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Two weeks after the Centers for Disease Control and Prevention withdrew a statement about the airborne transmission of the coronavirus, the agency replaced it Monday with language citing new evidence that the virus can spread beyond six feet. indoor.
“These transmissions occurred within enclosed spaces that had inadequate ventilation,” the new guidance said. “Sometimes the infected person breathed heavily, for example while singing or exercising.”
The incident was just the latest in a series of slow and often baffling scientific judgments by the CDC and the World Health Organization since the start of the pandemic. Despite evidence that wearing face coverings can help reduce viral spread, for example, the CDC did not endorse their use by the public until April, and the WHO did not do so until June.
On aerosols, tiny airborne particles, the CDC lagged even behind the WHO. In July, 239 experts studying aerosols asked the WHO to recognize that the coronavirus can be transmitted through air in any indoor environment and not only after certain medical procedures, as the organization had claimed.
In particular, the new CDC guidance softens an earlier statement that referred to coronavirus as “an airborne virus,” a term that may have required hospitals to treat infected patients in specialized wards and for hospital workers to treat infected patients. health wear N95 masks anywhere in the hospital.
Instead, the new council says that the virus “can sometimes be spread by airborne transmission” and can be spread by both larger droplets and smaller aerosols released when people “cough, sneeze, sing, speak or breathe.”
But while the virus can be transmitted through the air in some circumstances, it is not the primary way the virus spreads, the CDC said.
“I am a little concerned that they still distinguish between close contact and airborne transmission, which implies that airborne transmission only matters beyond six feet,” said Linsey Marr, an expert on airborne virus transmission at Virginia Tech in Blacksburg, Virginia. “Airborne transmission also occurs in close contact and is probably more important than large droplet spray.”
The reviews came as President Trump received treatment at Walter Reed National Military Medical Center in Bethesda, Maryland, for what may be a severe case of Covid-19, the disease caused by the coronavirus. (Mr. Trump was due to be released from the hospital Monday night.)
The administration is dealing with a growing number of such infections among Trump’s inner circle. Kayleigh McEnany, the president’s chief spokeswoman, announced Monday morning that she had tested positive for the coronavirus, the latest in a string of political figures heading into self-quarantine after what may have been a so-called super-spreading event in the White House previously. this month.
That ceremony, a celebration of the judge Amy Coney Barrett’s nomination to the Supreme Court involved close contact both indoors and outdoors by attendees who were not wearing masks.
Former Vice President Joseph R. Biden Jr. may also have been exposed to the virus during the presidential debate with Trump. But he continues to campaign, his aides said, because he failed to meet the CDC’s requirement for close contact, within six feet of an infected person.
But in a press release accompanying the new guidance, the CDC said: “People are more likely to be infected the longer and closer to a person with Covid-19.”
Trump spoke loudly and extensively during the debate, which experts say could have released 10 times more viruses than breathing alone.
The new language on the CDC website makes some of the same points as an earlier version, which quietly appeared on the CDC website on September 18 and was removed just three days later.
At the time, CDC officials said the document had been published in error and had not yet been approved through the agency’s rigorous scientific review.
In both documents, the agency emphasized the risk of infection in poorly ventilated indoor environments.
In such circumstances, the number of droplets and smaller infectious particles expelled by people with Covid-19 “became concentrated enough to transmit the virus to other people,” the agency said, including those who arrived in a room shortly after. that an infected person left.
But the new version took a more conservative tone on airborne transmission, saying that it is much more common for the virus to spread through close contact with an infected person than through airborne transmission.
Some experts praised the softer emphasis on airborne transmission.
“This is consistent with what epidemiological data have shown us: opportunistic and situational events occur in the air, but close contact is really what happens,” said Saskia Popescu, an epidemiologist at George Mason University Hospital in Fairfax, Virginia. .
But on Monday, a group of aerosol scientists, including Dr. Marr, argued otherwise in a letter to the journal Science. “There is overwhelming evidence that inhalation represents an important route of transmission,” the researchers wrote.
The new guidance takes on urgent importance as cold temperatures send people back indoors, where the risk of the virus spreading through the air is greatest.
The agency’s council also guides administrators of schools, offices, hospitals and other public buildings in preparing for winter by improving their ventilation systems and taking other precautions.
Scientists disagree on the precise definitions of droplets and aerosols, and their relative contribution to the spread of the virus. But they generally agree that droplets are heavier and fall more quickly to the ground, while lighter aerosols can stay in the air indoors.
Before it was withdrawn on September 21, the previous version of the CDC guidance was the agency’s first unequivocal acknowledgment that the virus was spread through the air. Before that, the agency had emphasized hand hygiene, wearing face covers and keeping six feet of clearance as the main ways people protect themselves.
All of those things matter, experts said, but it has been clear for months that, at least in poorly ventilated indoor spaces, the virus can travel more than six feet.
In August, for example, scientists reported that they had successfully isolated live viruses from aerosols collected at distances of seven and 16 feet from an infected patient in a hospital.
Some scientists have also said that aerosol spread is the most likely explanation for superprocessor events, such as a cluster of cases after a choir practice in Washington state, and perhaps the White House celebration for the judge. Barrett.
Indoor aerosol transmission may also explain the increase in southern states this summer, as people stayed indoors with air conditioning.
But the CDC had not openly recognized the risks of the coronavirus floating on particles adrift in indoor air, beyond indirect nods to the importance of ventilation in schools and businesses.
After the public outcry, the WHO agreed to review the new evidence and updated its guidance a few days later to reflect that the virus can remain in aerosols in crowded indoor spaces. But the CDC did not directly acknowledge this change, even though parts of its website have advised schools and businesses to update their ventilation systems.
The reversion on Sept. 21 to language emphasizing only droplets as vectors of transmission led some public health experts to speculate that the agency had been ordered to remove recognition of floating aerosols.
The agency had reversed its position on another recommendation, one that said close contacts of infected people did not need to be evaluated if they did not have symptoms. After The New York Times reported that the language had been dictated by appointees by politicians in the administration rather than by CDC scientists, the agency reverted to more scientific writing and recommended that all people who have been in contact an infected person should be tested, regardless of symptoms.
