From William Hughes-Games, Waipara, New Zealand
It doesn’t make sense to try to put a percentage figure on the achievement of herd immunity to covid-19 (23 January, p 12). Going to the extreme for illustration, in a sparsely populated country where you rarely interact with other people, your chance of getting covid-19 is slim, even if the incidence is somehow high.
On the other hand, in a country where you are shoehorned together on trains and in markets, you are more likely to catch the virus if even a small percentage of the population is infectious. We need some measure of how often you are likely to breathe in the air expelled by a fellow citizen multiplied by the incidence of the virus in the population.
