Airborne transmission of SARS-CoV-2 is evolving

It has long been clear that SARS-CoV-2, the virus that causes Covid-19, can be transmitted through the air through small droplets and aerosols. The difference between droplets and aerosols is that aerosols are really small (less than 5m), small enough to travel long distances and deposit in the lower respiratory tract.

Although the indirect evidence for airborne transmission is overwhelming, direct evidence has been more difficult to obtain.

In a new prepublication, NIH scientist Vincent Munster and colleagues report on a new and ingenious experiment that provides direct evidence.

Syrian hamsters are a good model of human SARS-CoV-2 infection

The experiment used the Syrian hamster (Mesocricetus auratus) as an animal model of what happens when a person is infected with SARS-CoV-2.

Of course, hamsters and humans are different organisms, so Munster first established that the two reacted the same to exposure to SARS-CoV-2. The researchers used a cell culture that had been processed so that some of the cells behaved at the molecular level like hamster cells while other cells behaved like human cells. Using a biotechnology called pseudovirus that is believed to work against such cells like SARS-CoV-2, the authors determined that hamster cells were neither more nor less likely to be infected with SARS-CoV-2 than cells human. In short, this experiment has shown that the Syrian hamster is probably a very good model of human infection with SARS-CoV-2.

Transmission of SARS-CoV-2 in hamsters in flight

Next, the researchers set up an experiment to see if the hamsters could infect each other from the air. The cages were set up with a tube connecting the cage of an infected hamster (“donor”) and the cage of a susceptible hamster (“recipient”). The hamsters could not move through the tubes, but the air flow would allow suspended particles, if any, to pass from the donor cage to the recipient cage. Experiments were made with connecting tubes of three different lengths, the longest measuring two meters. At this distance, the scientists determined that hardly any particles larger than 5 µm moved from the donor to the recipient’s cage. Despite this, all four susceptible hamsters were found to be infected. Since no large droplet could travel the entire two meters, transmission had to be through aerosols.

Alpha variant of SARS-CoV-2 is more prone to airborne transmission than the original strain

Now for the really interesting thing: these researchers did two experiments with both the original SARS-CoV-2 virus (which they call “lineage A”) and the Alpha variant (which they call by the scientific name B .1.1.7).

First, to determine if there are differences in transmission efficiency between the two strains, the scientists set up experiments over two different exposure periods (one hour and four hours) using the two meters. What they found was that the Alpha variant transmitted to all recipient cages with only one hour of exposure, while the original strain was transmitted only halfway through. receivers.

Second, to see if the more aerosol-prone Alpha variant was also competitively dominant, so that it would rule out the original strain, the researchers conducted a series of hamster-to-hamster infections. In the first generation, eight hamsters were inoculated with equal amounts of the original strain and the Alpha variant. After twelve hours, these were placed in cages with eight susceptible “sentinels” for a period of 24 hours. This second generation was then housed with eight new sentries (the third generation) for an additional 24 hours, followed by a series of transmissions from the third generation to the fourth.

Using molecular techniques, scientists determined which viruses the sentinel hamsters were infected with. In total, in 10 of 13 transmission events, the Alpha variant outperformed the original strain. By the third generation (a series of two inheritance events), the original strain had all but disappeared from the experience.

The result

Together, these experiments establish very important facts about the transmission of SARS-CoV-2 in Syrian hamsters.

1. Transmission is through aerosols, not just larger droplets.
2. Aerosol transmission can occur over long distances (two meters is a long way for a hamster) and over short periods of exposure (as short as an hour in these experiments).
3. The Alpha variant is more efficient at transmitting aerosols than the original strain.
4. The Alpha variant is more competitive than the original strain (which is probably why it has become dominant in the UK and elsewhere so quickly).

I asked Dr Munster what he thinks was the most important finding from this experiment. His answer: the experimental evidence needed to conclude that aerosols are probably one of the main routes of transmission of SARS-CoV-2.

So why is this important? According to Dr Munster, because vaccination remains limited in much of the world, SARS-CoV-2 mitigation strategies should continue to include preventive strategies focused on reducing exposure and blocking transmission. . This includes ventilation, air disinfection, social distancing, and wearing face masks.

The evidence for increased aerosolization in the Alpha variant is strong. With the world now beset by the new Delta and Lambda variants, one wonders how different these can turn out to be.