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Herd Immunity and Vaccine Effectiveness

Herd immunity, also called community immunity, is a form of indirect protection that occurs when a sufficient percentage of a population becomes immune to an infection, either through vaccination or previous infection. This immunity reduces the spread of the disease, lowering the risk for individuals who are not immune. Herd immunity is especially important for people who cannot be vaccinated due to medical reasons, such as newborns or those with weakened immune systems. Vaccine effectiveness, on the other hand, measures how well a vaccine prevents disease in real-world conditions. It is calculated by comparing disease rates between vaccinated and unvaccinated groups. Together, herd immunity and vaccine effectiveness are key to controlling and eliminating contagious diseases. Understanding these concepts helps explain how public health efforts, like vaccination programs, protect entire communities.

Figure (1)

Influenza Vaccine

Herd immunity happens when enough people in a group are immune to a disease, either from vaccines or past infections. This stops the disease from spreading easily, protecting those who are not immune. The more immune people there are, the harder it is for the disease to move through the group.

Vaccine effectiveness measures how well a vaccine stops a disease in real life. It compares how many people get sick in a vaccinated group versus an unvaccinated group. A vaccine that is 80% effective means 80% fewer cases in the vaccinated group.

Vaccines make people

Key Points

  • Herd immunity is a form of indirect protection that occurs when a sufficient percentage of a population has become immune to an infection, preventing the communicable pathogen from maintaining itself in the population.
  • Vaccine effectiveness is the percentage reduction of disease cases in a vaccinated group of people compared to an unvaccinated group under less-than-perfectly controlled conditions in a typical population.
  • Vaccine efficacy is the percentage reduction of disease cases in a vaccinated group of people compared to an unvaccinated group under the most favorable, ideal or perfectly controlled conditions, such as those in a clinical trial.
  • The basic reproduction number, denoted R_{0}, is the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection.
  • The effective reproduction number, R_{e} or R, is the average number of new infections caused by a single infected individual at time t in a partially susceptible population.

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