SARS-CoV-2, the novel coronavirus that causes COVID-19, is known to be spread by asymptomatic people via infectious aerosols. This incidence “deeply underscores the ongoing importance of regular widespread testing, wearing masks and physical distancing to reduce the spread of the virus,” according to an excerpt by experts at the University of California, San Diego (UCSD), US.

To recap, SARS-CoV-2 is a highly transmissible respiratory virus which causes respiratory tract infection(s) leading to a high viral load in respiratory secretions and saliva. Moreover, the virus can suppress the hosts’ innate immune response – particularly the interferon and cytokine response – which in turn leads to a higher level of viral replication than that by the 2003 SARS-CoV, also a member of the coronavirus family.

As a significant proportion of people may be pre-symptomatic or mildly symptomatic COVID-19 virus shedders, who are unlikely to be tested or isolated, it is necessary forcommunity-wide mask usage irrespective of symptoms, to reduce the infectivity of these COVID-19 cases.

UCSD highlights that these asymptomatic or mildly symptomatic people spread the virus silently in aerosols – while traditional respiratory disease controls (physical/social distancing) are designed to reduce transmission from large droplets produced in the sneezes and coughs of infected individuals, a recent study estimated that just a single minute of loud speaking might generate about 1,000-100,000 virion-containing aerosols (virus particles suspended in the air) which can remain airborne for hours and travel long distances; the small virus-containing aerosols can also bypass the immune system and penetrate deeply into the lungs, which can lead to more severe cases of COVID-19.

In addition, aerosols are known to travel further than six feet. Many factors affect aerosol spread indoors and outdoors, such as air flow and ventilation, the number of people in the space, time of exposure, sunlight, and temperature and humidity, so a six-foot perimeter indoors without a mask is most likely not enough if an asymptomatic person is actively shedding the virus. The possible infection can be likened to exhaled smoke – the distance from a smoker at which you can smell smoke indicates the distance at which you might also inhale virus-containing aerosols.

Another study by health authorities and scientists in the Hong Kong Special Administrative Region (HKSAR), China, showed that community-wide mask wearing may control the spread of COVID-19 by reducing the amount of emission of infected saliva and respiratory droplets from asymptomatic or mildly symptomatic individuals. The compliance of face mask usage by the HKSAR general public was 96.6%; and the COVID-19 incidence in HKSAR (129.0 per million population) was significantly lower (p<0.001) than that of other countries where mask wearing was less stringent, such as South Korea (200.5), Singapore (259.8),UK (831.5), US (1102.8), France (1151.6), Italy (2250.8), and Spain (2983.2).

UCSD reiterates that masks are essential: properly-fitted masks provide a protective physical barrier for healthy people and reduces the number of infectious viruses in the exhaled breath of asymptomatic individuals. Countries that have been most effective in limiting the spread of COVID-19 have implemented universal masking – Taiwan, where masking orders were quickly enacted and did not implement a lockdown during the pandemic, maintained a low incidence of approximately 441 cases and 7 deaths, and has not seen any domestically-transmitted COVID-19 cases for almost 50 days.