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Herd immunity is the idea that when a certain percentage of a population is vaccinated, those within the population who are not vaccinated are protected from the disease. This is a fairly straightforward concept; if 90% of a population is immune, then the chances that a carrier of the disease will come into contact with a non-immune member of the population (or "herd") is low.
The average number of people that an infected person can infect (R0) for a given disease is corresponds the percentage of a population that needs to be vaccinated to achieve herd immunity (1-1/R0). Hence, highly-transmissible measles (R0 = 12 to 18) needs a high percentage of the population to be vaccinated (92-95%). Conversely, Ebola has a relatively low R0 (1.5 to 2.5) and the percentage of the population that would need to be vaccinated to achieve herd immunity is estimated to only be 42-63%.
While herd immunity by vaccine in vastly superior, the term also covers immunity in a community where a sufficient number of people have caught and recovered from an infection.
For example, in 1962 Japan instituted mandatory vaccination against influenza in school children. This continued until 1987 when the program was changed to optional; it was abandoned in 1994. Approximately 85% of children were estimated to have been vaccinated. Several studies examined the mortality rates due to influenza during this period of time and revealed that the death rates due to influenza in all population groups was drastically decreased, including older adult populations. Since there was no increased rate of vaccination in these populations, the decreased mortality was caused by herd immunity effects of vaccinating so many children. Many researchers have written that this and other evidence argues strongly for mandatory vaccination programs worldwide as a means of controlling many diseases and preventing a pandemic.
The United States has mandatory vaccination according to an official Center for Disease Control schedule. Enforcement is left to the states. Most states have medical and religious exemptions. Some states make it very easy to opt out of vaccines, while other states strictly enforce compliance for school attendance.
In a nutshell
- The mathematics of herd immunity by Tom Lietman (2000) Mathepi.Com (archived from February 11, 2019).
- One number can help explain why measles is so contagious: A disease's R0 is the average number of people a person can infect in an unvaccinated population by Helen Thompson (1:00pm, May 28, 2019) Science News.
- www.polioeradication.org timeline of worldwide cases of polio Polio remains a problem in countries where vaccination is not sufficiently widespread.
- Control of Influenza
- Mandatory Vaccination of Japanese Schoolchildren Demonstrates "Herd Immunity"
- Herd Immunity