New monkeypox cases are declining in the United States, a trend public health officials and clinicians attribute to vaccination and changes in behavior.

Eligible individuals who did not receive the monkeypox vaccine were about 14 times more likely to become infected than those who received a first dose of the two-dose vaccine, according to new early data from the Centers for Disease Control and Prevention — a promising sign CDC Director Rochelle Walensky said provides “a level of cautious optimism that the vaccine is working as intended.”

The CDC is expanding eligibility for vaccination against a virus that has infected more than 25,000 people in the United States. Despite the decrease in new cases, severe infections have been showing up in recent weeks among men, the majority of them Latino and Black, according to CDC officials. Most have weak immune systems because they have advanced HIV. In many instances, these patients have had more than 100 lesions.

Is the monkeypox outbreak over?

The monkeypox outbreak is showing signs of slowing in the United States and around the globe.

Daily U.S. infections now average around 200 after peaking around 450 in mid-August. Health officials attribute the decline to vaccines and behavioral change among gay men who have reduced sexual activity, the most common mode of transmission.

But they’re also bracing for monkeypox to stick around as a background threat with periodic flare-ups.

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“Goal is eradication. Prediction is that we are going to get really close,” Demetre Daskalakis, the deputy coordinator of the federal monkeypox response, told reporters at an event promoting a Washington, D.C., vaccine clinic last week. “The hump is going to go down and we may have sporadic events that happen if we continue the great efforts of D.C. and other jurisdictions to get folks vaccinated.”

More about monkeypox, renamed mpox

Could the monkeypox outbreak get worse?

Experts warn that cases may rise again as people start to lower their guards — a similar dynamic seen during the COVID-19 pandemic after cases declined — or if the protection from vaccines turns out to be temporary.

Monkeypox “moved from being very visible to becoming invisible, but it’s still there,” said Peter Chin-Hong, an infectious diseases specialist at the University of California at San Francisco who has treated monkeypox patients. “Invisible doesn’t mean it’s not present. It’s just not up front and center in people’s consciousness.”

Federal health officials also say monkeypox continues to pose a threat to the United States if the virus continues to circulate globally. Authorities believe the current outbreak in the U.S. started with people returning from countries where the virus had already gained a foothold.

Monkeypox could also become endemic in the United States if it spills over into wild animal populations that periodically transmit the virus to humans, as is the case in parts of western and central Africa.

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Key questions about the latest version of monkeypox remain unanswered — including how effective vaccines are when administered under a new injection method and in preventing infections when the virus is transmitted during sex.

“This is a nascent outbreak and a nascent epidemic where we are still learning about it,” said Anne Rimoin, an epidemiologist at UCLA who has studied monkeypox in Africa for two decades. “We can’t have the hubris we understand everything about this virus and what we are going to see over an extended period of time just based on data from a short period of time.”

Who is still contracting monkeypox?

Globally, the monkeypox outbreak has been concentrated in men who have sex with men, and health authorities have yet to see widespread transmission in other groups.

But infections are no longer almost exclusively among gay and bisexual men. CDC data showed the share of new cases in men with recent sexual contact with other men declining throughout the summer. But infections are still heavily in that group, which made up two-thirds of cases with known sexual history and gender in the week ending Aug. 20.

Experts say it’s unsurprising that some children or women could contract the virus, especially if they are in the same household as someone in a high-risk group. But they are also less likely to transmit a virus that spreads through close intimate contact.

The outbreak is also increasingly affecting Black people as it drags on. They made up nearly half of cases in the week ending Sept. 11, while the share of cases declined among Latinos and remained stable among Whites.

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Are kids getting monkeypox?

Some worried monkeypox would spread in classrooms through hugging and other close contact among children after the new school year started.

While there have been isolated cases of students with monkeypox, there is no evidence of widespread outbreaks in K-12 schools.

The CDC reported only 19 cases in children between the ages of 6 and 15 as of Sept. 21. Other countries where students have been back in class longer also have not reported significant transmission in children.

