At the beginning of the month an article published in The New York Times It caused a chain of headlines around the world: the coronavirus itself is transmitted through the air, or so 239 experts claimed in a letter. published two days later. His goal: to convince WHO to change its recommendations for the control of the COVID-19 pandemic. Until then, the international organization had insisted that the transmission took place fundamentally by "Respiratory droplets" they quickly fall to the ground due to their weight.
The possibility of SARS-CoV-2 being transmitted through the air (aerosols) has been on the table since the start of the pandemic. To the public's confusion, it has become a semantic rather than a scientific discussion. The English term airborne (translated as air) evokes in many the image of an extremely contagious disease, but both the letter of the 239 experts and the reluctant update from the WHO they were very cautious. The former called for more ventilation, to avoid overcrowding indoors and to use ultraviolet filters and lights, without making any mention of the masks. The international body did not even change its recommendations.
This caution collides with the idea that the public imagination has of a disease airborne. Measles, a classic example of this type of pathology, can float in a room for hours after a patient has left it and infect whoever enters next. A single patient can infect an average of up to 18 people (his R0 is between 12 and 18).
Each patient with COVID-19, however, infects about 2.5 people on average, although the figure changes according to the estimate we consult. Does this mean that it is not actually airborne? In reality, the dichotomy or transmission by droplets or by air is false. As University of Iowa Epidemiologist Dan Diekema recalled in a recent article, "It is rather a continuum".
In other words, between the droplets that fall to the ground less than two meters away from their emitter and the pathogens that float in the air for hours and hours there are endless midpoints.
"If there is transmission by aerosols at distances of more than two meters it is probably something very unusual and it is not playing an important role in epidemiology," Diekema explains to elDiario.es. That would explain the caution of the 239 experts and the WHO when addressing the issue and that last week's announcement has not yet caused a paradigm shift in the management of the pandemic.
An article published this week in the magazine JAMA by researchers at Harvard University (USA) agrees with this argument. "There are few absolutes in biology, people produce both droplets and aerosols, and transmission can take place over a broad spectrum," the authors say.
"It is impossible to conclude that aerosol transmission never occurs and it is perfectly understandable that some prefer to fall on the side of caution. […]. However, the available evidence suggests that long-range aerosol transmission is not the dominant mode of transmission, "they conclude. They argued as arguments the low basic reproduction number (R0) of the COVID-19, which would suppose a infective dose very high in case the transmission by aerosols was important. They also pointed to the low secondary attack rate among close contacts, among other factors.
"The consensus among the vast majority of clinicians, epidemiologists, Centers for Disease Control and WHO is that aerosol-based transmission is the exception rather than the norm," explains the Harvard University researcher and co-author of the article in JAMA, Michael Klompas. The researcher at the University of Colorado at Boulder (USA) José Luis Jiménez disagrees. “It is possible that a disease that is spread by aerosols is not contagious, much, or anything in between. There is no reason to think that either the virus is like measles or, if not, then it is not spread by air at all, ”he explains. He gives the example of anthrax, which is spread by aerosols, but not between humans.
A lexicon fight ...
What many researchers do agree on is avoiding the use of the word “air” (airborne) for the interpretation that the public can make. "Health professionals tremble when they hear this word, they think of super-contagious diseases such as measles or chicken pox," explains Jiménez.
"[El término aéreo] It creates the impression that anyone can become infected just by breathing, when that is not the case. There is only risk in case of very close contact with an infected person or when spending a long period of time in a poorly ventilated space, "says Klompas. Jiménez prefers the term "opportunistic air" to convey that "it is less contagious" than measles. Also "aerosol", even safer and "more technically precise". Diekema also bets on this last option or, failing that, "transmission by small droplets".
“SARS-CoV-2 is not super-contagious by aerosols, nothing close to measles. It is not aerial in the medical sense of the term ”, Jiménez wrote a few days ago. "Aerosols are enough to infect when they are highly concentrated, for example in close contact situations. When diluted in a room with good ventilation do not [son capaces] to infect ”.
In other words, according to the researcher, there must be a combination of factors for infection to occur, as has happened in well-known examples such as transmissions in choirs. This would mean that, although the coronavirus was transmitted by the air, it did, in many cases, at distances similar to droplets.
For this reason, Klompas agrees with the term "opportunistic air" proposed by Jiménez and quotes Diekema's false dichotomy. "It is clear that people produce a mixture of aerosols and droplets that can infect people nearby, but it is also clear that long-range transmission is an exception," he says. "The reason is that the droplets don't travel long distances and the aerosols dilute quickly as they move away from the source."
Don't blur what's important
For this reason, Diekema considers that "we must focus on what measures are necessary, rather than on the semantics of the mode of transmission." In other words, “there is not much difference between droplet and aerosol transmission if both require close contact and the interventions are similar. The key question is at what distance is transmission more likely, ”explains the epidemiologist. "It is likely that this will be more than two meters in adequate conditions such as lack of ventilation, but not that it is long-range."
That is why he says that the message has not changed and lists the three main actions against the pandemic: "Physical distance, facial protection and avoiding crowded interior spaces." Klompas adds hand hygiene and environmental cleanliness. Measures that have been recommended and applied in many countries for months.
"We should be aggressive in closing interior spaces that are not well ventilated and where we see a lot of transmission, such as bars, restaurants and churches," says Diekema. Also "make the masks mandatory where it has not been done yet."
This does not mean that it is not advisable to add other measures, if only to follow the precautionary principle. Diekema, Klompas and Jiménez stress the importance of ventilation. They point to the use of HEPA filters and ultraviolet lights, although it is too expensive an alternative for some countries and it cannot be implemented in one day. In that sense, opening doors and windows is a quick and cheap alternative.