The proportion of a population that has to be immune in order to confer herd immunity on the rest of the population is a function of how contagious the virus is. Data from Wuhan is still preliminary, but mercifully, COVID-19 does not appear to be as contagious as measles. Its ‘reproduction rate’ has been estimated to be as low as 2.2 (every one person who gets the disease gives it to 2.2 people), which means you would reach herd immunity when 40 to 60 per ent of the population has been infected.
If there was a way to safely grant two-thirds of a society Covid-19 immunity, the pandemic would stop.
Covid-19’s mortality profile is peculiarly age-specific. Bird flu, swine flu, SARS and MERS outbreaks were all equal-opportunity killers. They showed no mercy to the young and healthy. This does not seem to be the case with Covid-19.