The basic aim of vaccination is to reduce the R naught (R0) factor of a pathogen.
r k mahapatra
R0 is a measure of transmissibility of the virus.
It is always observed that whenever a new pathogen enters a closed community, say an isolated island, it rapidly infects a large number of the residents causing an epidemic. As the infection progresses, members of the community become resistant to virus. Gradually the infection dies down. This cycle again repeats but after a gap. In the gap period, the islanders remain free from the epidemic.
The classic example is two measles epidemic in Iceland: 1846 and 1882. Halfway in between, in 1869, again measles entered Iceland but did not spread far.
The SIR Model
- S = Number of Susceptible
- I = Number of Infected
- R = Number of Recovered / Resistant.
A brilliant mathematician has created a mathematical model from SIR to predict the course of an epidemic in a community.
His name was Ronald Ross.
Ronald had shown exceptional talent in mathematics in his school career. His father was a military officer in the British Indian Army. He sincerely wished Ronald to join the British Indian Military as a doctor. Ronald on the other hand was interested in poetry, music and mathematics. But his father prevailed, and after a few unsuccessful attempts, Dr Ronald Ross entered the Indian Military Medical service. In his first posting at Chennai, he spent a lot of time writing poems than seeing patients. Then he went home, got married and came back to India with a brand new Microscope. This new toy changed Dr Ross. Though he was averse to use stethoscope, he buried himself with his microscope. He would make careful notes of his microscopy. His scientific notes are elegant write ups and can put a poet into shame.
One such note taken on a Friday, 20th August, 1897 at Secunderabad has become world famous.
The Nobel prize in Physiology and Medicine was awarded to Ronald Ross in 1902 for this piece of note.
By that time Ronald had completed his military career in India and had returned to England. In 1902, he became the Professor and Chair of of Tropical medicine in Liverpool. Here he developed the mathematic model of disease control on the SIR model.
Even today, our IIT Kanpur uses these models ( computer algorithm to predict the expected surges of Covid-19 to be known as the 4th wave.
Vaccination and R0
The R0 of Measles is 18 in a naive population. This will be far less in Bhubaneswar today. Why ? Because a large number of children here have developed immunity against Measles virus. When a large percentage of population acquire immunity by natural infection or on successful vaccination, this R0 falls.
When R0 = 1, the disease prevalence remains fixed. Every single case is replaced by one new case. When it falls below 1, the disease dies down.
Getting the natural infection to build Herd immunity is a costly affair. vaccination on the other hand improves the quality and quantity of ou adaptive response to that pathogen. In the first wave of Covid-19, a large number of precious lives were lost because of lack of herd immunity. Vaccination has changed that.
But we still believe that vaccination is a drug that should confer personal protection. Parents believe so. Otherwise they won’t come for vaccination of their kids. But we, paediatricians should know better.
In 1980, 2.6 million people died from measles. In 2020, it fell to 60,700, a 98% reduction following the introduction of measles vaccine. That is statistics. It so eloquently supports vaccination. Though a single course of measles vaccine may not protect the child 100%, but many such courses together will reduce the mayhem caused by the viruses. Getting vaccinated should be a sacred public duty.
You are not safe, unless everyone around you are safe.
Dr Ramakanta's Ideas 