Officials at the Centers for Disease Control and Prevention pushed back Friday against the idea that the monkeypox virus can spread through the air, saying the virus is usually transmitted through direct physical contact with sores or contaminated materials from a patient.
The virus may also be transmitted by respiratory droplets expelled by an infected patient who comes into physical contact with another person, they said. But it can’t linger in the air over long distances.
Experts on airborne transmission of viruses did not disagree, but some said the agency had not considered the possibility that respiratory droplets, large or small, could be inhaled at a distance from a patient.
The World Health Organization and several experts have said that while “short-range” airborne transmission appears to be uncommon, it is possible and warrants warning. Britain also includes monkeypox on its list of “high-consequence 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 occur,” said Linsey Marr, an expert on airborne viruses at Virginia Tech.
“I think the WHO has it right, and the CDC’s message is misleading,” she added.
In the United States, the monkeypox outbreak has swelled to 45 cases in 15 states and the District of Columbia, CDC officials said at a news conference. The global tally has risen swiftly 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 flulike symptoms before a characteristic rash appears. But some patients in 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 on Friday.
No deaths have been recorded yet in the current outbreak, she said.
Questions about the monkeypox virus of airborne transmission are important because the answers will turn into bear recommendations for masking, ventilation and other protective measures should the outbreak continue to grow.
The CDC said on Thursday that the monkeypox “is not known to be lingering in the air and is not transmitted during short periods of shared airspace.” The statement followed a New York Times article on Tuesday 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 described close, sustained physical contact with other people who were infected with the virus,” Dr. Walensky said on Friday. “This is consistent with what we’ve seen in prior outbreaks and what we know from decades of 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. In a 2017 outbreak of monkeypox in Nigeria, infections occurred in two health care workers who had no direct contact with patients, scientists said at a recent WHO conference.
A few patients do not have the current outbreak when or how they contracted the virus, CDC officials acknowledged.
The agency is right to reassure the public that the outbreak is not a threat to most people, because monkeypox is not 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 be a risk for immediate caregivers than anyone else, Dr. But Milton said, but cautioned that the possibility of denying “is the wrong way to do it.”
When a virus is present in the saliva or in the respiratory tract, as the monkeypox has been shown to be, it can be expelled in respiratory droplets while talking, singing, coughing or sneezing. Milton and other experts said.
The droplets may be heavy and quickly fall onto objects or people, or they may be small and light, lingering in the air for long periods and distances. The CDC’s assessment hinges on whether the virus is present in only large droplets or also in very small ones, called aerosols.
A similar debate unfolded at the beginning of the coronavirus pandemic, when the agency and the WHO focused on large droplets as the main route of transmission. But aerosols turned out to be a major driver.
The new CDC guidance on monkeypox described the respiratory droplets emitted by patients as “secrets that drop out of the air quickly.”
But the virus “can be present in any size of respiratory particles,” said Lidia Morawska, an air quality expert at Queensland University of Technology in Australia.
“In my view, there is no basis to the statement that the virus is transmitted only by large droplets and presenting infection risk only on close distances,” she wrote in an email.
Patients in the current outbreak seem to have become infected through close, sustained contact, CDC officials said on Friday. But this can be difficult to determine.
What to Know About the Monkeypox Virus
What is monkeypox? Monkeypox is a virus endemic in parts of Central and West Africa. It is similar to smallpox, but less severe. It was discovered in 1958, after outbreaks occurred in Monkeys for research, according to the Centers for Disease Control and Prevention.
When people are in close contact, it can be impossible to distinguish from a virus that was transmitted by touch, spray of large droplets or inhalation of aerosols, Dr. Marr said.
“In cases of transmission of this occurrence does not define how the virus got from one person to another,” she added. If the transmission can occur by the spray of respiratory droplets, “then it almost certainly happens by inhalation of aerosols, too.”
Still, most experts agree that whatever the contribution of inhaled aerosols, monkeypox does not seem to be transmitted over the distances that the coronavirus or the measles virus can.
“I agree that most monkeypox transmission occurs by touch – most likely direct touch between mucous membranes,” Dr. Milton said.
But the “CDC seems to be stuck on the old terminology,” he said. “We really need to talk about transmission using terms that clearly say how it happens – through touch, spray or inhalation.”
The CDC acknowledges the possibility of short-range airborne transmission in its advice to clinicians. The agency recommends that patients wear masks and that health care personnel wear them for N95 respirators, which are required to filter out aerosols.
It also cautions that “procedures should be performed to spread oral secretions into an airborne infection isolation room.”
There is evidence that monkeypox can survive in aerosols and that inhaled virus can cause disease in monkeys. Airborne transmission may not be ideal for the monkeypox virus, however.
Patients may not release much virus in aerosols, the virus may not remain infectious for long, or the amount of inhaled virus needed to infect may be too high, Dr. Marr said.
If that is the case, airborne transmission is likely to occur only among people who are close for long periods. Still, health officials in Britain, like those in the United States, have said that many patients do not seem to know when or where they may become infected.
If they were infected without close contact, “it is possible that airborne transmission is occurring more than we realize,” Dr. Marr said.