Faced once again with a rapidly spreading epidemic of mpox, the World Health Organization on Wednesday declared a global health emergency. The last time the W.H.O. made that call was in 2022, when the disease was still called monkeypox.
Ultimately the outbreak affected nearly 100,000 people worldwide, primarily gay and bisexual men, including more than 32,000 in the United States.
The W.H.O.’s decision this time was prompted by an escalating crisis of mpox concentrated in the Democratic Republic of Congo. It recently spread to a dozen other African countries. If it is not contained, the virus again may rampage all over the world, experts warned.
“There’s a need for concerted effort by all stakeholders, not only in Africa, but everywhere else,” Dr. Dimie Ogoina, a Nigerian scientist and chair of the W.H.O.’s mpox emergency committee, said on Wednesday.
Congo alone has reported 15,600 mpox cases and 537 deaths, most of them among children under 15, indicating that the nature of the disease and its mode of spread may have changed.
Here’s what to know.
Is this the virus we saw in 2022?
This is a different version of the mpox virus.
Mpox is a close relative of the smallpox virus. There are two main types: Clade I, the version that is dominant in Congo, and Clade II, a form of which caused the 2022 global outbreak. (A clade is a genetically and clinically distinct group of viruses.)
Clade I mpox is generally thought to cause more severe illness and to have a much higher mortality rate, which is one reason the W.H.O. is sounding the alarm now. Officials hope to contain this outbreak before it spreads to other continents.
The infection may resemble an ordinary respiratory illness at first but later blooms into a raised rash in the mouth, hands, feet or genitals. The virus spreads mainly through close contact — directly with the skin or fluids of an infected person, or with contaminated bed linens and other items.
Scientists learned during the 2022 outbreak that mpox can spread even in the absence of symptoms. And the rash may be mistaken for other diseases such as measles or chickenpox, particularly in young children.
Who is getting infected this time?
In the 2022 outbreak, mpox spread globally mainly among gay and bisexual men. Behavioral changes in that community helped to contain the virus, and vaccination at the time, or now, will help protect them.
Until recently, most cases in Congo resulted from consumption of contaminated meat or close contact with infected animals and people. But last year, scientists discovered a new subtype of mpox, Clade Ib, which appears to spread from person to person primarily through heterosexual transmission.
Most cases have been observed in prostitutes, truckers and other transient workers.
“Sex is probably the primary driver, and then the secondary driver is close contact and households,” said Dr. Jay Varma, the chief medical officer at SIGA Technologies, which manufactures tecovirimat, a drug used to treat mpox infection.
As with many other infections, most people with healthy immune systems are unlikely to become severely ill with mpox. Those who have weakened immune systems, including those living with H.I.V., are at highest risk of severe illness and death.
Older adults, who are typically more susceptible to infections, may be at least somewhat protected by their childhood vaccinations for smallpox, which ended in the United States in 1972.
Most of the deaths in Congo have been in children under 15, perhaps because their health may already be compromised by poor medical care, malnutrition and the many other pathogens they face.
Will the outbreak spread to the United States or Europe?
Many countries worldwide, including the United States, have continued to see patients with Clade IIb mpox, the version that caused the 2022 outbreak.
So far this year, there have been about 740 cases of mpox in the United States, more than double the number at this time last year, according to the Centers for Disease Control and Prevention.
Countries outside Africa have not reported cases of the deadlier versions of mpox ravaging Congo, although current tests may not distinguish between the different versions.
“I don’t think the risk right now for Americans is high at all, but what this is telling us is that we have to be vigilant,” said Dr. Trish Perl, an infectious disease physician at UT Southwestern Medical Center.
But previous epidemics, including of mpox, illustrate that an uncontrolled outbreak anywhere in the world may eventually turn up everywhere, Dr. Varma said. Since December, the C.D.C. has twice warned clinicians and the general public to remain alert for signs of mpox.
“I really think it’s only a matter of time before North America, Europe, etc., start to see cases,” he said, referring to the deadlier viral subtype. “Unless we invest in disease control everywhere, we’re going to remain always at risk.”
Will the vaccine still protect against mpox?
Two doses of the mpox vaccine Jynneos should protect against all versions of the virus, experts said.
Jynneos, made by Bavarian Nordic, was used in 2022 in the United States and Europe. The vaccine, initially developed against smallpox, should protect against mpox and all other members of that virus family, said Dr. Boghuma Titanji, an infectious diseases physician at Emory University in Atlanta.
Several studies have shown that antibodies prompted by the Jynneos vaccine wane and may be undetectable within a year. But other research has found that two doses effectively prevent severe illness, Dr. Titanji said.
In the United States, however, fewer than one in four people for whom vaccination was recommended got two doses.
“People were less interested in coming back and getting that second dose, or even starting the course of their vaccination,” Dr. Titanji said. “Maybe we will see an increase in uptick in vaccination, and this will serve as a reminder for people to come in and get vaccinated.”
In 2022, the federal government provided the shots at no cost. Jynneos is now commercially available, and some insurance companies may cover the cost.
For some patients, the shots may prove too expensive, Dr. Perl said. If mpox cases were to escalate, the U.S. government may again make the shots available free of charge, according to a federal official with knowledge of the situation.
Is the U.S. prepared for another bout with mpox?
Yes and no.
Scientists learned a lot about the virus in 2022 and have identified vaccines and treatments. But they do not fully understand how the deadlier virus is spreading in Africa, especially among children, or who is most at risk.
“This is very, very crucial when you think about designing a response strategy,” Dr. Titanji said.
There are few resources allocated to fighting sexually transmitted infections in the United States, said David Harvey, the executive director of the National Coalition of STD Directors.
Officials have not solved the problems that hobbled the response in 2022, including poor uptake of the vaccine and “a shockingly underfunded S.T.I. public health system,” Mr. Harvey said.
“Today, we worry about an mpox outbreak,” he added. “We’re already dealing with syphilis, and tomorrow there will be another outbreak of an S.T.I.”
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