dcyphr | Herd immunity – estimating the level required to halt the COVID-19 epidemics in affected countries


The Coronavirus Disease 2019 (COVID-19) has spread throughout China and many European countries. Many countries have shut down businesses, schools, etc. Another strategy is to develop herd immunity while protecting those at greatest risk for COVID-19.


This study used the effective reproductive number (Rt) instead of the basic reproductive number (R0). The Rt is the estimated number of people that an infected person can transmit the virus to, based on clinical data and testing. The researchers used the exponential data on the new daily cases. They conducted analyses using the software R. Then, they calculated the Pcrit value. The Pcrit is the minimum percentage of a population needed to be immune in order to stop the spread of the virus. They conducted analyses for the 32 countries that had over 100 COVID-19 cases as of March 13, 2020. They calculated Pcrit with the formula Pcrit = 1 - (1/Rt). Another reason for using Rt is that some individuals are already immune to the coronavirus.


All of the Pcrit values are shown in table 1. The Pcrit values range from 5.66% in Kuwait to 85% in Bahrain.

Here are the Pcrit values from some of the countries with the highest amount of cases:

Spain - 80.7%

The United States - 69.6%

Germany - 69.6%

Switzerland - 69.3%

France - 67.6%

Italy - 59.0%

Iran - 50.0%

Korea - 30.1%


Some people may have partial immunity to SARS-CoV-2. They may have antibodies against common seasonal viruses, such as OC43, 229E, NL63, and HKU1. These people seem to have milder symptoms or are asymptomatic if infected. Finally, a problem is that many people may die as a population achieves herd immunity. Fatality rates vary between 0.25%-3.0% of a population.