By Marsha Mullings, MPH
A vaccine is a biological tool that trains the body to fight an infectious agent that it has not yet encountered. Vaccines allow our bodies to respond to and prevent an infection from leading to illness by mimicking the body’s natural immune response. Vaccines are designed to replicate the body’s ability to counteract foreign agents (microbes) that infect it.
In the body’s natural immune response, special immune cells (memory cells) remember the configuration of each microbe it encounters and mount a rapid response if it is attacked by microbes it has encountered before. This system of attacking and then remembering different microbes is the body’s Immune System. When a new microbe attacks the body, the immune system cannot immediately fight off the invading microbe. Instead, it takes at least a week, and in many cases, two weeks or more for the immune system to “learn” the microbe’s configuration and then mount its response to the invader. In the interval between the infection and the immune system’s response, the microbe can produce millions of copies of itself, thereby overwhelming the body and causing illness. Some microbes are extremely powerful and can quickly overrun the immune response. Vaccines are designed to give the body an upper-hand by “tricking” the immune system into mounting an immediate response to a microbe, thus impeding the microbes’ action.
There are several types of vaccines. The most common are live-attenuated, inactivated, conjugate, and toxoid vaccines. There are also newer types of vaccines such as mRNA vaccines, which we will discuss in the following instalment.
Live-attenuated vaccines use a weakened form of the microbe to trick the body into stimulating an immune response to the target microbe. They typically produce a very strong immune response that confers lifetime immunity to the target microbe. The measles vaccine uses a weakened (attenuated) form of the measles virus to produce an immune response which lasts a lifetime.
Inactivated vaccines on the other hand, use a killed form of the microbe to produce the immune response. The immune response from an inactive vaccine is usually not as strong as produced by the live-attenuated vaccine. Inactive vaccines such as the flu vaccine usually do not confer lifetime immunity and require multiple booster shots.
Conjugate vaccines such as Haemophilus Influenza Conjugate (Hib) vaccine use specific sections of the microbe to produce an immune response to the target.
Toxoid vaccines use the toxin produced by the microbe (such as the tetanus and diphtheria bacteria), to produce an immune response. Both conjugate and toxoid vaccines induce a strong immune response, however both types of vaccines require booster shots for ongoing protection.
Vaccination is an ingenious tool to help the body in producing a rapid response to a target microbe. It essentially primes the body’s defense system for an enemy that it “knows” before its first encounter with the target. Vaccines have eliminated some of humanity’s greatest diseases from the realm of public health emergencies. Scourges such as Diphtheria, Polio, and Pertussis (Whooping Cough) have been almost eradicated among human populations. The race to eliminate diseases of modern time spur intense research into new vaccines and vaccine technologies. In the next instalment, we will discuss the development of the mRNA vaccine, which is currently being deployed in the fight against SARS-CoV-2 (coronavirus) and the COVID-19 pandemic.
For more information on Vaccines and Immunology, visit www.niaid.nih.gov/; www.cdc.gov; www.vaccines.gov