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 Belgium -0.45 35.4 0.99 5565.64
2 Bosnia and Herzegovina -0.43 43.5 0.75 3462.22
3 North Macedonia -0.39 39.94 0.63 2844.24
4 Moldova -0.36 35.3 0.75 1659.69
5 San Marino -0.36 42.19 0.14 126.28
6 Hungary -0.35 48.14 0.73 4131.18
7 Montenegro -0.34 39.58 0.79 2107.96
8 Bulgaria -0.34 39.72 0.8 3694.15
9 United Kingdom -0.33 30.28 0.96 723.64
10 US -0.32 28.57 0.88 1906.65