Christian Drosten, who's likely the origin of these numbers, has explained them a few days ago in a podcast. The math is incredibly simple. The idea is that ri

Christian Drosten, who's likely the origin of these numbers, has explained them a few days ago in a podcast. The math is incredibly simple. The idea is that right now you have an estimated transmission rate of 3 people infected by one person. (Of course this number has high uncertainties and can be changed by better isolating patients, but let's assume it's roughly correct.) If you want to stop the transmission this needs to go below one. The major thing that can stop transmission is immunity from patients that already had the disease (assuming, which is probably true, that most patients won't get infected twice, even though it may happen in rare cases). So getting from 3 to 1 means 2/3 of the population need to be immune.