 36 to 75 percent, while the effectiveness of two doses ranged from 66 to 86 percent. 

Martin Hirsch, professor of medicine at Harvard University, noted this difference is particularly more “dramatic” among immunocompromised individuals, who are less likely to develop as strong of an immune response as individuals with competent immune systems. This is especially troubling given what’s known about vaccination rates in the U.S.

“People who consider themselves at high risk and who are at high risk should get the primary vaccine dose,” Hirsch advised. “The data that I’ve seen in the United States is that only 23 percent of those considered at risk are now fully vaccinated.”

The CDC has so far not changed its recommendations on vaccine scheduling for mpox and has not specified whether previously immunized people should get a second course of shots ahead of this summer.

“CDC recommends that anyone at risk, anyone who has been infected, or anyone who has been exposed to mpox, get a total of two mpox vaccines,” the agency said when reached for comment.

Mora said individuals who have received an incomplete schedule of vaccines should see whether they can get a complete two-dose round. Those who got both doses, however, probably can wait before they inquire about further immunization.

“I don’t think there’s any indication that the vaccine efficacy wanes very quickly,” Hirsch said. “I don’t think people who have gotten their vaccines should be particularly concerned, unless they’re at particularly high risk.”

Groups that health authorities recommend get immunized include those who have been exposed to someone with mpox; men who have sex with men, transgender and gender-diverse people who have had more than one sex partner in the past six months; immunocompromised people; and those who work in high-risk settings, like clinics and hospitals.

In Hirsch’s opinion, the general population can hold off getting a shot for mpox because the risks could be greater than the benefit. He encouraged those with concerns to speak with their health care providers.

The mitigating strategies from last year still hold true, as well. Wash your hands; clean your clothes and linens if you’ve been infected; and avoid high-risk situations where skin-to-skin contact is more likely.