'Some people have 10 million times more virus than others': Are you a flu superspreader?

A growing body of research suggests that everything from the shape of your lungs to how you enunciate your Ts and Ks could make you a flu superspreader.

In January 2020, British businessman Steve Walsh began an international trip which would take him to Singapore and France before returning home to Brighton in the UK. Walsh's travels – which saw him contract Covid-19 and unwittingly infect about a dozen others he met on his journey – would go on to make headlines around the globe. 

Stories like these popularised the idea that some people could serve as Covid-19 "superspreaders". But virologists have long known that a small proportion of people play an outsized role in driving pretty much any outbreak of a respiratory pathogen – be it Covid-19, flu, respiratory syncytial virus (RSV), tuberculosis or even measles.

In fact, there's even a mathematical pattern: scientists have estimated that for a typical outbreak, 20% of people will be responsible for driving 80% of the infections. And research has shown that the amount of virus present in someone's respiratory fluids can vary enormously between infected people, according to Kylie Ainslie, an infectious diseases researcher at the Peter Doherty Institute for Infection and Immunity in Melbourne, Australia.

"Some people have 10 million times more virus than others," says Ainslie. "At the high end, concentrations of virus particles can reach a billion virus copies per millilitre."

But whether that translates into superspreading depends on a whole variety of factors, many of which have been uncovered through research spurred by popular cases like that of Walsh during the coronavirus pandemic.

Whether it is down to the characteristics or volume of your speech, the properties of your mucus or the humidity of the environment that you're in… do you know whether you're a flu superspreader?

Are you super sick?

To begin with, the stage of infection is key to someone's ability to superspread. People emit far more infectious particles with every breath or word spoken when they're at their sickest.

In a research project in 2021, microbiologists infected rhesus macaques and African green monkeys with Covid-19 and then analysed their breath, which is a mix of gases – including nitrogen, oxygen, carbon dioxide – and also microscopic bits and pieces of moisture. The monkeys went from breathing out 3,000-5,000 moisture particles for every litre of breath they exhaled, to 50,000-70,000 particles once infected.

Given that a single breath particle can carry anywhere from 200 to 300 viral particles, multiply that by 10, and it's easy to see how viruses can be transmitted from person to person very quickly, says Chad Roy, professor of microbiology and immunology at Tulane School of Medicine in New Orleans, who led the experiment.

"When you do the maths, it's staggering that we're not sick… all the time," says Roy. He suggested that while this was an experiment on a coronavirus, the same principle might also apply to other respiratory infections.

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It's not just the quantity of particles, though. The type of particles a person produces when they're particularly sick is also more conducive to transmitting an infection. Breath particles can be droplets ranging from five to 100 microns or larger in diameter, as well as aerosols, which are less than five microns in diameter, which is five millionths of a metre. And studies suggest that when someone is ill, they emit more small aerosol particles than larger droplets.

"This was a really important finding, because those particles can travel much further into the lung," says Roy. "So if you're the unlucky fellow who ends up inhaling those particles, they have a better probability of travelling deeper into the more susceptible regions of the lung." There are some suggestions that this could be a consequence of how viruses have evolved to increase their chances of spreading to more hosts. For example, throughout an infection, a virus will increase the rate at which it replicates. This causes more damage to the cells in the respiratory tract, breaking them down into smaller bits and pieces like aerosols, which then stay suspended in the air for longer periods.

Do you speak clearly?

There are other physiological factors involved as well. Research based on superspreading events involving the three coronaviruses Sars, Mers and Sars-CoV-2, suggests that your typical superspreader is more likely to be male and over 40. But the scientists couldn't really pinpoint the biological or behavioural reasons behind this pattern in the data, and there could have been bias due to who got tested the most.

Roy's 2021 monkey study also examined the breaths of 194 people infected with Covid-19 and concluded that superspreaders were more likely to be overweight. "Individuals who are overweight tend to produce more respiratory droplets when they breathe or cough," says Matthew Binnicker, director of clinical virology at Mayo Clinic in Rochester, Minnesota. This may be due to excess fat in the chest and abdominal cavity, preventing the lungs from fully expanding, resulting in a shallower and faster breathing pattern. Carrying extra body mass also requires more oxygen and produces more carbon dioxide, leading to a higher rate of respiration and more breath particles.

