Helene-Mari van der Westhuizen is a South African medical doctor and doctoral researcher in the Nuffield Department of Primary Care Health Sciences at Oxford University, studying TB infection control in rural settings in South Africa.

I was allowed to send “seven small items” for washing during my hotel quarantine at Heathrow airport in the UK. I was instructed to double-bag the clothes, which authorities would collect and place in quarantine, disinfect, wash and package before returning to me. 

That same morning, the US government’s Centers for Disease Control (CDC) and Prevention’s Emerging Infectious Diseases journal described an outbreak of Covid-19 at a hotel quarantine site in New Zealand. The spread was not through laundry; on the contrary, contaminated objects are considered low risk for Covid-19 spread.

In the New Zealand quarantine outbreak, the people who infected each other never met; they weren’t even outside of their hotel rooms at the same time. This was investigated using CCTV footage and genomic sequencing. 

Genomic sequencing is when scientists unravel the genetic code of a virus. SARS-CoV-2, the virus that causes Covid-19, is made up of a string of genetic code. When scientists decode the genes, they can figure out who infected whom with the virus. 

In a setting where there is no community transmission, establishing how transmission happens is one of the most valuable ways to understand the way Covid-19 spreads. 

The researchers looking at this quarantine example concluded that transmission most likely took place through the shared air of the hotel corridor when the respective guests opened their doors within a 50-second window of each other. They wrote: “Suspended aerosol particles were the probable mode of transmission in this instance, and the enclosed and unventilated space in the hotel corridor probably facilitated this event.”

Aerosols are microscopic liquid, solid, or semi-solid particles that are so small that they remain suspended in air that we can breathe in. 

The New Zealand researchers’ conclusion about aerosols means that we need to think differently about our Covid-19 preventative toolkit. And there are several studies with such findings.

The Lancet says: “There is consistent, strong evidence that SARS-CoV-2 spreads by airborne transmission. Although other routes can contribute, we believe that the airborne route is likely to be dominant. The public health community should act accordingly and without further delay.” 

We have been missing an important preventative tool

The often-repeated preventive Covid-19 measures are: wash your hands, wear your mask and keep a distance from other people. The UK’s public health campaign summarises this with the rhyming hands, face, space. 

But as our understanding of how Covid-19 spreads becomes more nuanced, a fourth crucially important measure has been added: fresh air. 

Is Covid-19 airborne?

Covid has turned terms like “R number” and “herd immunity” into dinner table discussions. Infection control isn’t far behind. 

So here’s a short Infection Control 101. In the case of Covid, there are three routes of  transmission that scientists have considered, each with a different list of preventative actions. These are: contact, droplet and airborne precautions. 

For contact precautions, it is important to avoid touching contaminated surfaces and to keep your hands clean. Droplet precautions assume that infectious droplets are heavy enough to fall to the floor around the infected person and require mask wearing only when in close contact with other people, with an emphasis on hand hygiene. Airborne precautions assume that the infectious particles float in the air like smoke and can spread across a room at a far greater distance, depending on how well-ventilated it is. 

At the start of the pandemic, Covid-19 was placed in the “droplet precaution” bracket. On 28 March 2020, the World Health Organization (WHO) tweeted: “FACT: Covid-19 is not airborne”. But during a novel pandemic many facts had to be revisited, scrutinised and re-evaluated. 

Infection control has been no exception. 

Some policy U-turns have been announced with great fanfare – for example, the shift in recommending the widespread use of masks. The acknowledgement that Covid-19 is airborne has been quieter, with an additional sentence slipped onto the WHO’s website overnight.

But the move to recognising airborne transmission has been unambiguous. The WHO clearly describes this new guidance where it states Covid-19 spreads through “particles [that] range from larger respiratory droplets to smaller aerosols. The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range).” 

In another example, the CDC has updated a scientific brief in May to include a focus on preventing “inhalation of virus”. Both of these statements emphasise how Covid-19 can spread through aerosol routes. 

A second component of aerosol spread is that it doesn’t exclusively happen via coughing or sneezing, but via breathing. Although it is possible for an airborne virus to spread around the corners of a room, the aerosols mainly infect other people in close proximity indoors. This is because with poor ventilation aerosols concentrate around the person who is the source. 

Thinking about a smoker in an indoor space offers a good analogy – the smoke will first collect around them, but over time it will spread to fill the room. 

The response to this shift in understanding should be dramatic

Recognising that Covid-19 also spreads via the air should provoke a much more extensive reaction. Our public health leaders and politicians should start by explaining this in media briefings using words that explicitly describe that Covid-19 spreads via the air, like cigarette smoke. 

In South Africa we have had several precautionary public health messages that indirectly address airborne spread (for example, widespread mask use, encouraging people to move gatherings outside and ventilation requirements for workplaces), but such messages still avoid the word ‘airborne’. 

We experienced most of our third wave during winter, when people tended to gather indoors. That’s why we need to communicate the danger of sharing air with other people in an enclosed space. 

Around the world, examples of indoor superspreader events are numerous, including gyms, choir practices, clubbing and indoor dining. For example, one restaurant outbreak in South Korea depicted how little contact is enough to become ill: a person sitting 6.5m away from the person who had infectious Covid-19 was infected after five minutes of exposure. 

