Are Aerosols the Dominant Mode of Transmission for Respiratory Viruses?

Check out this answer from Consensus:

The body of research indicates that aerosol transmission plays a significant role in the spread of respiratory viruses, including SARS-CoV-2 and influenza. While traditional views emphasized droplet and contact transmission, recent evidence supports the need to consider aerosols as a dominant transmission mode. This has important implications for public health measures, highlighting the necessity of improving ventilation, air filtration, and other interventions to mitigate the spread of respiratory infections.

The transmission of respiratory viruses has been a subject of extensive research, especially in light of the COVID-19 pandemic. Traditionally, it was believed that these viruses spread primarily through large droplets and contact with contaminated surfaces. However, recent studies have highlighted the potential significance of aerosol transmission, where small respiratory particles can remain airborne and infectious over longer distances and durations.

Key Insights

• Aerosol Transmission as a Dominant Mode:
  • Several studies suggest that aerosol transmission may be a dominant mode for various respiratory viruses, including SARS-CoV-2, influenza, and others. This is supported by evidence of virus-laden aerosols being capable of traveling long distances and remaining airborne for extended periods1 3 4 7.
  • The COVID-19 pandemic has particularly underscored the importance of aerosol transmission, with numerous superspreading events and differences in transmission dynamics between indoor and outdoor environments pointing towards aerosols as a significant transmission route1 2 3.
• Evidence from Air Sampling and Epidemiological Studies:
  • Air sampling studies have detected viral RNA in aerosols, and some have even cultured viable viruses from these samples, indicating the potential for infectious aerosols2 5 8.
  • Epidemiological studies have shown that enclosed environments with poor ventilation are associated with higher transmission rates, further supporting the role of aerosols in spreading respiratory viruses2 4.
• Challenges in Distinguishing Transmission Modes:
  • The distinction between droplet and aerosol transmission is often blurred, with some researchers advocating for a unified concept of airborne transmission due to the continuum of particle sizes and their varying infectivity6 7.
  • There is ongoing debate about the relative contributions of different transmission modes, with some studies indicating that measures targeting only droplet or contact transmission may be insufficient to control the spread of viruses like influenza and SARS-CoV-24 9 10.
• Impact of Environmental Factors:
  • Environmental factors such as ventilation, airflow, temperature, and humidity significantly influence the transmission of virus-laden aerosols. Effective ventilation and air filtration are critical in mitigating aerosol transmission1 8.

Are aerosols the dominant mode of transmission for respiratory viruses?

Linsey Marr has answered Uncertain

An expert from Virginia Tech in Aerobiology, Environmental Engineering, Infectious diseases

The virus can be released into the air through coughing, sneezing, talking, and breathing. Some people release more than others.

The virus is in droplets that range in size from smaller than we can see to large ones that fall to the floor quickly. There is no hard cutoff between “aerosols” and “droplets”.

At close range (imagine a cigarette smoke puff), the concentration of airborne virus will be quite high. As you get farther from the source, the concentration falls off rapidly.

If you are close, you are much more likely to inhale the virus. You are also more likely to be sprayed by large droplets that land on your face, which doesn’t happen if you’re farther away.

Viruses in small droplets can float around in air for many hours, but they will likely be quite diluted unless you’re in a small confined space. You could inhale these, but it’s much less likely than if you’re close to the person.

Airborne viruses will eventually settle on surfaces, like the floor and tables, and could be picked up if someone touches them. However, their ability to survive varies with temperature, humidity, and surface material.

They decay gradually over hours or days. It’s not like they all stick around for 9 days and then, poof, suddenly disappear. Think of them as fading away.

Are aerosols the dominant mode of transmission for respiratory viruses?

Melvin Sanicas has answered Unlikely

An expert from Takeda Pharmaceuticals International AG in Infectious diseases, Vaccinology, Global Health

We know that the novel coronavirus SARS-CoV-2 causing COVID-19 spreads from person to person through small droplets from the nose or mouth which are spread when a person with COVID-19 coughs or exhales. These droplets are larger than 5 microns in diameter and form a direct spray propelled up to 2 meters away from the infected person. Aerosols are droplets too but much smaller and may remain afloat for longer distances.

A Journal of the American Medical Association publication looked at hospital isolation rooms of 3 COVID-19 patients in Singapore and found no evidence of the virus in air samples but the air vent blades in 1 patient’s room tested positive. A second study, a preprint paper in the database medRxiv (not undergone extensive peer-review) examined the hospital environments in Wuhan, China and showed that in most rooms the virus was undetectable or low, but found the presence of the virus in aerosol form. It’s important to note that detection does not automatically mean what is detected can cause infection.

The basic reproduction number (sometimes called basic reproductive ratio, R0) of an infection is the expected number of cases directly generated by one case in a population where all individuals are susceptible to an infection. Measles has a Ro of 12-18, the highest of any infectious communicable disease because measles spread to others through coughing and sneezing, just like other respiratory viruses like influenza or coronavirus. The current estimates of the R0 for seasonal influenza is between 1 to 2 while COVID-19 estimates are between 2 to 3 in studies in China. Measles is much more infectious because it can live for up to 2 hours in an airspace where the infected person coughed or sneezed and infect a person who inhales the viral particles.

