The world is still not using one of its most effective weapons against Covid -- properly ventilating public spaces -- more than two years into the pandemic, experts warn.

At the moment there is a "fragile, armed peace" with Covid-19, said Antoine Flahault, director of the Institute of Global Health at the University of Geneva.

"In the hopes of stemming the tide of the pandemic and reducing mortality, we need to reduce the level of contamination, which the vaccine cannot do alone," he told AFP.

"We need a new phase -- improving the quality of indoor air."

Covid-19 is primarily transmitted through the air. It is carried in large droplets or fine aerosols when an infected person breathes -- and even more so when they talk, sing or shout.

In a closed off or poorly ventilated room, these aerosols can remain in the air for some time, moving around the space and greatly increasing the risk of infection.

While it is generally accepted that Covid can be transmitted within two metres (6.5 feet) via both droplets and aerosols, there is still no consensus on the importance of long-distance airborne transmission indoors.

A team of researchers from the UK Health Security Agency and the University of Bristol reviewed 18 studies in several countries on airborne transmission.

In research published in the BMJ this week, they found that people can infect each other when they are more than two metres apart.