Ruminations: COVID Infectivity : Time and Distance

"When you move further away, not only is he aerosol diluted down, there's also less infectious virus because the virus has lost infectivity [as a result of time]."
"People have been focused on poorly ventilated spaces and thinking about airborne transmission over meters or across a room."

"I’m not saying that doesn’t happen, but I think still, the greatest risk of exposure is when you’re close to someone,"
"It means that if I’m meeting friends for lunch in a pub today, the primary [risk] is likely to be me transmitting it to my friends, or my friends transmitting it to me, rather than it being transmitted from someone on the other side of the room."
Jonathan Reid, director, University of Bristol Aerosol Research Centre

"Airborne respiratory diseases are all transmitted the same way: pathogen is in tiny particles that we breathe out/give off when speaking."
They're small enough to stay in the air like smoke; you get more if you're closer but eventually the whole room is smoky."
Dr.Adam Squires, biophysical chemist, Bath University

A man receives a nasal swab during a test for COVID-19 at a streetside testing booth in New York on December 17, 2021. ED JONES/AFP via Getty Images

A new study out of the University of Bristol indicates that coronavirus loses infectivity soon after being exhaled. At longer distances, it becomes less likely to be contagious. According to the study researchers, the viruse loses 90 percent of its contagion capacity twenty minutes following its airborne state. Most of the loss occurs in the initial five minutes of the virus reaching air circulation.

Insights into the manner in which the coronavirus travels across air will be of assistance in guiding future containment measures. The study reinforces the theory that the virus is mostly transmitted over short distances, supporting social distancing and mask wearing as a means by which infections can be curbed.

Viral particles, concludes the study, rapidly dry out once they leave the moist, carbon-dioxide-rich environment of the lungs, thus restraining their capacity to infect other people. A determining factor in how swiftly particles become deactivted was found to be air humidity. As an example, shower rooms produce a slower wind-down than does an office environment.

Below 50 percent, air humidity levels similar to the type of dry air found in offices, sees the virus lose half of its ability to spread, within a five-second period. Once humidity rises to 90 percent, similar to what pertains in a shower room, the viruses loses infectiousness more gradually; over half of particles remain contagious following a five-minute interval.

Air temperature, according to the study, has no impact on virus infectiousness. The study would disprove early laboratory studies concluding SARS-CoV-2 could persist for 24 hours on cardboard and several days on plastic and stainless steel, should study findings be confirmed. In the passage of two years' experience with the pandemic it may just be that handwashing and wiping processes resulted in limited impact on diminishing transmission.

Both the World Health Organization and the U.S. Centers for Disease Control, initially declared the virus to be transmitted principally through large droplets; now both accept the importance of of airborne spread in virus transmission.