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Herd Immunity and Vaccine Effectiveness

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Terms you will need

  • herd immunityHerd immunity is when enough people in a group are immune to a disease, either from vaccination or past infection, so it stops spreading easily and protects those who are not immune.
  • vaccine effectivenessVaccine effectiveness is how well a vaccine works in real-world conditions, showing the percentage reduction of disease in vaccinated people compared to unvaccinated people.
  • vaccine efficacyVaccine efficacy is how well a vaccine works under ideal, controlled conditions, like in a clinical trial, showing the percentage reduction of disease in vaccinated people compared to unvaccinated people.
  • reproductive numberThe reproductive number is the average number of people who get infected by one person with a disease in a population where some people might already be immune.
  • basic reproductive numberThe basic reproductive number is the average number of people who get infected by one person with a disease in a population where everyone is susceptible and no one is immune.
  • effective reproductive numberThe effective reproductive number is the average number of people who get infected by one person with a disease in a population where some people are already immune.

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Herd immunity is a form of indirect protection that occurs when a sufficient percentage of a population becomes immune to an infection, either through previous infections or vaccination. This immunity reduces the likelihood of infection for individuals who lack immunity by limiting the spread of the pathogen. When herd immunity is achieved, the disease may gradually disappear from a population or be permanently reduced to zero if achieved globally. This concept is crucial for protecting individuals who cannot be vaccinated due to medical reasons, such as newborns or immunocompromised individuals. Vaccine effectiveness, which measures the percentage reduction of disease cases in a vaccinated group compared to an unvaccinated group, plays a key role in achieving herd immunity. High vaccine coverage and effectiveness are necessary to reach the herd immunity threshold, which varies depending on the basic reproduction number of the disease. Understanding herd immunity and vaccine effectiveness is essential for controlling and preventing the spread of contagious diseases.

Figure (1)

Influenza Vaccine

Herd immunity happens when enough people in a group are immune to a disease, either from past infections or vaccines. This stops the disease from spreading easily. This is important because some people cannot get vaccines for medical reasons.

The success of herd immunity depends on vaccine effectiveness. This is how well a vaccine prevents disease compared to not getting it. If a vaccine is very effective and many people get it, the disease can be controlled or even stopped.

But if too many people skip vaccines, the disease can spread again. It tells how many people one sick person might infect in a fully susceptible group. Diseases with higher R0 values need more people to be immune to stop them.

For example, a disease with R0 of 10 needs 90% of the population immune to reach herd immunity. Some people might not respond well to them, and immunity can fade over time. This means high vaccine coverage and regular boosters are needed to keep diseases under control.

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.

Terms

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