| Summary: | The data on COVID-19 mass screening, collected in medical institutions and test laboratories in fifteen European countries in time interval 2 March – 21 April 2020, permitted us to evaluate forming the herd immunity potential to SARS-CoV-2. Population infection rate (PIR) is taken as a factor strongly correlated with herd immunity. We used our own Susceptible–Infectious–Recovered modified epidemiological compartmental model with division into two parts that described two public policy modes of SARS-CoV-2 containment: 1. full lockdown mode (most European countries); 2. no-lockdown mode (Sweden and Iceland only). For 21 April 2020, the mean PIR for the European countries considered, is 9.66 per cent. For Europe, calculated maximal herd immunity level ranges from 14.18 ( r 0 = 1.6) to 25.52 ( r 0 = 5.6) per cent of population. The time range of 95 per cent herd immunity formation spans from 109 days ( r 0 = 5.6) to 407 days ( r 0 = 1.6) for different modes of containment and different scenarios of disease peak location. A concept of an effective reproduction number r eff. is introduced: r eff. = r 0 · c , where c is contagiousness probability. Its use may assist in herd immunity estimations and eliminate ambiguities connected with different evaluation of basic reproduction number r 0 for different samplings. The maximal possible value of r eff. is 0.552.
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