With the more infectious Delta variant driving South Africa’s third wave, there are concerns that a shorter period of contact can lead to infection. There was initial pushback against using the term ‘airborne’ when describing Covid-19 transmission, with people arguing it would cause public alarm. Yet, giving maskless indoor diners false reassurances about their safety with only perspex screens and hand sanitisers is also harmful. 

Perspex screens and plastic face shields offer no protection against an airborne infection, as aerosols float in the air and can easily bypass such a barrier. The main contributor to these superspreader events is that they took place indoors.

What better ventilation looks like in public spaces

Joseph Allen, an associate professor at Harvard University’s school of public health, says: “The focus should be on how you make indoors more like outdoors.” This involves improving indoor ventilation. There are simple steps that the CDC recommends for buildings, including classroom settings, that are easy to implement: 

1. Open windows and doors on both sides of the room. Consider using fans to increase the effectiveness of an open window. 
2. When sharing transport with someone who is not in your household, open the windows.
3. Switch heating, ventilation and air conditioning systems to maximise ventilation. Air conditioning systems often use settings that recirculate stale air.

To improve ventilation further, engineers and aerosol experts need to provide input through evaluating building infrastructure and its usual occupancy. They could guide several additional options to improve ventilation, including low-cost building adjustments such as wind-driven roof turbines, filtering air with portable air cleaners (for example, high-efficiency particulate absorbing – Hepa – filters are now mandatory for all schools in New York) and measuring carbon dioxide levels in an indoor space. Belgium has made carbon dioxide monitors mandatory for indoor venues such as gyms and restaurants to give real-time feedback on the estimated fraction of stale air that is being rebreathed by people in a room. 

Multiple preventative measures work better than picking a single one; for example, when evaluating school safety, Hepa filters and indoor mask wearing in combination remove more aerosol particles (90%), than Hepa filters alone (65%). 

Unfortunately, the market for air purifiers is rife with false promises, and expert input is critical to navigate this. South Africa has the expertise; we need to recognise the importance of getting people with the knowhow involved to improve ventilation. 

Looking at safe air inside health facilities

Health facilities are playing an important part in the roll-out of Covid-19 vaccinations, so we need to make sure they do not inadvertently spread infection. 

Most countries have had hospital outbreaks, including South Africa. For instance, in March last year, one patient admitted to the Netcare St Augustine’s Hospital in Durban, led to the infection of at least 135 patients and staff at the hospital complex. 

And improving ventilation in health facilities will not only be beneficial for Covid-19, but also to address the long-ignored high rates of occupational tuberculosis in health workers, another airborne pandemic prevalent in South Africa. Recent evidence suggests that a greater proportion of respiratory illnesses, that we formerly only used droplet precautions for, would benefit from improved airborne measures. This includes influenza and rhinovirus. 

Along with improved ventilation we also need an upgrade in health worker personal protective equipment. Australia has updated its infection control guidelines to recommend health workers use respirators when community transmission rates of Covid-19 are high. 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. They are often also called N95 respirators.

Australia’s guidelines are supported by research that showed health workers with access to respirators had better protection against Covid-19 than those who have only had access to surgical masks. A similar shift to respirators for South African health workers has been proposed in a position statement led by South African health workers and professional societies. 

Similarly, for the public, airborne transmission requires better masks. Although the initial focus on masks has been for source control (wearing your mask to prevent you from spreading infection from others) this means also looking at improving the protection it offers the wearer – so thinking about improving the fit of your masks (minimising gaps around the face) and improving the filtration. Read more here. 

All aboard?

Not everyone in the infection control community is on board with calling Covid-19 airborne. Some of this relates to the boundary historically considered to separate a droplet and aerosol (whether it is five or 100 micrometers) and different terms used by clinicians and aerosol physicists. Another common argument is – if Covid-19 is airborne, why are we not all infected? 

Yet, we know it is more complicated than that. 

South Africa’s TB epidemic is also airborne, but your risk of becoming ill depends on several factors, including the duration of exposure, how infectious the ill person is at the time, your health status and the ventilation of the space. 

In fact, South Africa has little to lose by embracing airborne prevention measures, and a lot to gain through dual benefit to TB. Improving the safety of air inside buildings is as important as making sure that tap water is not contaminated by infectious diseases. 

Open it up

The message that Covid-19 is airborne has spread slowly, prompting little effort to improve ventilation. Learning from superspreader events and hotel quarantine outbreaks, the evidence is pointing away from disinfecting groceries, using perspex barrier screens, and having elaborate processes for cleaning laundry. We should instead invest in improving ventilation, both through simple-to-use measures, but also by improving building infrastructure. 

But infection control is seldom about a single preventative tool – we need to combine what we know works: keep wearing masks, washing hands, social distancing, focus on accelerating the vaccination roll-out and add in fresh air. 

We know that using several tools together is better than focusing on just one measure. Improving indoor ventilation will have knock-on beneficial effects on preventing current and future respiratory pandemics, common respiratory illnesses such as flu, as well as productivity in workplace and school settings. 

There is an opportunity knocking on the window – we should open it. DM/MC

This story was produced by the Bhekisisa Centre for Health Journalism. Sign up for the newsletter.