Herd immunity is achieved when one infected person generates less than one secondary case on average. The percentage of the population needed to achieve herd immunity can be calculated from the basic reproduction number, R0 (i.e, the average number of persons infected by a case in the absence of control measures in a fully susceptible population). So when R falls to less than 1, then herd immunity is reached. The basic formula for calculating herd immunity is 1 - 1/R0. [1], [2], [3], [4], [5]

The estimated R0 for COVID-19 is between 2.5 and 3.5, which means around 60-72% of the population needs to be immune to provide herd immunity based on the assumption that everyone will have 100% immunity and the immunity will be long-term. The true percentage needed for herd immunity will also depend on the vaccine efficacy. If a vaccine is assumed to be 95% effective, then the required herd immunity level would be 63% to 76%. If we wanted a higher safety margin of R=0.5, then the range might increase to 84-90% of the population needed to be immunized. If a vaccine is assumed to be 80% effective, then 75-90% of the population needs to be immunized to achieve herd immunity. [1], [2], [3], [4], [5]

To complicate this further, the duration of protection against SARS-CoV-2 is not yet known. The durability of immune memory (through either infection or vaccination) is a critical factor in determining population-level protection and sustaining herd immunity. With seasonal coronaviruses, durable immunity has not been observed or has been short-lived. However, herd immunity can be sustained (even if periodic vaccination is required to do so) and outbreaks can be curtailed as long as the community maintains the necessary levels. [1], [2], [3], [4], [5]

Another complication in determining herd immunity involves understanding the true vaccine efficacy. There may be confounding factors in the efficacy data such as persons who did not get infected due to mask wearing, hand washing, and/or social distancing. If the true vaccine efficacy is lower than expected, then the required percentage needed for herd immunity may increase further. [1], [2], [3], [4], [5]