Is it necessary to vaccinate everyone to achieve population immunity?

• A. No; Not necessary to achieve 100% immunization
• B. Yes; 100% immunization is required
• C. Only for healthcare workers
• D. Immunity cannot be achieved through vaccination

Answer: (A)

Population immunity, or herd immunity, happens when enough people in a population are immune to a disease such that transmission chains are disrupted and the spread slows or stops, offering protection to those who remain susceptible. You don’t need every person vaccinated to achieve that protection. The key is the level of immunity required to prevent sustained transmission, which depends on how contagious the disease is (its basic reproduction number, R0) and how effective the vaccine is.

The higher the R0, the higher the threshold to reach, but that threshold is still typically well below universal vaccination. For example, a disease with moderately high transmissibility requires a large portion of the population to be immune, but not everyone. If the vaccine is not perfectly effective, you still aim to vaccinate enough people to push the effective immunity above the threshold; this often means slightly more than the simple threshold, not necessarily 100%.

It’s also important to recognize that some individuals cannot be vaccinated or may have waning protection, and population immunity helps shield those groups. So, the best answer is that vaccination does not have to be universal to achieve population immunity.