With Covid-19 spreading fast in the entire world, herd immunity is being considered as a strategy to tackle the disease in a few countries. What is herd immunity and can this really deal with Covid-19?


Herd immunity means that a group has sufficient number of individuals who are immune to a certain disease that even if an infected person entered the group, it would not spread easily. The herd as a whole has “immunity” against the disease even though all individuals are not immune. Enough people will not get the disease due to immunity gained either through vaccination or natural immunity that even those who are vulnerable are protected in that specific group. The immunity of the people in the group will prevent the spread of the disease.


Then arises the question of how many people should be immune in a group to prevent the spread of a disease. This completely depends on the contagiousness of the disease. 


In case of Covid-19 which is being considered a pandemic, this might not work as a strategy to prevent its rampant spread because it is very hard to predict things during a pandemic. The situation keeps changing fast that even some solid facts of which we were sure of like the reproductive rate, symptoms of infection need re-evaluation frequently.


There have been articles suggesting that herd immunity could be our response to the ongoing crisis due to the virus. Many of them are predicting herd immunity as a preventive strategy that may stall the surge in the spread of the disease.


The fact is herd immunity without a vaccine is not a preventive measure.


Let us understand this. Mumps is a highly infectious disease that causes many complications that can last for a lifetime but a highly effective vaccine has made the disease rare in the modern age.


It has a very high index of contagiousness that the basic reproductive rate (RO) of 10-12 leaves no one immune. This means every person who is infected will pass on the disease to 10-12 people.


When we analyse the situation, we will find that without a vaccination more than 90 per cent of the population is susceptible to the infection. Once this 95 per cent of the population gain immunity through a vaccine, the remaining less than 10 per cent of the people do not get sick because everyone else in the group is immune and there is no one to contract the disease from.


We can increase the number of people immune to a disease in a group only through vaccination which means herd immunity also depends on a certified vaccine to develop immunity in the group.


Vaccination not only makes people immune to infection but also stops the infected from passing on the disease to every other person, which they would have done without the vaccination.


Mumps is a much contagious disease than Covid-19 which has an RO of nearly 3 but nearly 70 per cent of the population is already infected with coronavirus which is why herd immunity can never be a means to flatten the curve without a vaccine.