— Initial exposure, strength of virus infection both seen as contributors to illness severity
Cruise ship passengers who embarked from the coast of Argentina in mid-March were unaware that they were living in a COVID-19 hotspot for more than a week after the ship departed.
The reason why these passengers were oblivious? Because a majority of the cruise ship's cases were asymptomatic.
Researchers are now pointing to this cruise ship outbreak, in which all passengers were provided surgical masks, as evidence that universal masking may result in a higher proportion of asymptomatic COVID-19 cases. Other outbreaks of mostly asymptomatic cases where widespread masking was implemented, in places like jails and meatpacking plants, provide epidemiological data that masks could reduce viral inoculum -- and as a result, decrease the severity of illness.
Writing in the New England Journal of Medicine, Monica Gandhi, MD, and George Rutherford, MD, of the University of California in San Francisco, hypothesized that widespread population masking may act as a sort of "variolation," exposing individuals to a smaller amount of viral particles and producing an immune response.
Gandhi told MedPage Today that the viral inoculum, or the initial dose of virus that a patient takes in, is one likely determinant of ultimate illness severity. That's separate from patients' subsequent viral load, the level of replicating virus as measured by copies per mL.
The "variolation" hypothesis holds that, at some level, the inoculum overwhelms the immune system, leading to serious illness. With less than that (and the threshold may vary from one person to the next), the individual successfully fights off the infection, with mild or no clinical illness.
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