CDC cancels airborne transmission of monkeypox. Some experts disagree.

On Friday, officials at the Centers for Disease Control and Prevention pushed back against the notion that the monkeypox virus could be spread through the air, saying the virus is usually transmitted through direct physical contact with a patient’s wound or contaminant.

The virus can also be transmitted through respiratory drops expelled by an infected patient who has had physical contact with another person, they said. However, it cannot stay in the air for long distances.

Experts on airborne infections of the virus did not agree, but some said that the agency did not fully consider the possibility of shortness of breath, large or small, breathing at a short distance from the patient.

The World Health Organization and a number of experts have said that the “short-range” airborne transmission of monkeypox seems unusual, that it is possible and with caution. Britain has included monkeypox in its list of “high-risk infectious diseases” that can be spread through the air.

“Airborne infections may not be the main route of transmission or not very efficient, but it can still happen,” said Lance Marr, an airborne virus specialist at Virginia Tech.

“I think the WHO is right, and the CDC’s message is misleading,” he added.

In the United States, the prevalence of monkeypox has risen to 45 cases in 15 states and the District of Columbia, CDC officials said at a news conference. The number has risen sharply worldwide since May 13, when the first case was reported, with more than 1,450. At least one and a half thousand cases are still under investigation.

Historically, people infected with monkeypox have reported flu-like symptoms before a characteristic rash appears. But some patients with the current outbreak have developed the first rash and some have had no symptoms at all, the agency’s director, Dr. Rochelle Walensky, said Friday.

No deaths have yet been reported from the current outbreak, he said.

Questions about airborne infections of the monkeypox virus are important because the answers will carry recommendations for masking, ventilation and other protective measures if the outbreak continues to increase.

The CDC said Thursday that monkeypox “does not appear to be in the air and does not transmit in a short period of time in shared airspace.” The statement followed an article in the New York Times on Tuesday in which scientists described the uncertainty surrounding the virus.

“What we do know is that those currently infected with the monkeypox have described close, long-lasting physical contact with other people infected with the virus,” Dr. Walensky said Friday. “It’s consistent with what we’ve seen in previous epidemics and what we know from studying these viruses and closely related viruses over the decades.”

But monkeypox is not poorly studied, other experts say, and occasional cases of airborne infections have been reported for closely related smallpox virus. In the wake of the 2017 Monkeypox outbreak in Nigeria, an infection occurred between two healthcare workers who had no direct contact with patients, scientists said at a recent WHO conference.

Some patients with the current outbreak do not know when or how they became infected with the virus, CDC officials acknowledged.

The agency is right to reassure the public that the outbreak is not a threat to most people, as monkeys are not nearly as contagious as the coronavirus, said Dr. John H. Snyder, an airborne virus specialist at the University of Maryland. Donald Milton says.

Dr. Milton said airborne infections are not likely to be a risk to anyone other than immediate caregivers, but warned that completely denying the possibility is “the wrong way to do it.”

When a virus is present in the saliva or respiratory tract, as monkeypox has been shown, it can be spread through the airways while talking, singing, coughing or sneezing, says Dr. Milton and other experts.

Drops can be heavy and fall quickly on objects or people, or they can be small and light, fixed in the air for long periods of time and distance. The CDC’s assessment depends in part on whether the virus is present only in large or very small droplets, called aerosols.

A similar controversy erupted at the beginning of the coronavirus epidemic, when the agency and the WHO focused on large droplets as the main route of transmission. But the aerosol turned out to be a major driver.

MonkeyPix’s new CDC guidelines describe respiratory droplets excreted by patients as “emissions that are quickly expelled from the air.”

But Lydia Morawska, an air quality expert at the Queensland University of Technology in Australia, said the virus could be present in “respiratory particles of any size”, not just large droplets.

“In my view, there is no basis for this statement that the virus is transmitted only by large droplets and only presents the risk of infection at close distances,” he wrote in an email.

Patients with the current outbreak appear to have been infected through close, prolonged contact, CDC officials said Friday. But it can be difficult to determine.

When people are in close contact, it is impossible to tell if the virus has been spread by touch, large droplets, or aerosol inhalation, Dr. Marr said.

“In such cases the incidence of infection does not define how the virus is passed from one person to another,” he added. If the infection can be caused by inhalation spray, “then it almost certainly also occurs through inhalation of aerosols.”

Nevertheless, most experts agree that monkeypox does not appear to be spread over distances that could be coronavirus or measles virus, regardless of the contribution of respiratory aerosols.

“I agree that most monkeypox infections occur through touch – probably direct contact between the mucous membranes,” said Dr. Milton.

But “the CDC seems to be stuck in the old terminology,” he said. “We really need to talk about the infection using terms that clearly state how it occurs – by touch, spray or inhalation.”

The CDC itself acknowledges the possibility of short-range airborne infections on the advice of physicians. The agency recommends that patients wear masks and that healthcare workers taking care of them wear N95 respirators, which are needed to filter aerosols.

It further warns that “the possibility of spreading oral discharge should be performed in an airborne transmission isolated room.”

There is evidence that monkeypox can survive in aerosols and that the respiratory virus can cause disease in monkeys. Airborne infections may not be ideal for the monkeypox virus.

Patients may not release too many viruses into the aerosol, the virus may not remain contagious for long, or the amount of virus inhaled may be too high to infect someone, Dr. Marr said.

If so, airborne infections can only occur in people who have been around for long periods of time. Yet, health officials in Britain, such as the United States, say many patients do not seem to know when or where they may be infected.

If they are infected without close contact, “it is possible that airborne infections are happening more than we realize,” said Dr. Marr.

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