BHEKISISA CENTRE FOR HEALTH JOURNALISM
New year, same rules: The science behind masks, ventilation and keeping a distance
Wearing a mask, washing your hands, good ventilation and keeping your distance all help to lower your chances of being infected by the virus that causes Covid-19. This is because they reduce how much virus you could be exposed to as you go about your day.
It’s been almost two years since a tiny virus brought the world to a grinding halt. And experts say that SARS-CoV-2 — the virus that causes Covid-19 — is likely here to stay. So how are we going to live with it? Because, let’s face it, we’re kind of over lockdowns, travel bans, not seeing our friends and not going into the office or school (who thought we’d ever say this?).
To get to a point where we return to some sense of our previously normal lives, we’ll need to use a combination of defences to snuff out the Covid-19 pandemic.
It’s all about the dose
When you exhale, speak, cough or sneeze, clouds of small respiratory droplets form in front of your face. These droplets can contain the virus that causes Covid-19 if you’re infected, and so you can spread the infection to someone else. The tricky thing with Covid-19 is that people can pass the virus on even if they don’t show symptoms — making it difficult to know who is and who isn’t infected around you.
The amount of virus that you are exposed to — in other words, the infective dose — plays a role in how sick you’ll get, says Kerrigan McCarthy, a specialist pathologist from the National Institute for Communicable Diseases (NICD). The NICD tracks the spread of Covid across South Africa.
So, to help us keep safe rather than be sorry, health authorities say we should combine vaccination and non-pharmaceutical measures (doing things like wearing face masks, avoiding crowds and ensuring proper ventilation indoors).
Non-pharmaceutical measures boost your protection because they reduce the amount of virus you come into contact with. And the less virus going around, the lower the chance of getting infected and becoming sick.
We sat down with McCarthy to unpack the science behind the things you can do to protect yourself.
Can I relax once I’ve had my vax?
Not quite. The SARS-CoV-2 virus constantly evolves and some of these changes have led to new forms, or variants, of the virus that can outsmart (at least, to some extent) the antibodies our bodies produce after vaccination or natural infection.
Covid antibodies’ main job is to protect us from getting infected with SARS-CoV-2, but variants such as Delta and Omicron are able to get around some of the protective properties of our antibodies and infect us regardless. That’s one of the reasons why Omicron spread so fast around the world over the past two months.
Getting infected after vaccination is called a breakthrough infection.
In a study in England in 2021 that included close to 970,000 household members (people who live at the same address), about 10% of unvaccinated people contracted the SARS-CoV-2 virus from a Covid-positive person in their household. In contrast, people who had already had at least one shot of the AstraZeneca or BioNTech/Pfizer vaccine had about a 50% lower chance of contracting Covid from someone they lived with.
This suggested that people who had been vaccinated were half as likely to contract the virus than those who had not had their jab. But because this study was conducted between January and February 2021, when the Alpha variant was dominant in England, it can’t be assumed that the same level of protection will hold for infection with the Omicron variant, which is currently dominant across the world and has far more mutations than earlier variants.
A 2021 Dutch study, done when the Alpha variant was the dominant form of SARS-CoV-2 circulating in the Netherlands, also showed encouraging results. Fully vaccinated people were approximately 70% less likely to transmit the virus to someone in their household than people who had not been vaccinated.
Although the Omicron variant seems to be much more transmissible than any other so far and there are larger proportions of breakthrough infections — mostly because of Omicron’s ability to circumvent our antibodies — it doesn’t look as if the variant is able to get around our T-cells as easily. (T-cells are the part of our immune system that protects us against falling seriously ill after infection with a germ.) That is why Covid jabs are really good at lowering your chances of getting severely sick if you have a breakthrough infection and so help to prevent you from ending up in hospital or dying from Covid-19.
Why mask up when we meet up?
Masks are a form of what scientists call “source control”. This means they reduce the number of infectious particles at the source — that is, the person who is infected.
Masks lower the number of viral particles in the air around someone’s face by trapping respiratory droplets and preventing airborne viral particles from travelling very far. So if an infected person wears a mask, they will spread fewer viral particles around than someone who isn’t wearing a mask. And if a contact also wears a mask, their chances of becoming infected are lower too, because the mask acts as a filter that lets air through but blocks a large amount of infectious viral particles from being inhaled (how much depends on the type of mask).
The logic is simple — and backed up by research: if there is less of the virus being spread from person to person, the chances of getting infected are lower — because it’s all about the dose.
In a 2021 study among university students in the US, the chance of testing positive for SARS-CoV-2 after contact with someone who had Covid was found to be five times higher if neither person wore a mask during their meeting than if both wore a mask (although the study didn’t specify which type of mask).
All masks are not the same. Cloth or surgical masks (the ones you buy at pharmacies) offer considerably less protection than N-95 masks, also known as respirators. A respirator looks like a mask, but is made of a thicker material specially designed to protect the wearer from inhaling aerosols because it filters incoming particles much more efficiently than a cloth or surgical mask.
How well your mask fits, so how tightly it fits on your face, also plays a role in how effective it is. Your mask should cover your nose and mouth so that there’s an effective seal to keep viral particles (at least partly) from escaping.
A large study that looked specifically at people wearing surgical masks and which involved just over 340,000 people across 600 villages in rural Bangladesh, showed that people who wore these masks were about 11% less likely to develop Covid-19 than those who didn’t wear a mask. This protection increased to almost 35% for people over the age of 60.
