How Respiratory Viruses Really Spread
How an infectious disease spreads from one person to another is a question so vital that if we get it wrong, we will fail to control its spread and may even make it worse than it has to be.
During the 19th century in London, people believed that miasma (‘bad air’) spread cholera, a diarrheal bacterial disease. So, stinky sewers were dumped into the Thames River, a major source of drinking water. This move ended up killing far more people, as in fact cholera spreads via contaminated food and water.
We have made similar mistakes with measles and tuberculosis, which are aerosol infections that we thought, for decades, were spread by droplets. And most recently, we have done the same with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the culprit behind the Covid-19 pandemic.
Droplets vs. aerosols
In July 2020, the World Health Organization (WHO) stated that SARS-CoV-2 spread via respiratory droplets that people emit by sneezing and coughing to object surfaces or another person within <1 meter apart. The same went for the U.S. Centers for Disease Control and Prevention (CDC), which also did not acknowledge that SARS-CoV-2 can spread via aerosols until October 2020, although only as a secondary route to droplets.
It was only recently that the WHO and CDC finally accepted aerosols as one of the main modes of SARS-CoV-2 transmission, on April 30, 2021 and May 7, 2021 respectively. The CDC is a step ahead of the WHO in this regard, stating that “surface transmission is not the main route by which SARS-CoV-2 spreads, and the risk is considered to be low.” But no news conferences were held, so the update did not get much attention initially.
Droplets are large particles that are heavy and drop from the air quickly, whether indoors or outdoors. Direct contact with such virus-laden droplets is from people; it’s indirect contact when it comes from fomites (contaminated objects and surfaces).
Aerosols, in contrast, are small particles that are light and linger in the air for some time, especially in poorly ventilated and crowded indoors. Aerosols thus travel farther than droplets — up to 8 meters vs. <1 meter — but they are rapidly dispersed in outdoor, well-ventilated, and fresh air.
This is also why superspreader incidents happen indoors (or a mix of indoor and outdoor). A database of over 2,000 superspreader events around the world has never recorded an outdoor superspreading event. But make no mistake, SARS-CoV-2 can still spread outdoors via close-contact droplets, although with a much lower risk of 18.7 times less than indoor spread.
The three most well-known aerosol infections are chickenpox virus, measles viruses, and tuberculosis bacterium, although the aerosol spread of the latter two was once denied for decades. Other respiratory infections, such as influenza and coronaviruses, are thought to spread via droplets.
When clinging to the status quo backfires
In early 2020, SARS-CoV-2 droplets were feared. We washed our hands and scrubbed objects and surfaces with disinfectants religiously in the name of coronavirus elimination. We obsessed over maintaining a physical distance of 1 meter apart. We didn’t think masks and air ventilation mattered. The CDC and WHO initially said that masks were unnecessary, not even indoors, if physical distance was maintained.
These are incorrect approaches, as most of us now know, but the damage has been done. Global Covid-19 cases and deaths have already surpassed 165 million and 3.4 million, respectively, this month. Not to mention that these numbers are most likely underestimates and do not consider disease morbidities (i.e., other medical problems caused), such as long Covid and risk of future diabetes (and other diseases).
“In my opinion, a lot of time, energy and money is being wasted on surface disinfection and, more importantly, diverting attention and resources away from preventing airborne transmission,” Kevin P. Fennelly, MD, a pulmonologist at the U.S. National Institutes of Health, said in November 2020. As The Atlantic also put it, “Hygiene Theater Is a Huge Waste of Time.”
By the end of 2020, global sales of disinfectants jumped >30% from 2019, totaling about $4.5 billion. Hundreds of millions of dollars were spent cleaning public transportation services in New York City alone in 2020, which clearly did not work as Covid-19 was raging there at the time.
Even in India now, where Covid-19 is running rampant and causing over 4,000 deaths daily, drones are being sent out to disinfect objects and surfaces. That money could have gone to addressing the widespread oxygen shortage in the hospitals in India, securing more vaccines, or improving indoor air quality.
Countries worldwide still ban outdoor activities but kept indoor conditions as they are, insufficiently ventilated and distanced — still adhering to one meter apart when aerosols can travel farther than that. But at least masks have been taken more seriously, which do a great job at blocking droplets and aerosols.
We have also become germaphobic. We avoid touching others or things others have touched. Schools, stores, and institutions still practice deep cleaning to instill in us a sense of safety. Although there is no harm in deep cleaning and the practice may even lower the risk of viral spread by a bit, it can take a psychological toll. Do we really have to live in such unnecessary fear? No wonder there’s such “pandemic fatigue,” our dwindling motivation to keep up with the infection control and prevention efforts.
