Herd immunity and vaccination coverage
Vaccination can protect a community by reducing transmission as well as protecting vaccinated individuals. The coverage needed for this indirect protection differs by disease, and gaps leave people exposed, including those unable to be vaccinated.
Indirect protection and coverage thresholds
Vaccination can protect unvaccinated people by reducing person-to-person transmission; this indirect protection is called community or herd immunity. Control of many vaccine-preventable diseases depends on a substantial proportion of the population being immune. A 2018 overview placed the coverage usually needed for community immunity at 75%–95% of the target population, whereas a 2019 account gave a disease-dependent range of 30%–95%. Herd immunity can be essential to successful vaccination campaigns, including measles control.
The immunity threshold is the proportion immunised that helps contain outbreaks; outbreaks can occur where coverage has not reached it. For highly contagious measles, a 2008 account put necessary vaccination coverage above 94%, while the 2019 Salzburg Statement on Vaccination Acceptance specified 95%.
Coverage gaps
In 2019, 85% of the world's infants were reported to receive a first measles vaccination, with fewer receiving the second dose. The Salzburg statement reported waning coverage in many populations and insufficient two-dose measles coverage for herd immunity in the United States and 34 countries of the WHO European Region in 2019. It warned that the re-emergence of measles could be repeated with other childhood diseases such as rubella.
People exposed when coverage falls
Insufficient immunisation allows disease to spread among unvaccinated people, immunosuppressed people and children too young to be vaccinated. Unvaccinated children have a higher risk of vaccine-preventable disease than vaccinated children and also put medically exempt people of all ages at risk. Many children under 12 months, and children who are immunosuppressed, risk disability or death if coverage falls too low.
Even vaccinated people can remain susceptible because vaccination does not make every recipient immune. Even at the coverage associated with herd immunity, protection is incomplete, and exposure from outside the community puts non-immunised people at risk.
Unvaccinated people remain members of the community and depend on others' vaccination for indirect protection. If too many refuse vaccination, outbreak risk rises, putting their own and other children at risk. A child's safety therefore depends partly on the proportion of the surrounding population that is immune.