If the basic reproduction number is high, which level of induced herd immunity is required?

• A. No induced herd immunity matters
• B. Induced herd immunity cannot influence outbreaks
• C. High induced herd immunity
• D. Low induced herd immunity

Answer: (C)

The important idea is that how contagious an infection is determines how much of the population must be immune to stop its spread. The herd immunity threshold is 1 minus 1 divided by the basic reproduction number (HIT = 1 − 1/R0). As R0 becomes larger, this threshold rises toward 1, meaning a larger fraction of the population must be immune. This is because higher transmissibility means each case generates more secondary cases, so to reduce the effective reproduction number below 1 you need more people immune. For example, with R0 around 3, about 67% would need immunity; with R0 around 5, about 80%; and with very high R0, the required immunity approaches nearly everyone. Therefore, when the basic reproduction number is high, a high level of induced herd immunity is required.