Ventilation and viral loads: the key misunderstandings of how coronavirus spreads

Science: it’s moved very very quickly this year. Due to the ongoing pandemic, streams and streams of new insights about COVID-19 have been published each week to help combat the virus.

There’s not only been major advancements in our understanding of how the virus spreads, but experts have also developed several coronavirus vaccines (such as the Pfizer vaccine).

Overall, it’s been a lot to keep up with. Which is why virologists including Dr Muge Cevik – University of St Andrews lecturer and member of the government's New and Emerging Respiratory Virus Threats Advisory Group (NervTag) ­– published a comprehensive meta-analysis of recent studies relating to COVID-19 in November.

And we mean comprehensive. Covering viral load dynamics, viral shedding and infectiousness, 79 studies investigating coronavirus published in the last 12 months were examined with the purpose of better understanding key factors behind COVID-19 transmission.

The review’s findings? Many. From identifying factors that dramatically raise the risk of infection, to the role of asymptomatic carriers, the paper highlighted key coronavirus insights – and misunderstandings.

Before the establishment of the new coronavirus variant across the UK, we spoke with Dr Cevik about what we can learn from the latest research.

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What are the key findings of the COVID-19 research review?

Basically, we noticed that the majority of infections occurred in indoor settings.

Up until maybe May or April, we thought that every interaction, every setting confers a similar risk. But it's becoming much clearer that not all activities or environments are the same when we look at transmission.

It's also becoming much clearer that the duration of contact is important. The longer you spend with an infected person, and the larger the gathering, the higher the risk.

If you're in an indoor setting and if you're doing various activities – singing or shouting or eating – you're producing many more droplets. And if there's not enough ventilation in the room, then those droplets could linger for longer.

There are some studies showing that just opening a window decreases the risk of infection. For example, [there was a study examining] a bus outbreak. And people sitting next to the window were not infected.

[The study, conducted by American not-for-profit corporation MITRE, tested aerosol dispersion in buses with windows and fans across 84 test runs. No passengers actually sat on the bus during the study – 28 sensors were used instead.]

There's a lot of focus on handwashing, which is also important. But we need to emphasise the fact that if you're spending a lot of time in an indoor space, if the door or window is not open, the risk is still higher ­– even if you wash your hands.

Opening windows is just as useful because we need to have what’s called six 'air changes' over an hour. If you open a window in a small room just two or three inches, that could circulate the air for five people.

Another key factor is socio-economic circumstances. Household size is quite important. For example, there's a study that came from France showing if you're living in crowded housing, your risk of getting infected is three times more than someone who has more space.

How could these findings change contact tracing strategies?

What we've learnt over the last few months is that COVID-19 tends to spread in clusters. But I think this knowledge has not yet fully entered our way of thinking. And it's not really entered our preventative practises because we're still kind of focussing on flu-like transmission dynamics where often each person transmits to another person.

What we're seeing with COVID is that the majority will not transmit the infection, but a small number of people will cause large clusters.

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So knowing where transmission is occurring and understanding high-risk environments could help us to come up with a far more nuanced and targeted approach. And it could help us to direct our contact tracing efforts. Because most people will have been infected by someone who also infected other people, often at the same time. That means we need to backwards contact – trace those people to identify the settings of transmission.

So, for example, South Korea found a small cluster linked to a nightclub and then they tested everyone who's been to that nightclub. This is not necessarily the contact tracing type we're doing at the moment in the UK because we're basically following up the contacts of the person.

Creating a strategy focussing on clusters – identifying venues linked to high risk – could help us to return to (a little bit) normal.

So, could we avoid stricter lockdown measures if we identify these high-risk environments?

Exactly. This is what Japan has been doing. They basically said to avoid crowds and close contact in closed spaces, especially if it involves talking and singing. That basically tells people where the majority of infections are occurring.

At the moment, the majority of public messages don't really make sense.

Not just in the UK. For instance, other countries have mandates to use masks outdoors. Maybe this could be useful if you're in a crowded environment outdoors. But if you're walking on an empty street with no social contact, you don't necessarily need to put a mask on.

I think these messages are quite harmful because people don't really understand where the real risk is.

Many people still fear going to a supermarket, whereas they sit in a restaurant for hours. The risk of transmission is very different.

In a supermarket, people often don't spend a lot of time and there's a huge space and often you don't come very close to people. But in restaurants, you're talking and eating in groups.

When you're in a group, people tend to feel much more comfortable, and they don't keep the same measures with people they don't know. That's where we're seeing the infection that's happening.

How much of a problem is asymptomatic transmission?

I think probably asymptomatic people – who basically showed no symptoms whatsoever – may not be contributing as much as someone who has become symptomatic or gets symptoms later on.

Asymptomatic people seem to be around 20 per cent of all infections, and they are probably are one third as infectious compared to those who have or go on to develop symptoms.

That tells us that we need to focus much more on symptomatic people: how we could identify them early, how we could test them early, how we could make sure that they isolate during their most infectious periods rather than trying to identify everybody.

There's been a lot of discussion about who to test ­­– whether we test asymptomatic people as well. I think asymptomatic contacts of any infected person need to be tested. But, potentially, you don't necessarily need to test everyone. Japan, for example, is not testing everybody at the moment – they're focussing on large clusters and environments.

One of the epidemiologists from Japan says that their approach is like looking at a forest and trying to find the clusters, not the trees. And he thinks that the Western world is getting distracted by the trees and got a little lost.

This interview has been edited for length and clarity.