Herd immunity, or group immunity, is the process by which a community is protected from disease by targeted vaccination. By reducing the risk of successful infection, disease outbreaks are prevented as there are too few vulnerable hosts for the illness to spread to. This group immunity starts manifesting when the population has an approximate 80% vaccination rate and is most effective at rates above 95%.
Remember, not everyone can get vaccinated due to compromised immune systems and allergies. These vulnerable members of society must be protected by creating an environment where diseases can't flourish.
Let's go ahead and look at an example of how herd immunity was used to eliminate smallpox in India. The population in India is too large for the medical infrastructure to supply sufficient vaccines to everyone. The number of doctors per capita is also insufficient to meet medical needs. As a result, something other than total vaccination had to be tried.
Vaccines were distributed to areas with smallpox outbreaks. Doctors then worked outwards to immunize the immediate family of the afflicted, followed by neighbors and coworkers. This created a "ring" of vaccinations around every known case of smallpox.
Several years later, the disease was eliminated through targeted group immunity. Once there was no possibility of spreading infection, the disease could only survive with the original carriers. The disease died out when either the individual's immune system overcame the illness or when the individual died.
Herd immunity doesn't exist to help the community on a small scale. It results in the complete eradication of a disease. Polio is on the verge of being eliminated in this way. Since it can survive outside the body for a limited time, isolated cases still emerge. By preventing fertile ground for this disease to flourish, these isolated cases do not result in a re-emergence of the disease.
The concept of one weak link in the protective chain resulting in a new outbreak is one vital fact that anti-vaxxers forget. If we strengthen our community's resistance through strong vaccination programs, we can overcome diseases that have stalked humanity since the very beginning.