Principles of immunityWednesday, September 01, 2021
Vaccination has been a pillar of public health and infection control since the efforts of Edward Jenner during the smallpox pandemic in 1796. Robust vaccination programmes have led to the virtual eradication of infectious diseases such as polio, rubella, measles, mumps, diphtheria, and tuberculosis across the globe. This has resulted in significant improvements in the quality of life and life expectancy of the entire human race.
Vaccination employs the principles of natural immunity, which is the body's natural, long-term protective response to invaders such as viruses, bacteria, and toxins. When these invaders enter the body of a person with a healthy immune system a cascade of events is triggered with the aim of eliminating the invader from the body and, more importantly, protecting the body in the event of subsequent exposure.
The primary defence mechanisms are T cells which regulate the immune response, neutralising proteins called antibodies, protective cells called cytotoxic T cells, and, most importantly, memory cells which lie in wait to mount secondary responses.
The secondary response is one that occurs after primary exposure to an invader. This response is rapid and robust as the memory cells quickly respond to a familiar invader, leading to the release of a high concentration of immune cells and antibodies. The result of this is immunity — the host is no longer susceptible to the invader once the immune system remains healthy. This is the aim of vaccination.
Vaccines expose the body, through various means, to whole or parts of an invader to trigger the primary response and to stimulate the production of memory cells, thus providing the vaccinated individual with immunity against the invader. Ideally, this immunity lasts for years, protecting the individual even within populations where the particular infection is common. This is why we can be safe sending vaccinated children to school and travelling to countries with various endemic diseases.
This brings us to the point of herd immunity. Herd immunity is resistance to the spread of an infectious disease within a population that is based on pre-existing immunity in the vast majority of individuals as a result of previous infection or vaccination. This essentially works because the vaccinated/recovered individuals protect the susceptible or unvaccinated individuals in the population. Typically the vaccinated/recovered individuals cannot catch or spread the infection and thereby provide a buffer against the disease within the population. The COVID-19 vaccines, though showing reduced efficacies in recent times ( https://www.cdc.gov/coronavirus/2019-ncov/variants/delta-variant.html), work in a similar manner.
Delbert J Robinson