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—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, termed the threshold proportion. If the level of immunity passes a certain threshold level, then the pandemic will start to perish as there are not enough new individuals to infect.

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. Herd immunity through vaccines has successfully controlled many lethal contagious diseases like measles, smallpox, polio etc. Some vaccines need booster injections after a certain period, failing which there is a chance for reduction in the immunity level in that individual and they may become infected with that disease. If the herd immunity threshold is disturbed in 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 as well. 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.

The second path of herd immunity is through natural infection which is acquired when a certain level of the population recovers from an infection and develops immunity. For example, people who survived the 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 for herd immunity? It depends on the contagiousness of the disease, mining that for those diseases which are more contagious, a greater proportion of the population is required to be immune to stop infection. For example, measles is a highly contagious disease and 94 percent of the population is required to be immune to disrupt the spread of this disease.

Defining the threshold for COVID-19 is critical, to calculate this, we need to know how many new people are infected by each infected individual on average. 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 that one person can infect two and half individuals (on average). So, in this scenario, the threshold for herd immunity is 0.6 or 60 percent. This does not mean that COVID-19 will die immediately after 60 percent of the population develops immunity against it; it may infect another 20 percent before it completely vanishes.

However, getting herd immunity against COVID-19 through natural infection needs some questions 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 coronavirus in recovered individuals. The second question is; how long will this immunity 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; 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 percent of the population (more than 200 million people) to recover from COVID-19 to control this pandemic, similarly, in Pakistan we may need around 150 million people to recover. 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 new cases until the development of a vaccine which can offer a safe way of getting herd immunity.