The long-overdue acceptance of aerosol spread
In July 2020, 239 international scientists wrote an open letter to the WHO outlining the evidence that respiratory viruses like coronaviruses can spread via aerosols, so the same most likely applies to SARS-CoV-2. The WHO, however, was not fully convinced and remained adamant that hand hygiene is the most essential measure for prevention and control of Covid-19.
But soon thereafter, more and more studies were published in reputable journals such as Nature that provide indisputable evidence for the aerosol spread of Covid-19 or SARS-CoV-2:
- Despite strict droplet precautions, frequent infections still occur among hospital staff and other places like restaurants. This makes aerosol spread over long distances indoors the most plausible explanation.
- Live or infectious SARS-CoV-2 have been found in air samples from hospital rooms of Covid-19 patients, as well as in an infected person’s car. (Live means that the virus is culturable in cultured cells in the lab, indicating an infectious potential.) Although some studies have failed to confirm the same, it can be expected given that isolating aerosolized viruses demands a complex set of skills with much room for failure.
- SARS-CoV-2 spread has been demonstrated in ferrets in separate cages distanced over one meter in a room. No direct or indirect contact took place.
- SARS-CoV-2 (non-live) has been detected in air sampled from exhaust vents of central ventilation systems in hospitals. (Non-live means dead genetic fragments of a virus that are not culturable in cells.)
- Kinetics (physical chemistry) studies have shown that aerosolized SARS-CoV-2 can be emitted via talking, singing, coughing, sneezing, and even the mere act of breathing.
- Infected persons who show no symptoms (asymptomatic) can still spread SARS-CoV-2 to others. How else can transmission occur besides aerosols when a person is not coughing or sneezing out droplets?
With such conclusive evidence at hand, the WHO and CDC can no longer remain in denial and have finally accepted aerosols as a significant driver of SARS-CoV-2 spread.
It took them over a year for many reasons.
For one, many scientists were not convinced. “They [proponents of aerosol spread] were rebuffed, not only by loudmouths on Twitter and on TV, but by other scientists who clung stubbornly to an outdated view of viral spread,” The Atlantic reported.
There’s the preconceived belief that aerosols spread far and wide, such as the measles and chickenpox viruses that are many times more infectious than SARS-CoV-2. But the tuberculosis bacterium, another familiar aerosolized infection, is as contagious as SARS-CoV-2. This is because other factors also determine the pathogen’s infectiousness, such as its resistance to dryness and other environmental conditions and the minimum infectious dose needed to cause disease.
Second, hospitals have always been careful of Staphlococcus aureus bacterium, respiratory syncytial virus (RSV), norovirus, or other infections that travel on surfaces, such as hands, bed rails, or medical devices. When Covid-19 arrived, the same precaution measures were practiced even more rigorously without a second thought.
Third, the WHO infection prevention and control committee hangs on the precautionary principle of not going against scientific dogma unless more definitive evidence shows otherwise. In the history of science, we always set “a higher standard of proof for theories that challenge conventional wisdom than for those that support it,” Zeynep Tufekci, Ph.D., associate professor and sociologist, wrote. “It is easier to keep adding exceptions and justifications to a belief than to admit that a challenger has a better explanation.”
A paradigm shift in science is always messy, with an initial backlash followed by ironic appreciation if the change survives the scientific scrutiny.
The groundless belief in fomite transmission
Why did authorities strongly believe that droplet or fomite spread was of utmost importance for Covid-19 for so long? (Recall that fomites are contaminated objects and surfaces.) Was there evidence? Yes, except that the evidence came from other viruses, not SARS-CoV-2. But little did we know that even such evidence was fundamentally flawed.
“I agree that fomite transmission is not directly demonstrated for this virus [SARS-CoV-2],” Benedetta Allegranzi, MD, the WHO technical lead on infection control, said in July 2020. “But it is well known that other coronaviruses and respiratory viruses are transmitted, and demonstrated to be transmitted, by contact with fomite.”
In an article published in The Lancet titled “Exaggerated risk of transmission of COVID-19 by fomites” in July 2020, Emanuel Goldman, Ph.D., a professor of microbiology and one of the earliest proponents of aerosol spread, critiqued the studies showing that various coronaviruses could live on object surfaces for a few days.
“In the studies that used a sample of 10⁷, 10⁶, and 10⁴ particles of infectious virus on a small surface area, these concentrations are a lot higher than those in droplets in real-life situations,” the professor wrote. “Hence, a real-life situation is better represented in the work of Dowell and colleagues in which no viable virus was found on fomites.” (Dowell et al. swabbed many hospital surfaces that came in contact with SARS patients and found only dead genetic fragments of SARS-1.) Thus, even “for the original SARS virus,” Prof. Goldman said, “fomite transmission was very minor at most.”
