Scientists are working on different strategies to curb COVID-19 pandemic. After drugs and vaccines, experts are now saying that herd immunity also has the potential to control this pandemic. This article will discuss herd immunity; how it can be achieved and different opinions about its potentials.
What is Herd Immunity?
When a large population of a community (herd) develops immunity against a disease, making the transmission of disease from person to person unlikely, 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 the population must be infected, called a threshold proportion. If the level of immunity passes a certain threshold level, the pandemic will start to perish as there are not enough new individuals to infect.
Routes to get herd immunity
There are two routes to get herd immunity for any disease including COVID-19 i.e. Vaccination and natural infection.
Vaccination
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. Some vaccines need booster injection after a certain period, failing of which may reduce the immunity level in that individual and he may become infected with that disease. If the herd immunity threshold is disturbed by such a way, it can result in the spread of that disease. For example, Measles has re-emerged in some parts of the world. Herd immunity through vaccination also has some challenges and opposition. For example, in Pakistan, some communities have refused to take Polio vaccines due to their fears about the risks or distrust on the benefits of the vaccine.
Natural Infection
The second path of herd immunity is through natural infection, which is acquired when a certain level of population recovers from infection and develops immunity. For example, people who survived in 1918 pandemic of Spanish Flu (Influenza) were immune against H1N1, another type of Influenza. The important question is what percentage of the population is required to be immune to herd immunity. It depends on the contagiousness of the disease meaning for those diseases, which are more contagious, a greater proportion of the population is required to be immune to stop the infection. For example, Measles is a highly contagious disease and 94 per cent of the population is required to be immune to disrupt the spread of this disease.
Experts believe that reaching herd immunity without a vaccine can be calamitous
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 nought). 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 a half individuals (on average). So, in this scenario, the threshold for herd immunity is 0.6 or 60 per cent. It does not mean that COVID-19 will die immediately after 60 per cent of the population develops immunity against it but it may infect another 20 per cent before it completely vanishes.
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. The second question is how long will this immunity protect the individuals from future infection. This also requires more investigation of 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 a large proportion of the population that need to be infected for the herd immunity threshold.
According to scientists, the US requires 70 per cent of the population (more than 200 million people) to be recovered from COVID-19 to control this pandemic. Similarly, in Pakistan, we may need around 150 million people to be recovered. This is a huge number and hospitals will be exhausted, moreover, this could lead to millions of deaths worldwide especially among chronically ill and older people. Experts believe that reaching herd immunity without a vaccine can be calamitous. Therefore, vagueness about the naturally developed herd immunity threshold and the consequences of illness due to COVID-19, limit us with only one way forward: Follow all SOPs provided by the WHO and our government to prevent the new cases until the development of a vaccine which can offer a safe way of getting herd immunity.
The writer is an Assistant Professor in Virology in the Department of Biology, College of Science, University of Bahrain