The Story of Vaccines: From Jenner’s Cowpox to mRNA

In 1796 an English country doctor scratched a little fluid from a cowpox sore into the arm of an eight-year-old boy. It was one of the most consequential experiments in history. Vaccination went on to wipe out smallpox, a disease that had killed hundreds of millions of people, and to bring polio, measles and many other killers under control. This guide explains how vaccines work, how the idea developed, what herd immunity means, and how India fought smallpox and polio.

Smallpox: the great killer

Smallpox was among the deadliest diseases in history. It caused high fevers and a rash of pus-filled blisters, killed roughly three in ten of those infected, and left survivors scarred or blind. It struck rulers and beggars alike and spread easily from person to person.

People noticed long ago that survivors did not catch it again. In parts of Asia, Africa and the Middle East, healers practised variolation: they gave a healthy person a small dose of material from a smallpox sore, usually through a scratch or by inhaling dried scab, hoping to cause a mild case that would protect them. In India this was done by traditional inoculators known as tikadars, and it was practised in China, too. The practice was risky, because some people died of the inoculation or spread the disease, but it saved many lives. Lady Mary Wortley Montagu, who had seen it in Istanbul, helped bring it to England in the early 1720s.

Jenner and the cowpox experiment

Edward Jenner, a doctor in Gloucestershire, England, heard a piece of country wisdom: milkmaids who caught cowpox, a mild disease from cattle, seemed not to get smallpox. In May 1796 he took matter from a cowpox sore on the hand of a dairymaid, Sarah Nelmes, and scratched it into the arm of James Phipps, the eight-year-old son of his gardener. Phipps had a mild fever. Weeks later Jenner exposed him to smallpox material, and the boy did not become ill.

Jenner published his results in 1798 and the technique spread rapidly across Europe and beyond. The word vaccine comes from the Latin vacca, cow. Today we would not accept an experiment on a child like this, and Jenner’s trial was tiny, but the principle was right and was soon confirmed on a large scale.

How a vaccine works

Try it: the herd immunity calculator

Vaccination protects the person who gets it, and also those around them. When enough people are immune, an infection cannot find enough new people to spread. The share needed depends on how contagious the disease is, measured by R0, the average number of people one case infects in a fully susceptible population. A simple estimate of the threshold is 1 minus 1 divided by R0.

Herd immunity in Python

for r0 in (1.5, 3, 6, 15):
    threshold = 1 - 1 / r0
    print(f"R0 = {r0:>4}: about {threshold * 100:.0f}% must be immune")

Output:

R0 =  1.5: about 33% must be immune
R0 =    3: about 67% must be immune
R0 =    6: about 83% must be immune
R0 =   15: about 93% must be immune

The more contagious the disease, the higher the bar. That is why measles, one of the most contagious diseases known, returns quickly when vaccination coverage slips even a little.

Try it: an outbreak on a grid

This toy model shows the same idea. Each square is a person. Vaccinated people (green) cannot catch the disease. One infected person (red) starts in the middle and can pass the infection to each neighbour with 60 percent probability. Change the vaccination coverage and press Run.

Milestones in vaccination

Pasteur extended Jenner’s idea to other diseases, and it was Pasteur who made “vaccine” a general term. The 20th century brought vaccines for diphtheria, tetanus, whooping cough, polio, measles and many more. Smallpox, the first disease to be targeted, became the first to be wiped out. The last natural case occurred in Somalia in 1977, and the World Health Organization declared the disease eradicated in May 1980.

mRNA vaccines: the new platform

Traditional vaccines use weakened or inactivated viruses, or pieces of them. An mRNA vaccine delivers a short strand of messenger RNA with instructions for making a single viral protein. The body’s own cells make the protein briefly, the immune system learns to recognise it, and the mRNA breaks down quickly. Katalin Kariko and Drew Weissman discovered that modifying the building blocks of mRNA stopped it from triggering harmful inflammation, a breakthrough that earned the 2023 Nobel Prize in Physiology or Medicine. The first mRNA vaccines were authorised for COVID-19 in late 2020.

Vaccines in India

India has a deep history with vaccination, from tika inoculation to modern campaigns. Smallpox cases were numerous in India in the 1960s, but a nationwide effort that included house-to-house searches brought them down, and India was declared smallpox-free in 1977. The Universal Immunisation Programme began in 1985 and was expanded by Mission Indradhanush from 2014. The Pulse Polio campaign, launched in 1995, used mass campaigns to give drops to children under five, and India recorded its last wild polio case in January 2011 and was certified polio-free in March 2014.

India is also a vaccine manufacturing giant. The Serum Institute of India in Pune, founded in 1966, is one of the world’s largest producers of vaccines by number of doses. During COVID-19 India produced Covishield, made by the Serum Institute, and Covaxin, developed by Bharat Biotech, and its vaccination drive began in January 2021 and administered more than two billion doses.

Mind map: vaccines at a glance

Common myths

• Myth: Vaccines cause autism. Large studies of millions of children have found no link, and the original paper that claimed one was retracted.
• Myth: Natural infection is always better. Natural infection with measles, polio or tetanus can cause severe harm or death, while a vaccine gives protection with far lower risk.
• Myth: Herd immunity means everyone must be vaccinated. It needs a high share, not everyone, and the share depends on the disease.
• Myth: Vaccines contain mercury that harms children. Some multi-dose vials once contained a mercury-based preservative at tiny doses; most childhood vaccines today do not.

Frequently asked questions

Keep exploring

Sources

• History of smallpox vaccination, World Health Organization
• Edward Jenner, Encyclopaedia Britannica
• The Nobel Prize in Physiology or Medicine 2023, Nobel Prize Outreach
• Ministry of Health and Family Welfare, Government of India

Featured image: Image: Gaston Mélingue, via Wikimedia Commons, Public domain. This guide is for general education and is not medical advice.