Herd immunity is a phenomenon that happens when most of the population is immune to a disease that causes indirect protection or herd protection to those who are not immune to the disease. For example, if 80% of a population is immune to a virus, four out of every five people who encounter someone with the disease won’t get sick which inhibits the ability of the virus to spread from person to person.
Herd immunity was first recognized in the 1930s by A. W. Hedrich, after the number of a new infection, decreased after many children became immune to measles. Despite this knowledge, the control and elimination of measles were unsuccessful, until mass vaccination using measles vaccines in 1960. Mass vaccination and discussions about disease elimination caused widespread in term “herd immunity” finally in 1970 a theorem was developed to calculate a disease herd immunity threshold.
To understand the concept of herd immunity threshold, we need to know about the basic reproduction number, it the number of cases each infected person of contagious disease generates. If the susceptible fraction is greater than the 1/R0, then the spread of disease is progressed but if the susceptible fraction is less than the 1/R0 the progress of the disease eliminated.
the proportion of the population susceptible to infection is ’S,’
S.R0 = 1
S can be written as S = 1 - p where p is the proportion of the population that is immune to the disease.
R0.(1 - p) = 1
1 - p = 1/R0
Hence, p = 1 - 1/R0
By getting this equation herd immunity is achieved.
So, In the end, the spread of disease can only be declined if the value of R0 is low as it stops the transmission. Herd immunity is advantageous to the people who are not able to develop the immune early old people, children, and people who are already suffering from another disease.
R0 OF WELL KNOWN DISEASES
Measles -- Airbone -- 12-18
Smallpox -- Social contact -- 5-7
AIDS -- Sexual contact -- 2-5
SARS-Cov -- Airbone droplet -- 2-5
HERD IMMUNITY FOR CORONA VIRUS PANDEMIC
Top on the list 2020 news is COVID-19, There is no vaccine for COVID-19 yet, the researchers are still working on it. In the future of this pandemic, we have a chance to develop herd immunity once a sufficient amount of people have developed immunity against the disease. Herd immunity can control the transmission of the virus and protect the people who are not immune to the virus. To accomplish herd immunity most of the population has to be infected by the disease but that can also increase the death rate in the population.
In the case of COVID-19, herd immunity cannot be executed as the researchers have not come up with the vaccine. We need most of our population to be infected with COVID-19 to achieve herd immunity, but this will raise the death rates in the population which may lead to the catastrophic event until the vaccine of the virus is not developed. Hence, herd immunity is not the best solution for COVID-19 yet.
So, in the end we can only fight COVID-19 if we think of us as ‘we’ and not ‘me’.
Acknowledgement: Siddhartha Negi for helping in research and writing this blog.