But whatever people did, Charity explained, the story ended only with a vaccine or herd immunity. There was no vaccine in sight; and the number of infections required to achieve herd immunity could be calculated, as it was a simple function of the reproductive rate. (The formula was 1 − 1/R0, where R0 was the reproduction number.) The simple truth that the formula captured: the more transmissible the disease, the more people needed to be infected before the herd was theoretically safe.
