A group of 239 scientists are asking the World Health Organization (WHO) to adjust its guidelines on the new coronavirus to address growing evidence that the virus can spread via airborne transmission—but the organization has indicated it's not yet convinced that airborne transmission is a common occurrence.
WHO says airborne transmission lacks 'solid' evidence
Earlier this year, a group of 36 experts urged WHO to formally acknowledge that the novel coronavirus can spread via airborne transmission, arguing that several so-called "superspreader" events had shown the virus likely was spreading through aerosols, or respiratory droplets in the air that are smaller than 5 microns.
"We've known since 1946 that coughing and talking generate aerosols," said Linsey Marr, an expert in airborne transmission of viruses at Virginia Tech University.
But WHO has maintained that the virus primarily spreads through large respiratory droplets that are expelled when an infected person coughs and sneezes—not through smaller, aerosolized droplets. In interim guidance WHO released June 29, the organization said airborne transmission of the virus is possible only after medical procedures that can produce aerosols.
Benedetta Allegranzi, WHO's technical lead on infection control, has acknowledged that "[t]here is a strong debate" on whether the new coronavirus can spread via airborne transmission. Allegranzi has said airborne transmission of the virus is "possible but certainly not supported by solid or even clear evidence."
As such, WHO in the interim guidance recommended handwashing as a primary strategy to prevent transmission of the virus, but not general use of face masks or coverings. The organization recommended using N95 masks and proper ventilation as strategies to protect against coronavirus transmission during only certain medical procedures.
Scientists say evidence increasingly shows coronavirus can spread via airborne transmission
But experts are once again arguing WHO's guidance ignores a growing body of evidence showing airborne transmission is a common way the new coronavirus spreads. In an open letter addressed to WHO and forthcoming paper entitled "It is Time to Address Airborne Transmission of Covid-19," the group of 239 scientists from 32 countries argue that there are several studies showing smaller respiratory droplets containing the novel coronavirus can linger in the air—and even float dozens of feet—and infect others.
The scientists contend that airborne transmission of the coronavirus is the only mechanism that can account for some superspreading events that have occurred as countries have reopened nonessential businesses, lifted stay-at-home orders, and relaxed social distancing measures intended to curb the virus' spread. They argue that airborne transmission of the virus is particularly dangerous in poorly ventilated or crowded indoor spaces and can occur even when people are distanced six feet apart from each other.
Donald Milton, a professor of environmental health at the University of Maryland and co-author of the letter, said WHO likely has delayed updating its stance on airborne transmission because it's difficult to identify aerosols and prove they can spread pathogens.
"It is easy to find virus on surfaces, on hands, in large drops," Milton wrote in an email sent to the Washington Post, adding, "But, it is very hard to find much less culture virus from the air—it is a major technical challenge and naive investigators routinely fail to find it." However, Milton wrote, "[b]ecause a person breathes 10,000 to 15,000 liters of air a day and it only takes one infectious dose in that volume of air, sampling 100 or even 1,000 liters of air and not finding virus is meaningless."
Lawrence Gostin, a professor of global health law at Georgetown University who provides technical assistance to WHO, said another possible reason why WHO has yet to add aerosols to its guidance on coronavirus transmission is because the agency "has to make recommendations for the entire world and it feels it needs irrefutable scientific proof before coming to a conclusion." Gostin said the "public, and even scientists, will lose full confidence in WHO without clearer technical guidance."
Paul Hunter, a member of WHO's infection prevention committee and a professor of medicine at the University of East Anglia, explained that measures intended to control airborne transmission of the new coronavirus could strain low- and middle-income countries with limited resources. Therefore, WHO may be hesitant to update its transmission guidance and recommendations without robust evidence supporting such a move, he said.
For instance, if the new coronavirus can spread via airborne transmission, people may need to wear face masks or coverings at all times when indoors to protect themselves—especially in places that are crowded or have poor ventilation, according to the New York Times. In addition, nursing homes, schools, and other businesses may have to install new filters and air flow mechanisms to reduce recirculating air in order to safely reopen, the New York Times reports.
"That's the balance that an organization like the WHO has to achieve," Hunter said. "It's the easiest thing in the world to say, 'We've got to follow the precautionary principle,' and ignore the opportunity costs of that."
Mary-Louise McLaws, also member of WHO's infection prevention committee and an epidemiologist at the University of New South Wales, said, "If [WHO] started revisiting airflow, we would have to be prepared to change a lot of what we do." She continued, "I think it's a good idea, a very good idea, but it will cause an enormous shudder through the infection control society."
A spokesperson for WHO said the organization is aware of the points raised by the scientists, and WHO's technical experts will review the plausibility of airborne transmission. In an email sent to the Los Angeles Times, Allegranzi wrote, "We value and respect their opinions and contributions to this debate."
However, Allegranzi added that, during weekly teleconferences held by WHO, a majority of a group of more than 30 international experts has "not judged the existing evidence sufficiently convincing to consider airborne transmission as having an important role in Covid-19 spread." Allegranzi wrote that airborne transmission of the new coronavirus "would have resulted in many more cases and even more rapid spread of the virus" than the world has seen so far.
Should people take precautions to protect against airborne transmission 'just in case'?
Although disagreement over whether the new coronavirus commonly spreads through the air remains, some providers and experts say people should take precautions to protect themselves against possible airborne transmission.
"There is no incontrovertible proof that [the novel coronavirus] travels or is transmitted significantly by aerosols, but there is absolutely no evidence that it's not," said Trish Greenhalgh, a primary care doctor at the University of Oxford. "[A]t the moment we have to make a decision in the face of uncertainty … [s]o why not just mask up for a few weeks, just in case" (Mandavilli, New York Times, 7/4; McAuley/Rauhala, Washington Post, 7/5; Read, Los Angeles Times, 7/4).