In public health, herd immunity (also known as population immunity) is point at which population becomes protected from an infectious disease. This happens when enough people have developed a defense—either through vaccination or previous infection—that virus or bacteria find it increasingly difficult to spread.
When most people are immune, the chains of transmission are broken, and disease hits a dead end.
How Herd Immunity Works
To understand herd immunity, it helps to look at how a virus moves through a group.
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Vulnerable Stage: In a population where no one is immune, a single sick person can easily infect several others, leading to an exponential outbreak.
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As more people become immune, likelihood of an infected person coming into contact with a susceptible person drops.
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Eventually, disease can no longer find enough new hosts to sustain its spread. This protects those who cannot be vaccinated, such as:
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Newborn babies.
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People with severe allergies to vaccine ingredients.
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Individuals with compromised immune systems (e.g., those undergoing chemotherapy).
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Herd Immunity Threshold
Percentage of population that needs to be immune to achieve this state is called threshold. This number isn’t a same for all as it depends entirely on how contagious disease is, measured by Basic Reproduction Number (R0).
| Disease | Contagiousness (R0) | Required Immunity Threshold |
| Measles | 12 – 18 | ~95% |
| Polio | 5 – 7 | ~80% |
| Influenza | 1 – 2 | ~33% – 50% |
The formula used to estimate this threshold (Ht) is:
There are two ways a community reaches herd immunity:
1. Vaccination
Vaccines create immunity without causing actual illness This is the preferred method for public health officials because it saves lives while building immunity.
2. Natural Infection
Immunity can be gained after recovering from disease.
Is Herd Immunity Permanent?
Not necessarily. Herd immunity can fade due to two main factors:
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Over time, body’s memory of virus may weaken, requiring booster shots.
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If a virus mutates significantly (creating new variants), immunity gained from older versions may be less effective.
Herd immunity is a collective effort. It isn’t just about protecting yourself; it’s about creating a safe environment for most vulnerable members of our society.