Herd Immunity and the Prevention of Epidemics
- Herd immunity occurs when a large percentage of a population becomes immune to an infectious disease, reducing its ability to spread within the community.
- This provides indirect protection to individuals who are not immune, such as newborns, the elderly, and people with weakened immune systems.
- When herd immunity is achieved, even if a disease enters a population, it is unlikely to cause an outbreak because there are too few susceptible individuals for sustained transmission.
How Does Herd Immunity Work?
- Direct Protection: Vaccinated individuals are immune and cannot spread the disease.
- Indirect Protection: Unvaccinated individuals are shielded because the disease has fewer opportunities to spread.
- Measles is highly contagious, with an $R_0$ of about 15.
- This means one person can infect 15 others in a susceptible population.
- To achieve herd immunity for measles, about 93% of the population must be vaccinated.
Herd Immunity Threshold
The herd immunity threshold is the minimum percentage of immune individuals needed to stop disease transmission.
- Measles requires about 95% of the population to be immune.
- Polio requires about 80-85% immunity.
The threshold varies based on the disease’s contagiousness, measured by the basic reproduction number ($R_0$).
Calculating the Herd Immunity Threshold
The formula for the herd immunity threshold is:
$$1 - \frac{1}{R_0}$$
- $R_0$: The average number of people one infected person will infect in a fully susceptible population.
- Higher $R_0$ means a higher percentage of the population needs immunity.
Relying on natural infection for herd immunity can lead to severe illness and high mortality rates.
Benefits of Vaccination
- Protects Vulnerable Individuals:
- Infants
- Elderly
