Herd immunity describes a situation when a large enough percentage of a population can no longer get sick from a virus so that the disease no longer spreads. There are generally two ways to accomplish this: either through a vaccine, or through natural exposure to the virus.
It appears that in Moscow, natural exposure to Sars-Cov2, aka Covid-19 may already have reached that level. Similar phenomena have been reported in other previous hot spots. This is good news, as it would anticlimactically end the Coronavirus problem in these places.
https://www.rt.com/russia/507942-half-moscow-residents-immune-covid/