Why does the herd have immunity?

Tech Term Tuesday.  When a significant proportion of a community (or herd) is immune to a contagious disease through prior illness or immunization, the remaining members of that community (such as newborns, the chronically ill, the elderly and those otherwise not immunized) also receive some level of protection. This is because the likelihood of coming in contact with an infected individual has been substantially decreased.

History and Epidemiology of Global Smallpox Eradication. From the training course titled "Smallpox: Disease, Prevention, and Intervention". The CDC and the World Health Organization. Slide 16.

Herd immunity begins to fail when immunization rates in the community fall below critical thresholds and the community is exposed to new members who are infected (such is the case with individuals travelling from international regions with a high incidence of vaccine-preventable illnesses). As a result, the choices of individuals can harm the broader community.

Sources: CDC Online Glossary and Merriam-Webster Medical Dictionary

Learn More: Herd Immunity Animation at The History of Vaccines website


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