Friday, 11 September 2015

All you need to know about ‘Herd Immunity’


Herd Immunity is a form of immunity that occurs when a significant portion of a population is vaccinated and in turn provides a measure of protection for the individuals who have not developed immunity. When this critical percentage of the population is already protected through vaccination against a virus or bacteria, it makes it difficult for a disease to spread because there are few vulnerable individuals left to infect.

The term herd immunity was first coined in 1923, recognized as a naturally occurring phenomenon in the 1930s when it was observed that after a significant percentage of children had become immune to measles, the number of new infections temporarily decreased, including among vulnerable children. Mass vaccination in order to actuate herd immunity has since become common and proved successful in preventing the spread of many contagious diseases.  During the smallpox eradication campaign in the 1960s and 1970s, the practice of ring vaccination, of which herd immunity is integral to, began as a way to immunize every person in a "ring" around an infected individual to prevent outbreaks from spreading.

This is highly effective in stopping the spread of a disease in a community. It is particularly crucial for protecting people who cannot be vaccinated. These include infants who are too young to be vaccinated, immunocompromised people, and those whose medical condition doesn’t allow them to receive vaccination (such as cancer patients).

However the percentage of a population which must be immunised in order to achieve herd immunity against a specific disease varies for each disease. Once a certain threshold has been reached, herd immunity gradually starts eliminating a disease from a population. This elimination, when achieved worldwide, results in the permanent reduction in the number of infections to zero, called eradication. Till date, two diseases have been eradicated using vaccination and herd immunity: smallpox and rinderpest.

However, when immunisation rates decline, it becomes difficult to retain herd immunity, leading to an increase in the number of new cases.

Following is an animated video by VaccinesToday, explaining how herd immunity works and what happens when it breaks down.





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