How coronavirus is transmitted and why masks may be useless

2024 Author: Malcolm Clapton | [email protected]. Last modified: 2023-12-17 03:44

Perhaps the best way to reduce the risk of infection is to ventilate the area more often.

In April, the results of a new study, Ten scientific reasons in support of airborne transmission of SARS ‑ CoV ‑ 2, were published that are changing the way COVID ‑ 19 spreads.

What is known about how the coronavirus is transmitted

Since the beginning of the pandemic, scientists have looked at three Questions and Answers: How is COVID-19 transmitted? probable routes of transmission of COVID-19.

1. Airborne. That is, with small drops of saliva that an infected person spreads when talking, breathing, coughing, sneezing.
2. Contact-household - through surfaces contaminated with viral particles and unwashed hands.
3. Aerosol. That is, with the smallest aerosol particles, which are also released during breathing and talking.

The contact-household route of infection was quickly recognized as unlikely by Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments. Scientists believed that the main method of transmission of coronavirus was airborne droplets. That is why the WHO and government health services insisted on wearing masks: filters are able to hold the largest droplets released during breathing and prevent them from spreading around.

But suspicions about the aerosol route of infection did not disappear. Because the experts could not explain the cases when people who did not have close contact with infected people fell ill with anything else. For example, bus passengers traveling on business half an hour or an hour after an infected person. Or patients of clinics who were not in the coronavirus department and even theoretically could not meet with those with COVID-19.

These cases could be explained only by the assumption that the virus is transmitted not only by drops, but above all with aerosols, for which walls, masks, or the time elapsed since the departure of a sick person is not so important.

What is an aerosol route of infection and how it differs from an airborne droplet

The difference is very subtle, but it is there. And it consists primarily in the size of the particles on which the virus spreads. It can be described very roughly as follows: drops are large and heavy, aerosol particles are small and light.

Where is the border that separates the droplets from aerosols, scientists have not yet decided. It is assumed that particles with a diameter greater than 5–100 µm (there is still debate about the exact value) are drops. Less is aerosol.

Size determines the difference in particle behavior.

• Heavy droplets that an infected person spreads around themselves cannot stay in the air for a long time: they quickly settle on the ground and other surfaces within a radius of a couple of meters from the source of infection.
• Light aerosol particles can hang in the air for hours, and when gusts of wind are blown, they can be transported tens of meters. That is, an infected person can infect even those people who are far from him, for example, on another floor of a hotel (if these floors are connected by a common ventilation system).

But it is difficult to unequivocally prove which of the modes of transmission of the infection is dominant. So far, doctors are only collecting information and making assumptions.

Why scientists decided that coronavirus can be transmitted by aerosol

Here are the arguments by which the authors of the study Ten scientific reasons in support of airborne transmission of SARS ‑ CoV ‑ 2, published in The Lancet, support the notion of aerosol transmission of the virus.

1. Superproliferators play a key role in the transmission of COVID-19

Super-distributors are people who manage to infect not one or two or even ten people, but hundreds at once. According to some reports, approximately 10% of those infected are responsible for 80% of all infections.

Scientists draw attention to the fact that all episodes of mass infections occurred not through close contacts, but at large-scale events, such as concerts, football matches, in restaurants. That is, where the super-distributor was separated from its "victims" by tens and even hundreds of meters. This is possible only if the virus did not spread with heavy droplets, but aerosolized.

2. Cases of transmission of COVID-19 “through the walls” were recorded

For example, in hotels where citizens who have arrived in a country are quarantined. Healthy people and those who are sick with coronavirus infection live in different rooms, do not contact or even see each other: they are separated by walls. Nevertheless, cases of infection are Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 during Border Quarantine and Air Travel, New Zealand (Aotearoa), and they can only be explained by the fact that the smallest viral particles are carried through ventilation systems.

The authors of the study recall that in approximately the same way, by excluding transmission by direct contact, it was possible to prove the aerosol route of measles transmission.

3. Asymptomatic carriers do not cough or sneeze, but infect others

People who do not have symptoms of COVID-19 are responsible, according to various sources, for 30–59% of all infections. Since such infected people usually do not cough or sneeze, that is, they do not emit large droplets into the surrounding air, their infectiousness can only be explained by the fact that the coronavirus is transmitted by aerosol.