More bizarrely, most studies showing fomite spread of respiratory viruses do not meet standards for real-world applicability, according to a 2021 paper of Prof. Goldman published in Applied and Environmental Microbiology. Reviewing these studies, as well as citing another two well-written and relevant 2018 and 2020 research reviews, the professor concluded that no convincing, real-world evidence exists for the fomite spread of respiratory viruses. This is except respiratory syncytial virus (RSV), where scientists in 1981 experimented with children volunteers to show that RSV can spread via surfaces.
Fomites “should be considered no more than a very minor component of this pandemic. It’s a terrible waste and sometimes even dangerous to continue expensive and time-consuming surface disinfection, and this is obstructing efforts to control the pandemic,” Prof. Goldman wrote. “Normal routine hygiene is all that’s needed.”
But pandemic or no, we should wash our hands after using the bathroom, before eating, and whenever necessary. Diarrhea and skin infection are things to avoid. It’s just that disinfecting every object and surface we encounter is simply going too far.
Another round of new evidence comes from a 2021 systematic review of 64 studies that studied the fomite spread of SARS-CoV-2, mainly in hospital settings. While many studies detected genetic fragments of SARS-CoV-2 from object surfaces, not one study has successfully recovered live SARS-CoV-2. That’s right, not one.
Why can’t SARS-CoV-2 seem to survive on fomites? “Due to the rapid decrease of water content in the evaporated droplets…the virus tended to deteriorate sharply, and virus concentration plummeted within a few minutes,” a 2021 research review explained. “Although a virus can be detected in a matter of hours, tens of hours, or days, the risk of transmission is negligible compared to when it first left the host.” Plus, coronaviruses are enveloped viruses that break apart very easily — with soap or drying, for example— compared to more resistant, non-enveloped viruses.
The insignificance of fomites in respiratory viral infections also makes sense if you think about it. Even before the pandemic, how often did people cough or sneeze directly at you? Basic manners remind us to sneeze or cough sideways, which should be sufficient because droplets drop quickly. It’s the aerosols we should be worried about.
Rethinking how respiratory viruses spread
It may be time to change our basic understanding of respiratory virology.
“Goldman is extending his crusade against fomite fear from COVID-19 to other diseases. The old story is that if you make contact with a surface that a sick person touched, and then you touch your eyes or lips, you’ll infect yourself,” The Atlantic reported. “While Goldman acknowledges that many diseases, especially bacterial diseases, spread easily from surfaces, he now suspects that most respiratory viruses spread primarily through the air, like SARS-CoV-2 does.” (Viruses are 10–100 times smaller than the smallest bacteria, so it is easier for viruses to fit and pack themselves in smaller aerosolized particles.)
Many other research groups have indeed found evidence of aerosol spread of the Middle East respiratory syndrome coronavirus (MERS-CoV), SARS-1, RSV, rhinoviruses, and seasonal influenza viruses that we thought were spread by droplets. The bitter part is that these pieces of evidence came from studies published between 1987 and 2018. Some of these studies were also not easily published; a 2011 paper on aerosolized influenza, for example, was rejected many times by major scientific journals.
Prof. Goldman is not alone in his views. “There is no evidence to support the concept that most respiratory infections are associated with primarily large droplet transmission,” Dr. Fennelly wrote in a 2020 paper published in leading scientific journal The Lancet. “In fact, small particle aerosols are the rule, rather than the exception, contrary to current guidelines.”
Yugou Li, Ph.D., a professor and head of the Department of Mechanical Engineering at the University of Hong Kong, led a 2004 study in the prestigious New England Journal of Medicine showing that aerosols were driving the spread of SARS-1. And Prof. Li’s team has also reasoned “that the public health establishment had it backward and that most colds, flu, and other respiratory illnesses must spread through aerosols instead.”
Another paper published in The Lancet last month from another group of six distinguished researchers (including Prof. Tufekci) also arrived at a similar conclusion when reviewing the existing literature: “Decades of painstaking research, which did not include capturing live pathogens in the air, showed that diseases once considered to be spread by droplets are airborne.”
It’s not only cholera, measles, and tuberculosis that we got their mode of disease spread wrong. We got it wrong with most, if not all, respiratory viruses. Let Covid-19 be the last time, and let’s be wiser when the next one comes. There’s no need to fear touch or outdoor activities. Rather, wearing masks, maintaining good quality indoor air, and getting vaccinated safely should be our priorities.