HERD IMMUNITY

Subject: Science & tech

Context: Director-General of ICMR Dr. Balram Bhargava said: “If we’re able to vaccinate a critical mass of people and break virus transmission, then we may not have to vaccinate the entire population.”

Concept:

• Herd immunity is a form of indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individuals who are not immune.
• Initially, herd immunity, an important tool in epidemic control, was proposed as a means to overcome the pandemic.
• Only a certain proportion of the population needs to be infected in order to stop large outbreaks, either through naturally-acquired disease, or through vaccination.

Challenges in achieving Herd Immunity through Partial Vaccination:

• The levels of immunization needed for herd immunity are determined by how the virus spreads in the population, and makes the assumption that spread is homogenous. But SARS-CoV-2 virus spread exhibits a high level of uneven transmission.
• This is the reason why there have been a number of super-spreading events where some infected individuals spread the virus to very a large number of people while most infected individuals transmit the virus only to a few or none.
• With COVID-19 vaccines, we will also need to reach age groups which are not currently targeted as part of the large-scale immunisation efforts. This will be a design, logistics and implementation challenge. All of these aspects are important to consider while considering the country-wide COVID-19 immunisation plans.
• Considering that the government has already listed out the high-priority groups that will receive the vaccine, the issue of choosing other sections of the population that needs to be vaccinated to achieve herd immunity will be ethically challenging.
• Another contentious area is the question of vaccinating those who have already been infected. The immunity offered by the natural infection is probably long term. Till date, we are not aware of the added benefits or risks of vaccinating the people who are already infected.