Q.30 In a population, the probability of a susceptible individual getting infected with
SARS–CoV–2 is low when a majority of individuals in the population becomes
immune to this virus. This phenomenon is known as _______.
(A) innate immunity
(B) adaptive immunity
(C) active immunity
(D) herd immunity

The correct answer is (D) herd immunity. This phenomenon describes indirect protection for susceptible individuals when a large proportion of the population gains immunity, reducing SARS-CoV-2 transmission.​

Option Analysis

Innate immunity refers to the body’s first-line, non-specific defenses like skin barriers and phagocytes, present from birth and not tailored to specific pathogens like SARS-CoV-2.​
Adaptive immunity involves antigen-specific responses developed during an infection or vaccination, including T and B cells, but applies to individuals rather than population-level effects.​
Active immunity occurs when the body produces its own antibodies through infection or vaccination, contrasting with passive forms, yet it does not inherently describe community-wide protection.​
Herd immunity arises when enough immune individuals (via infection or vaccination) block pathogen spread, lowering infection risk for susceptibles, as in the SARS-CoV-2 example.​

Herd immunity occurs when a majority of individuals in a population become immune to SARS-CoV-2, drastically lowering the probability of susceptible individuals getting infected. This protects vulnerable groups without direct immunity.​

Key Mechanism

Immune individuals act as barriers, reducing the pathogen’s reproduction number (R_e) below 1, halting epidemics. For SARS-CoV-2, thresholds vary (e.g., ~67% for R_0=3), achieved via natural infection or vaccination.​

Comparisons

CSIR NET Context

This concept tests epidemiology in life sciences exams, distinguishing individual from community immunity. Vaccination accelerates herd immunity safely over natural spread.​