Officials at the Centers for Disease Control and Prevention on Friday contested the idea that the monkeypox virus can spread through the air, saying the virus is usually transmitted through direct physical contact with a patient’s wounds or contaminated materials.
The virus can also be transmitted through respiratory droplets expelled by an infected patient who comes into physical contact with another person, they said. But it cannot remain in the air for long distances.
Experts on airborne virus transmission didn’t disagree, but some said the agency hadn’t fully considered the possibility that respiratory droplets, large or small, could be inhaled at a shorter distance from a patient.
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The World Health Organization and several experts have said that while “short-range” airborne transmission of monkeypox appears uncommon, it is possible and precautions are needed. Britain also includes monkeypox on its list of “serious infectious diseases” that can spread through the air.
“Airborne transmission may not be the predominant mode of transmission, nor is it very efficient, but it could still occur,” said Linsey Marr, airborne virus expert at Virginia Tech.
“I think the WHO is right and the CDC’s message is misleading,” she added.
In the United States, the monkeypox outbreak has grown to 45 cases in 15 states and the District of Columbia, CDC officials said at a news conference. The global number has quickly risen to more than 1,450 since May 13, when the first case was reported. At least 1,500 cases are still being investigated.
Historically, people with monkeypox have reported flu-like symptoms before a characteristic rash appears. But some patients in the current outbreak developed the rash first, and some didn’t have these symptoms at all, Dr. Rochelle Walensky, the director of the CDC, on Friday.
No deaths have been recorded in the current outbreak, she said.
Questions about airborne transmission of the monkeypox virus are important, as the answers in turn address recommendations for masking, ventilation and other protective measures if the outbreak continues to escalate.
The CDC said Thursday that monkeypox “is not known to be airborne and is not transmitted during short periods of common airspace.” The statement followed a Tuesday New York Times article in which scientists described uncertainties about the transmission of the virus.
“What we do know is that those diagnosed with monkeypox in this current outbreak have described close, ongoing physical contact with other people who have been infected with the virus,” Walensky said Friday. “This is consistent with what we have seen in previous outbreaks and what we know from decades of studying this virus and closely related viruses.”
But monkeypox is poorly studied, other experts said, and occasional episodes of airborne transmission have been reported for the closely related smallpox virus. A 2017 monkeypox outbreak in Nigeria saw infections among two healthcare workers who had no direct contact with patients, scientists said at a recent WHO conference.
Some patients in the current outbreak don’t know when or how they contracted the virus, CDC officials conceded.
The agency rightly reassures the public that the outbreak poses no threat to most people because monkeypox isn’t nearly as contagious as the coronavirus, said Dr. Donald Milton, an expert on airborne virus transmission at the University of Maryland.
Airborne transmission is unlikely to pose a risk to anyone other than those immediate caregivers, Milton said, but warned that completely denying the possibility is “the wrong way to go.”
When a virus is present in the saliva or airways, as monkeypox has been shown to be, it can be expelled in respiratory droplets when you speak, sing, cough or sneeze, Milton and other experts said.
The droplets can be heavy and fall quickly on objects or people, or they can be small and light and remain airborne for long periods of time and distances. The CDC’s assessment depends in part on whether the virus is only present in large droplets or also in very small ones called aerosols.
A similar debate erupted early in the coronavirus pandemic as the agency and WHO focused on large droplets as the main route of transmission. But aerosols turned out to be an important driver.
The new CDC guidance on monkeypox described respiratory droplets emitted by patients as “secretions that fall rapidly from the air.”
But the virus “can be present in airway particles of any size,” not just large droplets, said Lidia Morawska, an air quality expert at Australia’s Queensland University of Technology.
“In my view, the statement that the virus is only transmitted by large droplets and only poses a risk of infection at close range is irrelevant,” she wrote in an email.
Patients in the current outbreak appear to have been infected through close, ongoing contact, CDC officials said Friday. But that can be difficult to determine.
When people are in close contact, it can be impossible to distinguish whether a virus was transmitted through touch, a large droplet spray, or aerosol inhalation, Marr said.
“The occurrence of transmission in such situations does not define how the virus got from one person to another,” she said. If transmission can occur through droplet spray, “then it almost certainly occurs through inhalation of aerosols.”
Still, most experts agree that regardless of the contribution of inhaled aerosols, monkeypox does not appear to be transmitted over the distances that the coronavirus or measles virus can travel.
“I agree that most monkeypox is transmitted by touch — most likely through direct contact between mucous membranes,” Milton said.
But the “CDC seems to be sticking with the old terminology,” he said. “We really need to talk about transmission in terms that clearly state how it happens — through touch, spray or inhalation.”
The CDC, in its advice to clinicians, recognizes the possibility of short-range airborne transmission. The agency recommends that patients wear masks and that medical staff caring for them wear N95 respirators, which are needed to filter out aerosols.
It also warns that “procedures likely to spread oral secretions should be performed in an airborne infection isolation room.”
There is evidence that monkeypox can survive in aerosols and that inhaled viruses can cause disease in monkeys. However, airborne transmission may not be ideal for the monkeypox virus.
Patients may not release a lot of virus in aerosols, the virus may not stay infectious for long, or the amount of inhaled virus required to infect someone may be too high, Marr said.
If this is the case, airborne transmission is likely only between people who have been close for a long time. Still, health officials in the UK, like the United States, have said many patients don’t seem to know when or where they may have been infected.
If they were infected without close contact, “it’s possible that airborne transmission may have occurred more than we realize,” Marr said.
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