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Superspreaders are also more likely to be loud talkers, keen singers, or perhaps even someone with anger issues.

Researchers in California measured the quantity and size of the breath particles that 48 people produced when speaking at different volumes in four different languages: English, Spanish, Mandarin, and Arabic. They found that loud speech generated up to 50 times more aerosols than quieter speech, likely because the vocal cords open and close more frequently during louder speech, allowing more particles to form at the voicebox. 

And people who enunciate their words more harshly might also be more likely to be superspreaders, according to a study published in early 2020 looking at the vocalisation of letters. "If you produce the sounds 'T', and 'K', and 'P', you probably emit a few more droplets, while the louder you speak, the more you will likely produce," says Werner Bischoff, professor of infectious diseases at Wake Forest University School of Medicine in North Carolina. "People have their habits, but the close talker who is very loud should be avoided."

Interestingly, that 2020 study also noted that the vowel sounds in words like "need" and "sea" produced more breath particles than the vowel sounds in words like "saw" and "hot", for instance, or "blue" and "mood".

(Read more about how diseases spread when we sing and talk.)

What size are your lungs?

Pre-adolescent children are less likely to be superspreaders, for example, because they're breathing much smaller volumes of air in and out, as their overall lung size, airway size, and the number of air sacs they have are much smaller than those of an adult. 

However, even in adulthood, there's still considerable variation in our lung capacity and the amount of air we're able to exhale. Genetic differences can mean that some people have larger lungs than others by nature, while doing more physical activity during childhood can lead to larger and stronger lungs throughout adulthood. On the other hand, having childhood asthma or exposure to chronic air pollution can actively stunt lung development. This all makes a key difference when it comes to superspreading, since those with a greater lung capacity who breathe more volumes of air in and out are more likely to be shedding and transmitting more virus, says Bischoff.

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Superspreading is also thought to be influenced by the characteristics of a person's respiratory mucus, the jelly-like substance all over the insides of our nose and lungs designed to trap pathogens. Everybody has different mucus viscosity and texture, and a different recipe of what makes up their mucus microbiome. "Some people's mucus may trap and inactivate viruses more effectively, while others have mucus that allows more virus to remain infectious and transmissible," says Ainslie.

Are you hanging out in superspreader environments?

Still, many individuals likely have all the physiological qualities of a superspreader, yet aren't actually responsible for that many infections. That's because superspreading is also determined by a variety of behavioural and social factors, ranging from the number of close contacts that a person has to the amount of time they spend in poorly ventilated environments where their aerosols are more likely to linger, according to Osnat Livne-Shtraichman, director of the pulmonary institute at Rabin Medical Centre in Israel.

Superspreading is more likely to take place during group gym or exercise classes, for instance, due to the combination of a closed environment and the fact that we produce more than 130 times more aerosols or droplets while breathing heavily during exercise than we would do at rest.

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Studies also suggest the humidity of the environment plays a significant role: in very dry settings, the fluid within breath particles quickly evaporates, making the particles smaller and more concentrated with the virus. This also enables them to stay airborne for a longer period of time, increasing the likelihood that they will be taken up by another person.

If anything, reports of superspreaders being more likely to be men over 40 may actually reflect context rather than biology, according to Livne-Shtraichman. "Midlife adults are more likely to hold high-contact roles at work or in social settings, increasing their risk of being at the centre of transmission clusters," says Livne-Shtraichman.

None of these superspreading factors exists in a silo, of course, and a lot more research is necessary to accurately pinpoint who is a superspreader well before they become one, according to Roy. But as experiments like these help researchers understand more about superspreading, they are laying the groundwork for, one day, profiling a superspreader in advance. Likely by testing the fluidity of their mucus, or analysing their speech patterns.

"Maybe that would be something very interesting for someone to know about themselves," says Roy. "And they could then take extra care in terms of reducing risk of transmission to others, or it may further embolden them to get vaccinated."

In the meantime, it's perhaps wise to stay well clear of noisy talkers and karaoke bars this flu season.

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