4. Coronavirus spreads better indoors than outdoors

Outdoor Transmission of SARS ‑ CoV ‑ 2 and Other Respiratory Viruses: A Systematic Review is about 18 times more likely to become infected while being in the same room with an infected person than by contact with a sick person on the street. If the same room is ventilated, the risks are drastically reduced It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19).

According to the authors of the study, this confirms the version of the airborne, rather than airborne, transmission of the virus.

5. In hospitals where airborne transmission of infection is excluded, cases of infection are still recorded

Personal protective equipment used by doctors reliably protects against saliva particles. That is, if the virus spread by airborne droplets, the doctors would not get sick. But they get sick.

This can only be explained by the effect of aerosols, against which the same masks are powerless.

6. Viable virus was detected in air samples

To at least some scientists,. It should be noted that the results of these studies are ambiguous and the virus could not be found in other samples.

But the authors of the study remind that isolating the virus from the air is generally an extremely difficult task. And, for example, measles and tuberculosis, whose aerosol transmission has been established, is still not possible to detect in air samples.

7. Coronavirus found in ventilation systems of hospitals

Viral particles would not have been able to get there in any other way besides aerosol.

8. Animals in the experiments managed to infect by aerosol

So, in one study SARS ‑ CoV and SARS ‑ CoV ‑ 2 are transmitted through the air between ferrets over more than one meter distance, scientists connected two cells using a complex ventilation system. Healthy ferrets were sitting in one of them, and sick ferrets in the other. This system of curved pipes ensured that the droplets of liquid secreted by infected animals did not reach healthy ones. Nevertheless, after some time, the latter were also infected.

9. There is no convincing evidence that the aerosol route of transmission of infection is excluded

Yes, people who are in the same room with an infected person do not always become infected. But, as the researchers suggest, this is due to the fact that a combination of several factors is necessary for infection.

For example, it is not enough to stand next to a sick person. It is necessary that he still excrete a fairly large number of viral particles.

10. Evidence that airborne transmission is the main route is not convincing

Scientists have suggested that the infection is transmitted through drops, after they found out that most often the infection occurs through close contact. They say that saliva particles scatter nearby, so an infected person especially easily shares the virus with those who are nearby.

However, there may be another explanation. For example, this: the coronavirus still spreads by aerosol, but a lighter aerosol, flying a long distance, often dissolves in air masses and becomes less concentrated and infectious.

Can the aerosol route of transmission of coronavirus be considered practically proven?

No. Although the above arguments are weighty, they are still not convincing enough. More research is needed to confidently say that COVID-19 is transmitted primarily by aerosol.

However, WHO is already confidently claiming Questions and Answers: How is COVID-19 transmitted? that the spread of the virus can occur, including by aerosol. And it emphasizes the possibility of infection in poorly ventilated, that is, poorly ventilated rooms.

What does airing have to do with it

In fact, this is the only way to protect against aerosol-transmitted viruses.

We repeat once again: neither masks, nor walls, nor social distancing will save you from aerosols. Such tiny particles can hang in the air for hours and be carried tens of meters with the wind.

If we accept the version that the aerosol route of infection is the key, then the traditional methods of protection against coronavirus - masks, social distancing, disinfection of surfaces - recede into the background. And the first thing is the ventilation of the premises: the moving air efficiently and quickly removes aerosols away.

Does this mean that masks can not be worn, and hands can not be washed?

No. As we have already said, the hypothesis that the aerosol route of transmission of the virus is the key one does not yet have sufficient evidence. In addition, this version does not exclude that COVID-19 can be transmitted by airborne droplets. So masks are likely to play a role in protecting against infection.

There is another reason not to abandon the already familiar hygienic safety measures. As practice shows, since the beginning of quarantine, the number of diseases transmitted by airborne droplets has significantly decreased. So, this winter the flu has practically disappeared - and the massive wearing of masks and social distancing may have played a role in this. Well, the habit of washing your hands is likely to reduce the frequency of traditional summer rotavirus and enterovirus infections: intestinal flu, Coxsackie virus diseases, and others.

In general, information on the possible aerosol route of infection should be taken into account. To supplement the existing coronavirus prevention measures with ventilation, and not to abandon them altogether.