Conditional Herd Immunity

The basic reproduction number, R0, for COVID-19 is estimated to be 2.5. With this level of R0, herd immunity is reached when 60% of the population have been infected or recovered. During the COVID-19 epidemic, governments are encouraging/ordering the population to vary their behavior so that the effective value of R is considerably less than 2.5. This raises the prospect of a population reaching herd immunity with only 20% of the population being infected or recovered. This would be conditional herd immunity as it would be conditional on the population maintaining the lower value of R even after the number of infections starts to drop.

Based on a detailed analysis we present the countries listed in order of the expected time to reach conditional herd immunity.

Rank Country Years to Herd Immunity Immunity (%) R values TII (ppm) Strategy
1 Peru -1.14 98.1 1.03 1956.68
2 US -0.68 34.63 0.87 5051.42
3 Brazil -0.64 45.15 1.21 2672.86
4 United Kingdom -0.41 34.3 1.08 7124.12
5 Sweden -0.39 23.08 0.68 797.9
6 Israel 0.17 14.85 0.83 3815.71
7 Singapore 2.71 1.47 1.45 1058.42
8 Australia 3.28 0.81 1.28 852.04
9 New Zealand 132.03 0.1 0.93 18.87