University Of Bristol Study Shows SARS-CoV-2 Loses Ability To Infect Within Minutes In Air Subjective To The Environment’s Relative Humidity
: A new study by researchers from the University Of Bristol shows that the SARS-Cov-2 coronavirus loses its ability to infect within minutes in the air subjective to the environment’s relative humidity (RH). In drier or RHs below 50% environments such as offices and malls, a near instant loss of infectivity in 50–60% of the virus is found whereas in higher humidity areas such as bathrooms and steam rooms, at 90% RH the viral stability is sustained for the first 2 minutes, beyond which it decays to only 10% remaining infectious after 10 minutes.
The aerosol microenvironment is highly dynamic exposing pathogens, such as the SARS-CoV-2 virus when exhaled in respiratory aerosol, to extreme conditions of solute concentration, pH and evaporative cooling. Yet surviving this environment is a key step in the transmission of such pathogens.
Properly comprehending the impact that airborne transport has on pathogens and the influence of environmental conditions on pathogen survival can inform the implementation of strategies to mitigate the spread of diseases such as COVID-19.
The study team reports changes in the infectivity of the airborne virus over timescales spanning from 5 s to 20 minutes and demonstrate the role of two microphysical processes in this infectivity loss: particle crystallization and aerosol droplet pH change.
Understanding the factors that influence the airborne survival of viruses such as SARS-CoV-2 in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission.
study team presented measurements of the stability of SARS-CoV-2 in aerosol droplets (~5-10µm equilibrated radius) over timescales spanning from 5 seconds to 20 minutes using a novel instrument to probe survival in a small population of droplets (typically 5-10) containing ~1 virus/droplet.
Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to ~10 % of the starting value was observable for SARS-CoV-2 over 20 minutes, with a large proportion of the loss occurring within the first 5 minutes after aerosolization. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallize at RHs below 50% leading to a near instant loss of infectivity in 50–60% of the virus.
At 90% RH however, the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 minutes, beyond which it decays to only 10% remaining infectious after 10 minutes. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilization of CO2 from bicarbonate buffer within the droplet.
Interestingly three different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.
The study findings were published on a preprint server and is currently being peer reviewed.
The study was the world’s first simulations of how the virus survives in exhaled air.
The study findings re-emphasize the importance of short-range SARS-CoV-2 transmission, with physical distancing and mask-wearing likely to be the most effective means of preventing infection.
Ventilation, though still worthwhile, is likely to have a lesser impact.
Lead author, Professor Jonathan Reid, director of the University of Bristol’s Aerosol Research Centre told media, “People have been focused on poorly ventilated spaces and thinking about airborne transmission over metres 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. When you move further away, not only is the aerosol diluted down, there’s also less infectious virus because the virus has lost infectivity as a result of time.”
Till now, many assumptions about how long the virus survives in tiny airborne droplets have been based on studies that involved spraying virus into sealed vessels called Goldberg drums, which rotate to keep the droplets airborne.
Utilizing this method, American researchers found that infectious virus could still be detected after three hours. Yet such experiments do not accurately replicate what happens when we cough or breathe.
The study team from the University of Bristol instead developed apparatus that allowed them to generate any number of tiny, virus-containing particles and gently levitate them between two electric rings for anywhere between five seconds to 20 minutes, while tightly controlling the temperature, humidity and UV light intensity of their surroundings.
Professor Reid said, “This is the first time anyone has been able to actually simulate what happens to the aerosol during the exhalation process.”
The research findings suggest that as the viral particles leave the relatively moist and carbon dioxide-rich conditions of the lungs, they rapidly lose water and dry out, while the transition to lower levels of carbon dioxide is associated with a rapid increase in pH. Both of these factors disrupt the virus’ ability to infect human cells, but the speed at which the particles dry out varies according to the relative humidity (RH) of the surrounding air.
When RH this was lower than 50% ie similar to the relatively dry air found in many offices, the SARS-CoV-2 virus had lost half of its infectivity within 10 seconds, after which the decline was slower and more steady. At 90% humidity ie roughly the equivalent to a steam or shower room, the decline in infectivity was more gradual, with 52% of particles remaining infectious after five minutes, dropping to about 10% after 20 minutes.
Importantly the study findings found that the temperature of the air made no difference to viral infectivity, contradicting the widely held belief that viral transmission is lower during warm weather.
Professor Reid further added, “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.”
He said that this highlights the importance of wearing a mask in situations where people cannot physically distance.
Importantly the same effects were seen across all three Sars-CoV-2 variants the team has tested so far, including Alpha.
The study team hopes to start experiments with the Omicron variant in the coming weeks.
Thailand Medical News
would like to add that preliminary studies have shown that the Omicron variant has unique conformational structures and also folding modes, possibility indicating that it has evolved to withstand harsher environmental conditions. The study involving specially the Omicron variant would be more interesting.
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