Keeping COVID  infection risk low from Omicron variants now requires room ventilation rates 50 times greater than in 2020 when the pandemic arrived, researchers have revealed.

Building scientists and air quality specialists have assessed the ventilation rates of rooms required to keep the risk of infection under one per cent, concluding that Omicron and its variants require ventilation some 50 times greater than the virus did when the world first encountered it in 2020.

Despite the manifest benefits of vaccination, the risk of airborne transmission in confined spaces might be higher, and so the corresponding precautions regarding ventilation needed to be re-appraised, according to the study published in the journal Building Simulation.

The researchers also found that air purifiers were ineffective in reducing transmission when used in scenarios without N95 masks.

For all three variants of concern, ventilation rates had to increase substantially to ensure an infection probability of under one per cent, they said. The ventilation rate is defined as the rate at which fresh air flows into a room or building, measured in cubic meters per hour.

For Alpha, this meant ventilation rates of 650-1,200 cubic meters per hour for 15 minutes of exposure, and 8,000-14,000 cubic meters per hour for three hours of exposure.

For Delta, the rate jumped up further still, to 2,200-6,800 cubic meters per hour for 15 minutes, and 26,000-80,000 cubic meters for three hours.

For the Omicron variant (not distinguishing between sublineages), the ventilation rate hit 5,400-17,000 cubic meters per hour for 15 minutes and 64,000-250,000 cubic meters per hour for three hours.

"Without any additional measures, this means the Alpha variant requires a ventilation rate some four times more aggressive than the ancestral strain," said Bin Zhao, professor at Tsinghua University in Beijing, "while the Delta and Omicron variants require ventilation rates roughly 20 times and 50 times greater. This is difficult to meet for actual building engineering."

However, the good news is that if both the infected person and the susceptible person are wearing N95 masks, the ventilation rate required to keep the risk of infection to under one per cent decreases to about one hundredth of these reported values, said the researchers.