Experts quickly focused on differences among patients, such as age and co-morbidities, that can affect their chances of severe illness. But the details of two outbreaks on cruise ships spurred Gandhi to think that viral dose could be another important determinant of the course of the new illness.
In February, one of the first outbreaks of COVID-19 outside of China occurred on the Diamond Princess cruise ship docked in Yokohama, Japan. Of the 634 people on board who tested positive, about 18% of infections were asymptomatic.
In March, an Argentinian cruise ship found itself in a similar predicament, but of the 128 people on board who eventually tested positive, 81% were asymptomatic.
A key difference, Gandhi noted, was that on the Argentinian ship, surgical masks were issued to all passengers and N95 masks to all staff as soon as the first passenger became sick.
More recently, an Oregon seafood processing plant where workers were required to wear face masks reported an outbreak of 124 cases, 95% of which were asymptomatic. Similarly, in a Tyson chicken processing plant outbreak in Arkansas where workers were provided mandatory masks, 455 out of 481, or nearly 95% were asymptomatic.
To Gandhi, these case studies suggest that if more people wore masks, we could see less serious illness from COVID-19 and a higher proportion of asymptomatic cases, currently estimated to be around 40% of cases by the Centers for Disease Control and Prevention.