The term particle / PFU [plaque forming units] ratio refers to the number of viral particles required to form one plaque in a plaque assay. It is a measure of the efficiency by which a virus infects cultured cells. The ratio of total particles to plaque forming units for influenza and other RNA viruses is on the order of 10:1 to 100:1 which means only ~1–10% of virions are thought to initiate productive infection of a cell under low multiplicity conditions of infection. We do not know the particle / PFU ratio for SARS-COV-2 but it will probably be similar to influenza and other RNA viruses. 

Aerosol transmission may be possible if a person is exposed to elevated aerosol concentrations in closed spaces as we have seen in healthcare workers in China and Italy who got infected. That is why Infection Prevention and Control (IPAC) recommendations include airborne precautions should be used when aerosol generating medical procedures (AGMPs) are planned or anticipated to be performed on patients with suspected or confirmed COVID-19.

In conclusion, aerosol transmission is unlikely to be the primary force driving the current pandemic or else we would have seen more cases of COVID-19.

Are aerosols the dominant mode of transmission for respiratory viruses?

Hannah  Sassi has answered Likely

An expert from University of Sydney in Microbiology

In short, yes. Aerosols and large droplets (sneezing, vomiting, coughing, talking, laughing) can spread the virus through air easily and allow the virus to settle. There is no “safe distance” for this, as the virus can travel, depending on air carriages/wind, several hundreds of meters in some cases (outdoors). In a normal scenario (office building, school, public transportation) an aerosol will travel just feet/meters. It is highly dependent on the force of the sneeze/cough/laugh/breath, which then depends on the person!

But the second problem then becomes when the virus settles on high touch surfaces. Viruses can survive a wide range of times on fomites, depending on the virus and the environmental conditions. With very little physical contact in public, most people contract viruses from fomites. Several studies have found that a large proportion of virus from surfaces can transfer to hands, and subsequently your face/nose/mouth. This then puts you in direct contact with viral particles.

The only way to control for these events is you regularly wash and sanitize your hands, avoid touching high touch public surfaces (elevator buttons, door handles, hand rails) when you can, and disinfect/sanitize your own high touch surfaces regularly (mouse, keyboard, mobile phone, etc).

Are aerosols the dominant mode of transmission for respiratory viruses?

Donald Milton has answered Uncertain

An expert from University of Maryland in Aerobiology, Infectious diseases, Medicine

This is the wrong question and cannot be answered clearly without creating confusion. It is not a matter of identifying one dominant mode of transmission. It is likely that airborne transmission by aerosols (droplets and droplet-nuclei that remain suspended on air currents) is important. But, it is also true that there is no sharp cut off between aerosols and larger respiratory droplets that don’t travel very far. Short-range airborne transmission via the higher concentration of aerosols near a case is likely important, as are respiratory droplet transmission and fomites and contact. There is also evidence for fecal-oral transmission. All these routes are likely important an no one mode is likely to be dominant, as far as we know now.

Are aerosols the dominant mode of transmission for respiratory viruses?

Daniel Verreault has answered Uncertain

An expert from Université Laval in Aerobiology, Microbiology, Infectious diseases

The importance of the different modes of transmission, including person-to-person, surface-to-person, droplet and droplet nuclei, is unclear and is different for every virus strain. Some viruses may be mostly transmitted through contact (person-to-person and surface-to-person) while others may be mostly transmitted through aerosols (droplets and droplet nuclei). There is no clear answer to this question and only time and further research will allow us to determine the dominant mode of transmission of various viruses, including SARS-CoV-2.

For aerosol transmission, we have to consider two types of aerosols, large droplets and droplet nuclei, then again, the size threshold separating the two types of aerosols is unclear. Large droplets are mostly generated by coughing or sneezing, these droplets settle rapidly and do not travel more than 2 to 3 meters, whereas droplet nuclei are much smaller and can remain airborne for much longer periods, droplet nuclei are formed by the desiccation of larger droplets. The probability of these particles to cause infection depend on a variety of factors including environmental conditions and the ability of the virus to withstand these conditions.

It is not possible, at this moment, to determine which is the dominant mode or transmission for respiratory viruses in general or for SARS-CoV-2.

Are aerosols the dominant mode of transmission for respiratory viruses?

Jose Cabeda has answered Likely

An expert from Universidade Fernando Pessoa in Immunology, Molecular Biology, Genetics, Virology, Microbiology

Aerosols are the start of most fomites. In that regard they are the dominant mode of transmission. This does not mean that they are the major form of transmission, as this varies imensely on the virus and the environment.

Viral stability on fomites and in aerosols is highly variable and dependent at least on humidity, UV light and environmental polutants. All these can inpact on the viral stability and infectivity.

Nevertheless, aerosols are an important mode of transmission whenever close contact with an infected person that is producing unprotected aerosols is in place. Also, if such a person has produced aerosols in a closed environment, the contaminated aerosols they may stay airborne for a long time increasing the risk of contamination even in the absense of direct contamination. Furthermore, as mentioned in the start of this answer, even in the case where the aerosols have finally landed on a surface they can still produce contamination, now in the form of aerosol derived fomites.