Federal health officials warned Thursday that there was a significant risk the mpox virus will resurge this summer and reiterated calls for people at high risk to get vaccinated if they have not already done so.
The warnings came after a cluster of 21 mpox cases were identified in Chicago this month, one of the first major clusters since the outbreak in the U.S. died down last fall, and as three different estimates of the mpox vaccine’s effectiveness were published Thursday afternoon.
Although the methods and precise findings of those studies varied, they collectively showed that two doses of the vaccine, known as Jynneos, were effective at preventing symptomatic disease. The vaccine is made by Danish manufacturer Bavarian Nordic.
“Even though no vaccine is perfect, even imperfect immunity in many people still leads to a lot of immunity in the population, and … with that increased immunity we are less likely to have any outbreaks at all or have smaller outbreaks,” Demetre Daskalakis, the White House’s national mpox response deputy coordinator, told reporters. “However, it is important to say that without renewed prevention efforts, especially vaccination, we are definitely at risk of a resurgence, in fact a substantial risk of resurgence.”
The warnings from the White House and the Centers for Disease Control and Prevention echoed those from the World Health Organization last week. Although the agency ended the mpox global health emergency, Director-General Tedros Adhanom Ghebreyesus urged countries to remain vigilant especially as the summer approached.
The concern is highest for men who have sex with men, who were hit hardest by the virus last summer. Pride Month in June and other summer festivals for the LGBTQ+ community could lead to increased transmission, experts have warned.
The virus can lead to painful lesions on the skin and mucous membranes, although the disease can on rare occasions be more severe, particularly in people with compromised immune systems. There were 42 deaths associated with the outbreak that began last year.
Transmission can happen by contact with respiratory droplets or infected skin, although last year’s outbreak was largely spread through sexual contact.
When WHO declared the global emergency last July, it was still unclear how effective Jynneos would be against the virus. The virus had only ever led to small outbreaks, largely in Central and West Africa, and so the shot had been approved in 2019 based on data from animals intentionally exposed to the virus and data from healthy volunteers that showed it was safe and induced antibodies against the virus.
Early reports were mixed, especially as some jurisdictions decided to only use fractional dosing to stretch limited supply.
The three studies released Thursday, however, all indicated that giving two doses of the vaccine was between 66% and 88.5% effective, depending on the study. Data on a single dose of the vaccine, however, were more mixed. A CDC study across 12 jurisdictions found one dose was 75.2% effective, but a New England Journal of Medicine study of electronic health records found that was only 36% effective.
Officials said the reports also indicated that giving lower doses of the shot by injecting within the skin, as U.S. officials directed health care professionals to do last summer in another effort to stretch supply, was as effective as giving higher doses subcutaneously.
“Different methodologies mean different views and they do inform us in the same way, which is one dose is good and two doses is better,” said Daskalakis. “This definitely tells us that these vaccines work and that our strategy of vaccinating people and getting both doses really remains core to our efforts to prevent.”
More than 1.2 million people have received one dose of Jynneos, said Christopher Braden, the CDC’s mpox response incident manager. But less than a quarter of the population considered at risk have been fully vaccinated.
The agency’s modeling showed that areas where less than 35% of at-risk individuals have received at least one dose “are more likely to have outbreaks which could be as large or even larger than those in 2020,” said Braden.
Despite the new data, key questions about the vaccine remain, including how well it prevents infection and how long protection lasts — including how well people last year will be protected today.
Most of the people infected in the cluster were previously vaccinated with at least one dose, including at least nine people who received two doses, although officials cautioned there were alternative explanations for why they may have been reinfected besides waning durability. Officials are also examining whether the doses they received were compromised or mishandled in some way, as well as if the virus they were infected with had mutated in any way to avoid immunity from vaccination.
At least three other ongoing studies, including one in the Democratic Republic of Congo and two in the U.S., may give researchers an early glimpse. Officials also said symptoms were milder in the Chicago patients who had mpox after vaccination, suggesting the vaccine may still have attenuated their disease.