Understanding Herd Immunity with Interesting Facts.
                     UNDERSTANDING HERD IMMUNITY WITH 
INTERESTING FACTS
What is herd immunity?
When most of a population is immune to an 
infectious disease, this provides indirect protection 
or population immunity (also called herd immunity 
or herd protection) to those who are not immune to 
the disease.
Source: www.jhsph.edu
Concept and Purpose
Herd immunity prevents an infected individual from 
introducing the pathogen into the population or 
minimizes the extent or speed of transmission once 
it has entered the population.
Source: www.jhsph.edu
Necessary Conditions
The realization of herd immunity is not easy. Only 
when “sufficient proportion” of the population have 
“sufficient immunity” can the pathogen be 
prevented to spread to susceptible individuals.
Source: www.liebertpub.com
Restrictive Circumstances
It is impossible for a population to achieve herd 
immunity against all pathogens, which is limited by 
the following three conditions: 
1. The pathogens are fairly species specific. 
2. The pathogens are spread contagiously by fairly 
direct means. 
3. Host exposure or vaccination confers fairly 
strong immunity
Source: www.liebertpub.com
Challenges
Even though the two necessary conditions and 
limitations are met, the ability to achieve herd 
immunity still faces the challenge of heterogeneous 
transmission and vaccine efficacy.
Source: www.liebertpub.com
Heterogeneous transmission
Heterogeneous transmission is common in most 
human and animal populations. 
Specifically, the presence of super-spreaders and 
highly susceptible populations makes it difficult to 
estimate the proportion of individuals who need to 
be immunized to reach the herd immunity threshold 
so as to destroy and eliminate the corresponding 
pathogens
Source: www.liebertpub.com
Vaccine efficacy
Poor vaccine preparation and storage, improper 
vaccination timing, and so on may lead to 
population immunity lower than expected. 
This low level of immunity may even promote the 
further spread of the pathogen, as it may make 
patients' clinical symptoms less obvious, and policy-
makers relax their vigilance.
Source: www.liebertpub.com
SARS-COV
For SARS-COV, it has been shown that only 
antibodies against S protein can neutralize the virus 
and prevent infection.
Therefore, all SARS-COV-2 vaccines under 
development contain at least part of the S protein. 
As mentioned earlier, the choice of different vaccine 
platforms will show different effectiveness and 
safety.
Source: www.liebertpub.com
Spread of COVID-19
With the relentless spread of COVID-19, we need to 
correctly understand the relationship between herd 
immunity and COVID-19 from the aspects of 
pathogen, population, and the behavior taken by 
the population.
Source: www.liebertpub.com
The bottom line
The realization of herd immunity depends on the 
sufficient proportion of population in the population 
to obtain sufficient immunity, whereas the 
acquisition of immunity mainly depends on the 
mature and effective vaccination program.
Source: www.liebertpub.com