Scientists are working on different strategies to curb the Covid-19 pandemic. After drugs and vaccines, experts are saying that herd immunity also has the potential to control this pandemic.
When a large population of a community (herd) develops immunity against a disease, making the transmission of disease from person to person unlikely, and as a result the whole community becomes secure, not just those who are immune, this is called herd immunity. To get this, a certain percentage of population, called threshold proportion, must be infected.
There are two routes to get herd immunity for any disease including Covid-19 i.e. vaccination and natural infection.
A vaccine is an ideal approach to get herd immunity because vaccines provide immunity without illness or complications that may lead to death in case of natural infection. Herd immunity through vaccines has successfully controlled many lethal contagious diseases like measles, smallpox, polio etc.
The second path to herd immunity is through natural infection which is acquired when a certain level of population recovers from an infection and develops immunity. The important question is that what percentage of population is required to be immune for herd immunity.
Defining the threshold for Covid-19 is critical. To calculate this, we need to know that on average how many new people are infected by each infected individual. This number is called R0 (R naught). If you obtain R0, then you can put it in a simple formula to calculate the threshold required for herd immunity i.e. 1-1/R0. As it was announced that R0 for Covid-19 was 2.5 (it may vary from country to country), it means one person can infect two and half individuals (on average). So, in this scenario, threshold for herd immunity is 0.6 or 60 per cent.
However, getting herd immunity against Covid-19 through natural infection needs some questions to be answered. First, it is not clear yet that the individuals recovered from Covid-19 have developed immunity against future infection. There are some reports of reinfection in recovered persons.
Therefore, detailed studies are required to verify the protective immunity against the coronavirus in recovered individuals. Second question is how long this immunity will protect the individuals from future infection. This also requires more investigation on recovered individuals. Let us assume that Covid-19 recovered individuals develop long-lasting immunity, but another question is about the consequences of Covid-19 in large proportion of the population that need to be infected for herd immunity threshold.
According to scientists, US requires 70pc of the population (more than 200 million people) to be recovered from Covid-19 to control this pandemic.
This is a huge number and hospitals will be exhausted, moreover, this could lead to millions of deaths worldwide.
Experts believe that reaching herd immunity without a vaccine can be calamitous. Therefore, we should follow all SOPs provided by the WHO to prevent the new cases until the development of a vaccine.
Dr Muhammad Nauman Zaid is Assistant Professor in the Virology Department of Biology, College of Science, University of Bahrain