Herd immunity occurs when so many people in a population are immune to a disease that the pathogen can no longer spread easily. Even people who are not immune are then protected, because they are unlikely to meet an infected person.
This matters because some people cannot be vaccinated or do not respond well to vaccines:
• newborn babies too young for certain vaccines
• people with weakened immune systems, for example during chemotherapy or with HIV
• people allergic to a vaccine ingredient.How many people must be immune depends on how infectious the disease is. The basic reproduction number, R0, is the average number of people one infected person passes the disease to in a population with no immunity. The proportion that must be immune, the herd immunity threshold, is:threshold = 1 − 1 / R0 Worked example: measles has an R0 of about 15. Threshold = 1 − 1/15 = 0.93, so about 93% of the population must be immune. For a disease with R0 = 4, the threshold is 1 − 1/4 = 0.75, or 75%.If vaccination rates fall below the threshold, outbreaks can return. For example, measles cases in England rose after uptake of the MMR vaccine fell in the late 1990s and early 2000s.This is why vaccination programmes aim for high coverage in every community: pockets of unvaccinated people allow a pathogen to keep circulating.
This matters because some people cannot be vaccinated or do not respond well to vaccines:• newborn babies too young for certain vaccines
• people with weakened immune systems, for example during chemotherapy or with HIV
• people allergic to a vaccine ingredient.How many people must be immune depends on how infectious the disease is. The basic reproduction number, R0, is the average number of people one infected person passes the disease to in a population with no immunity. The proportion that must be immune, the herd immunity threshold, is: