But he posited a "wilder" hypothesis as well -- that a "
proportion of the population might have pre-existing immunity to #SARSCoV2, potentially due to prior exposure to 'common cold' coronaviruses".
Balloux said that might explain issues like cases where there is no transmission between spouses.
Earlier this month, an American study in the journal Cell suggested between 40 and 60 percent of the population could be immunized against COVID-19 without ever being exposed to it.
"Clearly some individuals are more susceptible to the disease than others; after being infected some individuals have severe clinical symptoms and might even die, while others might show very little in terms of clinical symptoms," they told AFP by email.
"Our study suggests that preexisting immunity might be one of the factors to be considered; but at this point is simply an hypothesis that needs to be addressed with further experiments."