Explained: How Can Herd Immunity Be Achieved

As the total number of cases for COVID-19 reach 25 million, there is a constant chatter about achieving herd immunity against the virus. Herd immunity is when a large proportion of the population has developed immunity against a contagious, infectious disease. This can be achieved either through vaccination or natural infection.

While scientists across the world, including the likes of Johns Hopkins University believe that around 70% of the population needs to be infected to develop herd immunity against SARS-CoV-2, the World Health Organization believes that 50% of the population achieving immunity will suffice to control the spread and tackle COVID-19.

However two of the WHO experts, Director General Tedros Ghebreyeseus as well as Director of Health Emergency Services, Mike Ryan feel that globally we are very far from achieving herd immunity. The Director General has also stated that herd immunity is not a salvation. In order to achieve herd immunity globally, the WHO has created the ACT Accelerator to ensure the equitable distribution of vaccines and medicines.

What Is Herd Immunity?

Herd Immunity is when a large proportion of the population is protected from being infected by a contagious disease either by being vaccinated against the pathogen or contracting the infection and developing antibodies for the virus.

For SARS-CoV-2, several vaccines are at various stages of development. Experts believe that till a proper vaccine is available, people should follow physical distancing norms and proper hygienic practices.

In herd immunity, after a person has been vaccinated or naturally infected, they either develop antibodies to fight the pathogen or T cells that identify the pathogen to raise an immune response by the body when it encounters the pathogen again. As the number of immune people in the population increases, the rate at which the virus can reproduce decreases. This decreasing reproductive number of the virus contributes in controlling the spread of the disease.

COVID-19, however, poses a different challenge. As the duration for which immunity can last for the virus is still being studied, cases of reinfection and waning antibodies have been reported across the world. Vaccination for COVID-19, just like in the case of flu, may require booster shots which may not be everlasting. The severity of a reinfection could be milder than the first infection as seen with the flu.

Natural infection on the other hand could overwhelm the existing health infrastructure globally. Reports of hospitals not being able to manage an increased caseload at the peak of the infection were witnessed in Mumbai, New York and even Italy.

Currently, various sero-survey studies have been conducted globally to understand the prevalence of the spread of the virus. Most of them only analyse for presence of antibodies against SARS-CoV-2 explaining whether a person has been infected. They do not take into account a subset of neutralizing antibodies that particularly act on the virus. These numbers gauge the spread but are not enough to ascertain herd immunity, experts in the WHO believe.

While Spain has sero-prevalence in the range of 4.6-5%, CDC in the US has reported a sero-prevalence ranging between 2-10% in 10 regions. India's sero-survey conducted by the ICMR found a 30% spread while specific studies in Delhi and Mumbai have shown a prevalence of 28.35% and 33%.

The higher prevalence does not reflect herd immunity as the number of daily cases in India are still high. Spain has witnessed a second outbreak, while the US is also reporting 35-60,000 new cases daily.

Which Diseases Have We Achieved Herd Immunity For?

Measles which requires a 95% herd immunity is one of the contagious diseases eliminated in areas that followed vaccination for measles. Similarly, small pox, diphtheria are other diseases that were reduced and eliminated following herd immunity techniques.

View our video on herd immunity below: