Take away: As with most things in nature, there are always exceptions – transmission occurring at greater distances than 3 ft and evidence of aerosolization have been reported.
Discussion: In scientific terms, this virus is still very new so the data supporting an optimal physical distance to prevent transmission remains scarce. In the absence of data, public health agencies have used what they understand about this virus and similar viruses to infer a “best” answer. Public health agencies try to simplify the recommendation to a single answer, but the reality is much more complex.
According to reports the WHO bases their recommendation for 1 meter (~3 ft) distancing off of an understanding that SARS-CoV-2 behaves like similar respiratory viruses that are primarily transmitted via larger droplets (as opposed to smaller aerosols). Assuming most spread is via droplets, the WHO reportedly follows the results of a 1934 study indicating most respiratory droplets fall to the ground within 3 feet.
However, as with most things in nature, there are always exceptions – transmission occurring at greater distances than 3 ft and evidence of aerosolization have been reported.
The evidence basis for the CDCs guidance for 6 feet of separation is less clear, but probably reflects lower risk tolerance, or greater weight to evidence of aerosolization or wider droplet spread.
Even with further study, there may never be a clear answer for optimal physical distancing. This is because, (1) the area of high risk for transmission is probably dependent on the specific conditions of the interaction (e.g. loud talking, windy environment), and (2) the “optimal” distance is based on risk tolerance. There is no single distance between individuals where risk of transmission drops off precipitously to zero.
All evidence indicates that greater distances are safer but, for example, consider how restrictive a physical distancing recommendation of >50 ft would be. In the end, because we can’t control how far others stand away from us, we ask governments to consider these tradeoffs and deliver a “best” answer to guide their citizenry.