Summary
Indirect protection of unvaccinated people when enough of a population is immune.
In depth
The threshold is 1 - 1/R0: for measles (R0 about 15) about 93 to 95 percent need to be immune. It protects people who cannot be vaccinated (infants, immunocompromised). Pockets of low vaccination can allow outbreaks.
Effective when immunity is spread evenly. It depends on vaccine effectiveness and duration. Waning immunity or new variants alter thresholds.
Example: outbreaks cluster in communities with low coverage.
When enough people are immune, the chain of transmission breaks and the infection cannot spread widely, which indirectly protects babies, pregnant women and immunocompromised people. The threshold depends on R0 and on how well the vaccine blocks transmission, not just disease. Measles needs roughly 95 percent coverage, polio 80 to 85 percent and seasonal influenza lower levels.
Declining coverage leads to outbreaks, such as measles outbreaks in communities with low vaccination. Immunity must be spread evenly, since clusters of unvaccinated individuals allow local outbreaks. Waning immunity and new variants change the threshold over time.
Remember: herd immunity is reached by vaccination, not by letting infection spread, which causes avoidable illness and deaths.
Frequently asked questions
What is Herd Immunity?
Indirect protection of unvaccinated people when enough of a population is immune.
Which area of biology covers Herd Immunity, and how advanced is it?
Herd Immunity is one of the 52 topics in Immunology & Disease. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Basic Reproduction Number, Immunosenescence, Vaccines.
Can you give an example of Herd Immunity?
Measles outbreaks occur when coverage drops below 95 percent.
What should I learn before Herd Immunity?
Herd Immunity is a Beginner topic, so no earlier topic is required. Other Beginner topics in Immunology & Disease that pair well with it include Immune System, Viruses, Innate Immunity.
What topics are related to Herd Immunity?
The closest topics to Herd Immunity are Basic Reproduction Number, Immunosenescence, Vaccines, Virus Evolution. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Herd Immunity?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Herd Immunity. To practise the whole of Immunology & Disease, use the Immunology & Disease quiz.
How is Herd Immunity different from Basic Reproduction Number?
Herd Immunity: Indirect protection of unvaccinated people when enough of a population is immune. Basic Reproduction Number: R0 is the average number of new infections one case causes in a fully susceptible population. Read both side by side and note what each one explains.
How should I study Herd Immunity step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Basic Reproduction Number and explain the difference aloud. Finally, take the Immunology & Disease quiz and come back to this page for any question you miss.