This was a randomised controlled study, which means that the effect of an intervention (in this case, the wearing of surgical masks) was compared between two groups of similar people. One group received the intervention while the other did not. The participants in each group were randomly selected, which means they were chosen in a way that characteristics such as age and gender, which could have a bearing on the study’s outcome, are equally represented in both groups.
When using a cloth mask, it’s best to use one with three layers of material, which is what the South African government recommends. Helene-Mari van der Westhuizen previously reported for Bhekisisa that the type of material is also important. Adding a removable filter made of polypropylene in the cloth mask or layering a cloth mask and surgical mask on top of each other could help the mask offer more protection to the person wearing it. Polypropylene is a type of plastic that can be pulled into fibres that can be woven into wearable fabrics. Because these fibres are water-absorbent, they are good at catching respiratory droplets.
The easiest way to know if your cloth mask has three layers is to check with the manufacturer. The US government’s Centers for Disease Control also recommends checking whether your mask can block out light when you hold it close to a light source, to get a rough idea of how good your mask is at filtration.
Why keep people at arm’s length?
Because it’s all about the dose. “Respiratory droplets fall to the ground [after a while] and if a person is further away, they’re likely to get a smaller dose [of the virus],” explains McCarthy. So, with more distance between you and an infected person, the lower the chance of breathing in a large dose of virus, and so the lower the chance of getting sick.
By now, it is also well established that SARS-CoV-2 does not spread only through particles that are heavy enough to land on surfaces, but also through airborne particles that are light (and small) enough to hang in the air.
This “aerosol spread” means that infection doesn’t happen only through coughing or sneezing, but also through breathing. Although it is possible for an airborne virus to spread around the corners of a room, the aerosols mainly infect people nearby. This is because with poor ventilation, aerosols concentrate around the person who is the source.
Thinking about a smoker in an indoor space is a good analogy — the smoke will first collect around them, but over time it will spread to fill the room.
A 2020 study of air samples taken around Covid patients confirms that a higher dose of virus exists in the air close to an infected person. Researchers found that the air near patients’ noses contained up to 48 copies of the virus per litre of air, whereas the concentration dropped about 10 times in samples taken some distance away from the hospital bed. Samples taken at the door to the room showed just over two copies of virus per litre of air.
Many factors influence how far respiratory droplets can travel. When you sing, sneeze or cough, you exhale with more force, which makes the droplets go further and hang around in the air longer than when you simply breathe in and out. Although tiny vaporised droplets can travel up to eight metres after you sneeze, larger droplets tend to fall fairly quickly within a distance of one or two metres — about the length of your arm. As larger respiratory droplets will contain more viral particles if someone is infected, the guideline of keeping about two metres apart makes sense, because it reduces your chance of being exposed to a large amount of virus from the infected person.
But researchers caution that there should be some flexibility in how guidelines are applied, depending on the setting. Too short a distance could risk avoidable infections, whereas too long a distance could be unnecessarily disruptive to work and social lives.
Should I choose an indoor gathering or an outdoor event?
It depends — and you should choose wisely. The concentration of virus-carrying particles is almost always lower outdoors than indoors, because, as McCarthy explains, a slight breeze or airflow outside naturally dilutes the concentration of infectious particles.
But, she says, “a packed soccer stadium, with every seat filled, is not likely to be safe, even though it’s outdoors”. In a crowded setting, like a sports stadium filled to capacity and with spectators shouting and cheering close to one another, people can easily inhale someone else’s respiratory droplets. And if the person next to you is infected, being so close can up your chances of becoming infected too. Because — yes, we’ll say it again — it’s all about the dose.
McCarthy suggests having a number of empty seats between people or even leaving adjacent rows empty as a countermeasure at outdoor events. Simply put, “stadiums shouldn’t be full”, she warns.
So what about indoor gatherings? “We’re at the mercy of the ventilation systems of that venue,” says McCarthy.
In indoor settings without good airflow, the concentration of virus-containing particles can be high. And higher concentrations mean there is more chance of inhaling a large dose of virus and getting sick if you’re in that space.
McCarthy says we should try to control the airflow as much as we can when we gather inside, for example by opening windows. Good airflow helps to “dilute these particles and reduce the chances that I might breathe in an infectious particle”.
In indoor settings such as in a mall or at the movies, the air conditioning system controls the number of air changes per hour. This is the number of times the air in a room is completely replaced by air from outside in an hour. More frequent air changes reduce the likelihood that you can inhale viral particles from a sick person and get infected.
Says McCarthy: “If we are to make indoor settings safer, facilities managers must know the importance of good ventilation and how to maintain the systems that clean or recycle air.”
What if I can’t avoid close contact?
Then do what you can — taking at least some Covid precautions is better than none. We should think of non-pharmaceutical measures being like different parts of an anti-Covid armour, giving us an extra layer of protection with each piece we put on.
In spaces where groups of people gather — whether at school, the office or church — it’s best to try to keep about two metres apart. But if that’s not possible, aim for the most feasible distance and add some of the other protective layers, like opening windows and wearing face masks correctly.
It might feel odd to wear a mask when singing at church, but McCarthy says it’s important to adhere to this practice because “we are breathing more than we normally would and with greater force, [which means] the droplets are probably being dispelled further”. And to reduce our exposure to a high dose of virus particles, wearing masks, sitting a safe distance apart and ensuring proper ventilation are good bets. DM/MC
Additional reporting: Mia Malan, Helene-Mari van der Westhuizen
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