Herd Immunity. Population Immunity. Herd Protection. You must be hearing these terminologies ever since the hunt for a solution to control the ongoing Covid-19 pandemic started. Let’s try to understand the basics of herd immunity, its threshold, and how vaccination has worked in the past.
Crux of the Matter
It is a concept seen by epidemiologists, in which a population can be protected from a certain virus if a threshold of vaccination is reached.
Alternatively, it can also be achieved when individuals become immune by contracting and then recovering from an infection naturally. This provides indirect protection to those who are not immune to the disease yet.
Can You Give An Example?
Yes. For polio, if 80% of a population is immune to the virus, 4 out of every 5 people who encounter a sick person won’t fall ill. Thus, the community spread of the disease would decline and then disappear.
Does Herd Immunity Threshold Vary?
Yes. Herd immunity for high threshold disease like measles required about 92 – 95% of a population to be vaccinated. For a low threshold disease like influenza, 33 – 44% were vaccinated.
How Do Vaccines Function?
Vaccines train our immune systems for the time when we are exposed to the actual disease, by creating Y shaped proteins called antibodies, which fight off the virus, without making us sick. Vaccines can either be prophylactic (to prevent the effects of a future infection by a natural pathogen) or therapeutic (to fight a disease that has already occurred like cancer).
How Can We Not Fall Sick?
A vaccine isn’t harmful to our body as it just contains an agent, which is either the weakened or killed forms of the microbe/its toxins/its proteins.
Herd Protection From Once Common Diseases
Before vaccines were developed, kids with weakened immune systems could still get infected. Diseases like Measles and Chickenpox are now rare in communities with high vaccination coverage.
Where It Doesn’t Work?
After viruses mutate and create new variations, the antibodies from a previous infection provide short-time protection. Also, herd immunity applies only to contagious diseases like flu and not infectious diseases like Tetanus.
- The modern word “immunity” comes from the Latin immunis, meaning exemption from military service, tax payments, or other public services. The term “immunes”, is also found in the epic poem “Pharsalia” written around 60 B.C. by the poet Marcus Annaeus Lucanus to describe a North African tribe’s resistance to snake venom.
- Variolation used to immunize individuals against smallpox (Variola) with material taken from a patient or a recently variolated individual, in the hope that a mild, but protective, infection would result. Material from infected patients was inserted in people through skin scratches which resulted in immunity against smallpox. The method was first used in China, India, and the Middle East before it was introduced into England and North America in the 1720s.
- Pulse Polio is an immunization campaign established in India to eliminate polio by vaccinating all children under the age of five years. Tamil Nadu was the first Indian state to become 100% polio-free through the pulse strategy.