Herd Immunity

Herd immunity (from Latin collectivus — collective and immunitas — non-susceptibility, exemption) is a key epidemiological concept. It describes a phase of population protection in which most people possess personal immunity to an infection. This makes further widespread transmission impossible.

Etiology and pathophysiology

Viruses and bacteria require a continuous chain of transmission from an infected individual to a susceptible one to survive. The contagiousness of a pathogen is measured by the basic reproductive number (R0) — the number of people that one infected person will transmit the pathogen to on average in a completely non-immune environment.

If an individual in the chain is vaccinated or has antibodies from a previous illness, they become a biological “dead end” for the pathogen. The virus enters their body but is immediately destroyed by the immune system and is not released into the external environment.

When the number of such protected individuals reaches a critical threshold, the pathogen cannot be transmitted from person to person, and the disease is not registered in the population or community:

• for the highly contagious measles — the threshold is 95%;
• for poliomyelitis — around 80%.

Clinical significance

The formation of herd immunity through mass vaccination is the only tool in medicine for the global eradication of diseases (for example, humanity eradicated smallpox entirely this way).

From a clinical perspective, this mechanism acts as a “social shield.” It is essential for protecting patients who cannot be vaccinated due to medical contraindications.

Such vulnerable categories include:

• newborns;
• pregnant women;
• recipients of donor organs or bone marrow;
• patients with HIV or undergoing severe chemotherapy.