Superspreading events were flagged early in the pandemic, with coronavirus-positive individuals unwittingly infecting others at choir practices or on cruise ships.
While the close proximity of a coronavirus patient to others will inevitably raise the risk of transmission, certain individuals were thought to exhale a higher number of infected droplets from the mucus in their airways.
Results reveal some of the individuals exhaled three times more aerosols than others, with just 18% of the volunteers behind 80% of the total particles emitted.
This was particularly true among the older volunteers and those with an elevated body mass index (BMI).
Aerosols are considerably smaller than droplets, with the latter measuring as little as 0.005mm across. The relatively heavy droplets are quickly subject to gravity, potentially contaminating surfaces, while lighter aerosols linger unseen in the air for up to several hours.
The coronavirus primarily spreads via coughs and sneezes, however, respiratory particles are also emitted when a person breathes and talks.
During normal breathing, a rush of air flows over the thin mucus that lines the respiratory tract, breaking it up into droplets and aerosols.
The extent to which this occurs is thought to be partially driven by an individual's diet, age and whether they have the coronavirus.
To learn more, the Harvard scientists studied the particles exhaled by 194 healthy volunteers, aged 19 to 66.
The results show 35 individuals, less than 20% of the group, made up 80% of the total aerosol production; defined as exhaling at least 156 particles per litre of air.
This is "analogous to the conventional definition of superspreading of airborne infectious disease", the scientists wrote in the journal PNAS.
"Within this high producing group, we noted that ∼80% of the 'superspreader' (of aerosol) production was generated by approximately half of the group, that is, 18 individuals."
The scientists looked closer at the 146 volunteers of a known age and BMI.
The results further show the 73 individuals of the lowest age and BMI "exhaled significantly less aerosol" than the remaining 73 participants with the "highest BMI-years".
All the participants under 26 or with a BMI of less than 22 – 18.5 to 24.9 is considered healthy – were "low spreaders".
Statistics have repeatedly flagged elderly people and those who are overweight or obese are more likely to develop coronavirus complications.
The volunteers' sex was not linked to their aerosol production, despite men being more at risk of ill health with the coronavirus.
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Three coronavirus vaccines are approved in the UK, with more than 15.9 million people being immunised with their first dose to date.
Experts are also working to develop drugs that cut the risk of infection, severe disease and death.
The Harvard scientists added "it may be advisable for the scientific community to additionally focus on management of COVID-19 [the disease caused by the coronavirus] through the restoration of airway lining mucus barrier function and notably, in the reduction of the propensity of airway lining mucus to disintegrate under the force of natural breathing.
"Exhaled aerosol numbers appear to be not only an indicator of disease progression, but a marker of disease risk in non-infected individuals.
"Monitoring (as a diagnostic) might also be an important strategy to consider in the control of transmission and infection of COVID-19 and other respiratory infectious diseases, including TB [tuberculosis] and influenza."
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