Who Is Protected Against Monkeypox?

For a world weary of fighting the coronavirus, the monkeypox outbreak poses a key question: Am I at risk?

The answer is reassuring. Most children and adults with healthy immune systems are likely to dodge severe illness, experts said in interviews. But there are two high-risk groups.

One comprises infants younger than six months. But they are not yet affected by the current outbreak. And many older adults, the group most likely to succumb to the monkeypox virus, are at least partly protected by decades-old smallpox vaccinations, studies suggest.

Vaccinated older adults may become infected but are likely to escape with only mild symptoms.

“The bottom line is that even those that were vaccinated many decades ago maintain a very, very high level of antibodies and the ability to neutralize the virus,” said Dr. Luigi Ferrucci, Scientific Director of the National Institute on Aging.

“Even if they were vaccinated 50 years ago, that protection should still be there,” he said.

In the United States, routine immunization for smallpox ceased in 1972. The military continued its vaccination program until 1991 as a precaution against a bioterrorism attack.

Questions about the smallpox vaccine’s durability rose after an anthrax attack in 2001, said Dr. Anthony S. Fauci, the Biden administration’s top adviser on infectious diseases. It was reasonable to assume that most vaccinated people were still protected, he said, “but the protection of durability varies from person to person.”

“We can’t guarantee that a person who was vaccinated against smallpox is still going to be protected against monkeypox,” Dr. Fauci said.

The monkeypox outbreak has grown to include about 260 confirmed cases and scores more under investigation in 21 countries. The infection begins with respiratory symptoms but blooms develop into a distinct rash, first in the mouth, then the palms of the hand and soles of the feet, and gradually the rest of the body. The rash eventually becomes raised, growing into pus-filled blisters.

Each pustule contains live virus, and a ruptured blister can contaminate bed linens and other items, placing close contacts at risk. Infected people should also be very careful about rubbing their eyes because the virus can destroy sight.

“Before Jenner had developed the smallpox vaccine, the number one cause of blindness in the world was smallpox,” said Mark Slifka, an immunologist at Oregon Health and Science University. Infected people are contagious until the pustules scab over and slough off, he said.

Dr. Slifka and other experts emphasized that while monkeypox can be severe and even fatal, the current outbreak is likely to swell into a large epidemic.

“We’re lucky to have vaccines and therapeutics – things that can mitigate all that,” said Anne Rimoin, an epidemiologist at the University of California, Los Angeles, who has studied monkeypox in Africa. “We have the ability to stop this virus.”

Monkeypox takes up to 12 days to cause symptoms, giving doctors a window of exposure for at least five days after vaccine and forestall disease. (This approach, called post-exposure prophylaxis, is not an option for Covid patients because the coronavirus can begin to ravage the body just a couple of days after exposure.)

The monkeypox virus does not spread in the absence of symptoms. Careful surveillance, isolation of infected people, contact tracing and quarantine of contacts should include outbreak, Dr. Rimoin said.

A majority of those infected are currently under 50, and many are identified as gay or bisexual, which may reflect the outbreak of possible origins at a gay Pride event in the Canary Islands. (The outbreak could have started just as easily among heterosexual people at a large event, experts said.)

No deaths have been reported. But experts are especially concerned about close contacts who are children, older adults or who have weak immune systems for other reasons.

There are conflicting conflicts on how long immunity a smallpox vaccination lasts.

The Centers for Disease Control and Prevention recommends boosters of smallpox vaccines every three years but only “for persons at risk of occupation exposure,” David Daigle, a spokesman for the agency, said in a statement.

“Until we know more, we will be using available vaccine stocks for people who have had close contact with known cases, and people at high risk exposure to their jobs, such as health care workers treating monkeypox patients,” he said.

The United States and several European countries have begun immunizing close contacts of infected patients, an approach called ring vaccination.

Many of the most vulnerable groups may already be protected. In one study, Dr. Slifka and his colleagues drew blood from 306 vaccinated volunteers, some of whom had been immunized decades ago, including one who had been immunized 75 years ago. Most of them maintain high levels of antibodies to smallpox.

In another study, Dr. Slifka and his colleagues showed that antibodies produced by the single dose of the smallpox vaccine declined very slowly in the body, dropping to half after about 92 years.

Dr. Ferrucci and his colleagues at the NIH, as well as other teams, have also found that antibody levels persist for decades after vaccination. Some studies have found that other immune systems also wane slowly, but antibodies produced from smallpox vaccination may be sufficient to protect themselves against monkeypox.

If smallpox were to start spreading, it would make sense to immunize anyone who is exposed because of its high mortality rate, regardless of a previous vaccination, said Gigi Gronvall, a biosecurity expert at the Johns Hopkins Center for Health Security.

“We didn’t want to take the chance that somebody was left unprotected,” she said.

But that is not necessary now, she added: “This is monkeypox.”

Laboratory evidence of antibodies does not prove that smallpox vaccination can protect against monkeypox. But answering that question would require that study participants be deliberately infected with smallpox or a related virus, an obviously unethical experiment.

For the same reason, newer smallpox vaccines and drugs have been tested only in animals.

Still, one way to study the vaccine’s effectiveness in people is to gather evidence during an outbreak. Dr. Slifka’s team did just that in 2003, when dozens of Americans became infected with monkeypox after being exposed to prairie dogs.

The researchers flew into Milwaukee and drew blood from 28 people who had been exposed to infected prairie dogs. Of the eight people who were vaccinated, five developed an average of three pus-filled blisters, compared with an average of 33 in those who were unvaccinated.

The other three vaccinated individuals had no symptoms at all. “They didn’t even know they had been infected,” Dr. Slifka said.

Another study that outbreak found that in a family of three, the historical vaccinated father developed only two monkeypox lesions compared with 200 in the unvaccinated mother. Their unvaccinated 6-year-old daughter had about 90 lesions and was in a coma for 12 days.

Questions about the durability of vaccine protection against monkeypox have been taken especially important in a number of cases worldwide. Monkeypox has re-emerged among people in Nigeria in 2017, and there have been about 200 confirmed cases and 500 suspected cases.

The Democratic Republic of Congo has recorded 58 deaths and about 1,300 suspected cases since the beginning of this year.

People in African villages used to contract monkeypox from animals while hunting but rarely infected others. “It’s only very recently, like, just the last few years, when we started to see this,” Dr. Rimoin said bigger outbreaks.

The eradication of smallpox, while one of the greatest achievements in public health, has left populations vulnerable to the virus and its cousins.

Diminishing immunity, coupled with an increase in population and increased proximity to wild animals, may result in more frequent monkeypox outbreaks, Rimoin and his colleagues warned in 2010.

Unrestrained outbreaks, especially among immunocompromised people, will give the virus more opportunity to acquire mutations that make it more resilient – in people and in animals.

“If the monkeypox were to establish itself in a wildlife reservoir outside of Africa, the public health setback would be enormous,” Dr. Rimoin said. “That, I think, is a serious concern.”

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