College campuses present more of a concern because students are more likely to be sexually active and in smaller sexual networks where a virus can more efficiently circulate. But there have not been reports of widespread transmission at the collegiate level either, and federal officials have said the risk is low.

How well does the Jynneos vaccine work?

There has been limited data on how well Jynneos, the primary vaccine being used in the U.S. outbreak, works against monkeypox in real-world conditions. On Wednesday, the CDC released data showing rates of monkeypox cases by vaccination status from July 31 through Sept. 3. Among 32 jurisdictions, unvaccinated individuals had 14 times the risk of monkeypox disease compared with people who had received their first vaccine dose 14 days or more earlier.

The data does not take into account differences in behavior or patients’ underlying medical conditions. Officials also don’t know how many individuals received the vaccine using the traditional method or the alternative method, known as intradermal vaccination, that allows a single-use vial to be split into five injections because of the vaccine shortage.

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Walensky said the early findings, and similar results from studies in other countries, suggest that even one dose of the monkeypox vaccine offers at least some initial protection against infection. But laboratory studies show that immune protection is highest two weeks after the second dose of vaccine, Walensky said during a White House monkeypox briefing Wednesday. “It may be that second dose that provides durable protection.”

Rimoin said it will be critical to get additional data on vaccine performance by how the shots are given. “We will need to understand more about durability of protection over time and how much we can attribute to vaccine-induced protection vs. behavior change,” she wrote in an email.

Who is eligible for the monkeypox vaccine?

The CDC has expanded the vaccine to groups of people with the highest potential for exposure to monkeypox. On Wednesday, the agency said these groups can get the shot before they are exposed to the disease, something many states and local health departments are already doing. Until now, the agency had recommended vaccination for people after exposure.

Among the people who can now get the vaccine:

— Gay, bisexual and other men who have sex with men, transgender or nonbinary people who in the past six months have had a new diagnosis of a reportable sexually transmitted disease, such as acute HIV, chlamydia, gonorrhea or syphilis.

— People who have had sex in the past six months at a commercial sex venue or at a large public event in a geographic area where monkeypox is spreading.

— Commercial sex workers.

— People who anticipate experiencing the above risks.

Some individuals are declining to get vaccinated because of the stigma associated with the visible but temporary mark — often a blister. To counter that stigma, new guidance from CDC is allowing people who don’t want to risk a visible mark on their forearm to get the shot on skin by their shoulder or upper back, areas more frequently covered by clothes.

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How sick are people getting from monkeypox?

Most people recover from monkeypox, but in recent weeks, public health officials say they are hearing more about severe infections. All of the severe cases have been in men, with a median age of 32, said Agam Rao, a CDC monkeypox expert, during a recent call with clinicians hosted by the CDC and the Infectious Diseases Society of America. The majority have been Latino or Black.

Most have frail immune systems because of advanced HIV, she said. “These patients have had progressive illness. They’ve had over a hundred lesions in many cases, new lesions, despite being on” the antiviral treatment TPoxx.

Two otherwise healthy men in their 30s developed encephalomyelitis — inflammation of the brain and spinal cord — suggesting that neurological complications are a potential outcome of monkeypox infections, according to a recent CDC report. Both were treated with TPoxx and recovered but have difficulty walking.

How many people have died from monkeypox?

Monkeypox has not been as deadly in the latest global outbreak compared with earlier ones in parts of Africa where the case fatality ratio has been 3 to 6 percent.

In the United States, authorities have confirmed one death attributed to monkeypox in a Californian who was severely immunocompromised and are investigating the cause of death in a Texan with monkeypox who also had severe underlying health issues and a weakened immune system.

Officials say people who are immunocompromised, including those living with HIV, should be especially vigilant in seeking care because they face a higher risk of severe complications including death.

Worldwide, authorities have recorded just 12 deaths in countries that have not historically reported monkeypox out of nearly 66,000 cases.