Last week 237 scientists from various disciplines signed a letter to a journal suggesting that infection with CoVid 19 could be from airborne transmission and not just by droplets. They called on the World Health Organisation to acknowledge this and advise accordingly. In this blog I explain what this means and summarise the arguments for and against their suggestion. I then consider what are the practical consequences if these scientists are right.
What is the difference between droplet and airborne transmission?
In simple terms, we breathe out drops of different sizes. When we cough, or sneeze, (even talk loudly or sing!) we breathe out large droplets. These can land on surfaces, or if we are close enough land on someone’s eyes, nose or mouths. If these droplets carry the virus then this can lead to infection. Because droplets can only travel for a short distance, by keeping a safe distance, thought to be around 2 metres, and especially washing hands in case they have touched surfaces on which there are active droplets, most carriage of infection could be stopped. Indeed, that was the advice at the beginning of the pandemic. In general, this has also been the advice for years to avoid catching infections such as flu.
The scientific question is based on the fact that even when we breathe out normally, we breathe out smaller droplets that hang around in the air for longer times and longer distances. These smaller droplets are referred to as aerosols (like the air freshener sprays we use at home). The scientists who signed the letter suggested it is possible that the CoVid virus can also spread in these smaller drops and hence be able to lead to infection at larger distances than 2 metres. This is illustrated below: Source: (https://www.eurekalert.org/pub_releases/2020-04/quot-ipe041520.php
There are some key questions:
1. If someone is carrying the virus can they spread the virus via aerosols as well as by droplets?
There is only limited research on this and the data have not been subject to peer review (ie the quality of the research judged by experts to assess its quality). In one research study, Chinese scientists studied the breath of patients with CoVid 19 and looked for evidence of the virus in air samples as well as in their actually exhaled breath. What they found was that there were viral traces in 17% of samples of directly exhaled air (ie the ‘large drops’ in the picture above) but also in 4% of samples of ‘wider’ air (the ‘small drops’). This was a small study but the conclusion was there are traces of the virus in the air from the breaths of people with CoVid
2. Does this type of study prove that the exhaled air contains active virus?
This is a source of dispute between scientists. The studies such as the one above only show that there is viral genetic material (RNA) and cannot prove the actual presence of the ‘whole’ virus. RNA for example may be found in swabs of people who are recovering from infection but that does not mean they are infectious. The problem is that whilst it is easy to investigate for the presence of RNA, it is very difficult to prove the presence of the whole virus.
3. How far can the aerosols spread?
Forgetting CoVid for the moment, there is a general question which is how far aerosols can spread, and the short answer is we don’t know. and depends on so many factors including how they were produced ranging from normal breathing to speaking, sneezing and coughing. Droplet size is also important: larger droplets will fall to the ground closer to the person than smaller droplets. Droplets in aerosols can certainly travel more than 2 metres before dropping
4. How long would the virus last for in the aerosol?
The next question is in a person who has CoVid and is breathing out small droplets in an aerosol how long would a virus survive? This depends on where you are. In laboratory studies, 90% of viruses cannot survive more than 6 minutes outside in sunlight. Contrast that with being inside a poorly ventilated room with no UV light, and the virus within aerosol drops could survive for two hours
5. Would you receive a big enough dose of the virus to actually get infected?
Thus far much of the above evidence is circumstantial. It is known that we produce aerosols when we breath out normally, and that virus traces can be found in exhaled breath. What is not known is how much virus is needed to trigger an infection. To give an idea of magnitude, a droplet of one ml can contain millions of virus particles and one minute of loud speech could generate 1000 virus-containing droplets, which can remain airborne for 8 minutes. There is also the concern that unlike even other coronaviruses eg SARS, in the laboratory aerosols with CoVid19 can remain infectious for 6 hours. It is important to remember that studying laboratory-based aerosols may not replicate what happens in real life
6. Is there any real life evidence that aerosols can cause CoVid at a greater safe distance than we were led to believe?
At the moment it is anecdotal and based on records of cases occurring in people who were in contact but not close contact. There was an interesting study of people who caught the infection in a restaurant. They proposed that those who did, and who did not, contract the infection could be explained by air conditioning flows rather than their proximity to each other. A now infamous choir practice in Seattle emerged with 52 members being infected. Although it is possible that one affected member spread the virus to all the others by direct droplet, an alternative view is that this person was exhaling aerosols throughout the evening and the high concentration of virus in the exhaled air lead to the high rate of infections
This all sounds quite scary how worried should we be?
Obviously more people would be exposed to the aerosol from an infected person than to their large droplets. Further, even before lockdown measures, an infected person typically spread the virus to less than 3 others (that is, the R value was under 3). Given how many people we are in contact with, if aerosol exposure was important then we might have expected a much higher transmission rate.
In addition, the risk of infection, even by airborne route, depends on the prevalence of the infection in the population. As the number of new cases is decreasing, so does the likelihood of coming into contact with someone who is carrying the virus.
What are the conclusions:
Like so much else with this pandemic despite our desire for hard facts there is much we don’t know.
- My conclusion is that whilst the large majority of infections occur from our contact with large droplets, the scientists are right to be cautious in not excluding the theoretical risk of aerosol transmission, even from normal activities in an asymptomatic person.
- Statistically the chance of becoming infected in this way is small, and especially small in areas where there have been no recent cases
- Outside and in well ventilated spaces and especially where there is UV light, social distancing should offer the necessary level of protection.
- SAGE’s advice that when indoors, especially in poorly ventilated places, people should wear face masks, avoid face to face interactions, and loud talking or singing seems sensible
Interested in further reading:
“Mounting evidence suggests coronavirus is airborne-but health advice has not caught up”. Nature, 8 July 2020
“How coronavirus spreads through the air: what we know so far” Scientific American, 12 May 2020
First10EM: http://first10em.com/aerosols-droplets-and-airborne-spread/
Making sense of CoVid-19 with Silman 