On Friday, officials at the Centers for Disease Control and Prevention rejected the idea that the monkeypox virus could spread through the air, saying the virus is normally transmitted through direct physical contact with sores or contaminated materials from a patient.
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 stay in the air over long distances.
Experts in airborne viruses disagreed, but some said the agency had not fully considered the possibility that respiratory drops, large or small, could be inhaled at a shorter distance than patient.
The World Health Organization and several experts have said that while “short-range” airborne transmission of monkeypox seems uncommon, it is possible and requires caution. Britain also includes smallpox in its list of “high-impact infectious diseases” that can spread through the air.
“Airborne transmission may not be the dominant route of transmission or very efficient, but it could still happen,” said Linsey Marr, an expert on airborne viruses at Virginia Tech.
“I think the WHO is right and the CDC’s message is misleading,” he added.
In the United States, the smallpox outbreak has risen to 45 cases in 15 states and the District of Columbia, CDC officials told a news conference. The overall count has risen rapidly since May 13, when the first case was reported, to more than 1,450. At least 1,500 cases are still under investigation.
Historically, people with monkeypox have reported flu-like symptoms before a characteristic rash appears. But some patients with the current outbreak have developed the rash first, and some have not had these symptoms at all, Dr. Rochelle Walensky, the agency’s director, said Friday.
No deaths have been reported in the current outbreak, he said.
Questions about airborne transmission of monkeypox virus are important because the answers, in turn, will go to recommendations for masking, ventilation, and other protective measures in case the outbreak continues to grow.
The CDC said Thursday that the monkey’s smallpox “is not known to remain in the air and is not transmitted for short periods of shared airspace.” The statement was followed by an article in the New York Times on Tuesday in which scientists described the uncertainties about the transmission of the virus.
“What we do know is that those diagnosed with monkeypox in this current outbreak described close and sustained physical contact with other people infected with the virus,” Dr. Walensky said Friday. “This is consistent with what we’ve seen in previous outbreaks and what we’ve known for decades about studying this virus and closely related viruses.”
But monkeypox is little studied, other experts said, and occasional episodes of airborne have been reported for the closely related smallpox virus. An outbreak of smallpox in 2017 in Nigeria resulted in infections in two health workers who had no direct contact with patients, scientists said at a recent WHO conference.
Some patients in the current outbreak do not know when or how they contracted the virus, CDC officials acknowledged.
The agency is right in assuring the public that the outbreak is not a threat to most people because the monkey’s smallpox is not as contagious as the coronavirus, said Dr. Donald Milton, an expert on virus transmission. air at the University of Maryland.
Airborne transmission is unlikely to be a risk to anyone other than immediate caregivers, Dr. Milton said, but warned that denying the possibility altogether “is the wrong way to do it.”
When a virus is present in saliva or the airways, as has been shown to be smallpox in the monkey, it can be expelled in respiratory drops when spoken, sung, coughed or sneezed, said Dr. Milton and other experts.
Drops can be heavy and fall quickly on objects or people, or they can be small and light, lingering in the air for long periods and distances. The CDC’s assessment depends in part on whether the virus is present only in large droplets or in very small droplets, called aerosols.
A similar debate took place at the beginning of the coronavirus pandemic, when the agency and the WHO focused on large drops as the main route of transmission. But aerosols turned out to be a major engine.
The CDC’s new guide to monkeypox described patients’ breathing drops as “secretions that come out of the air quickly.”
But the virus “can be present in respiratory particles of any size,” not just large drops, said Lidia Morawska, an air quality expert at Queensland University of Technology in Australia.
“From my point of view, there is no basis for claiming that the virus is transmitted only by large drops and that it only poses a risk of infection at close distances,” he wrote in an email.
Patients in the current outbreak appear to have been infected through close and sustained contact, CDC officials said Friday. But this can be difficult to determine.
What you need to know about the monkeypox virus
Card 1 of 5
What is the smallpox of the monkey? Smallpox is an endemic virus in parts of central and western Africa. It is similar to smallpox, but less severe. It was discovered in 1958, after outbreaks of monkeys kept for research, according to the Centers for Disease Control and Prevention.
What are the symptoms? Monkeypox creates a rash that begins with flat red markings that rise and fill with pus. Infected people may also have a fever and body aches. Symptoms usually appear between six and 13 days, but can take up to three weeks after exposure and can last between two and four weeks. Health officials say smallpox vaccines and other treatments can be used to control an outbreak.
How infectious is it? The virus spreads mainly through body fluids, skin contact, and respiratory droplets, although some experts suggest that it may occasionally pass through the air. It does not usually cause major outbreaks, although this year it has spread in unusual ways and among populations that have not been vulnerable in the past.
Do I have to worry? The virus is likely to spread during sexual intercourse, but the risk of transmission in other ways is low. Most people have mild symptoms and recover within a few weeks, but the virus can be fatal in a small percentage of cases. Studies also suggest that older adults may have some protection against smallpox vaccines from decades ago.
When people are in close contact, it may be impossible to tell if a virus was transmitted by touch, spraying large drops, or inhaling aerosols, Dr. Marr said.
“The occurrence of transmission in these situations does not define how the virus passed from one person to another,” he added. If the transmission can be caused by the spray of respiratory drops, “then it is almost certain that it also occurs by inhalation of aerosols.”
However, most experts agree that whatever the intake of inhaled aerosols, the monkey’s smallpox does not appear to be transmitted at distances that could be coronavirus or measles virus.
“I agree that most of the transmission of monkeypox is caused by touch, probably direct contact between mucous membranes,” Dr. Milton said.
But “the CDC seems to be stuck with the old terminology,” he said. “We really need to talk about transmission using terms that clearly state what’s going on: by touch, spray, or inhalation.”
The CDC itself recognizes the possibility of short-range air transmission in its advice to physicians. The agency recommends that patients wear masks and that their healthcare staff wear N95 respirators, which are needed to filter aerosols.
He also warns that “procedures that may spread oral secretions should be performed in an airborne isolation room.”
There is evidence that monkeypox can survive in aerosols and that the inhaled virus can cause disease in monkeys. However, airborne transmission may not be ideal for monkeypox virus.
Patients may not release too many aerosol viruses, the virus may not stay infectious for long, or the amount of inhaled virus needed to infect someone may be too high, Dr. Marr said.
If this is the case, airborne transmission is likely to occur only between people who are nearby for long periods of time. However, health officials in Britain, such as the United States, have said that many patients do not seem to know when or where they may have become infected.
If they became infected without close contact, “airborne transmission may have occurred more than we thought,” Dr. Marr said.