Herd immunity is a key concept for epidemic control. It states that only a proportion of a population needs to be immune - through overcoming natural infection or through vaccination - to an infectious agent for it to stop generating large outbreaks. A key question in the current COVID-19 pandemic is how and when herd immunity can be achieved and at what cost. Herd immunity is achieved when one infected person in a population generates less than one secondary case on average, that is, the average number of persons infected by a case dropping below 1 in the absence of interventions. Taking these considerations into account, there is little evidence to suggest that the spread of SARS-CoV-2 might stop naturally before at least 50% of the population has become immune. With flu pandemics, herd immunity is usually attained after two to three epidemic waves, each interrupted by the typical seasonality of influenza virus and more rarely by interventions, with the help of cross-protection through immunity to previously encountered influenza viruses, and vaccines when available