Lines snaked around New York streets when a 1947 outbreak of smallpox led to vaccination of 6 million people in less than a month.

The eradication of smallpox is the one undisputed success story in the long history of humankind’s fight against disease.

Undisputed, did I say? That smallpox is gone is beyond a doubt. No-one has seen it outside of a laboratory since 1977,  which makes the last case almost old enough to be historical, if one of us were to write a book about it. (Fifty years or more before the present day is usually suggested as the timespan for ‘Historical’, though we might want to review that in the light of how different the ’70s and ’80s are from the present.)

Yet some argue that vaccination was not the reason for the disappearance of the disease; that it was getting milder as the population grew healthier; that even at its height, the vaccination campaign only reached 10% of the populations of the countries were vigorous vaccination campaigns took place.

Some smallpox was always milder

Point 1: as I discovered when I researched smallpox for To Mend the Broken-Hearted, smallpox always came in two varieties. Variola minor had a death rate of 1%; variola major, on the other hand, killed 30% on average. It’s true that, by the mid-20th century, variola minor was the predominant strain in the United States and the United Kingdom. Nonetheless, variola major continued to scythe its way through communities in the rest of the world, killing 300 million people and occasionally making a visit to the supposedly safer countries, courtesy of international travel. Here are just two examples. An overseas visitor to New York started a massive vaccination campaign in 1947, after he infected 12 people. Two, including the visitor, died.  In 1962, a traveller from Pakistan started a smallpox outbreak in Wales.

Twenty-five people contracted smallpox, and six of them died, including a nine-month-old baby. [https://www.bbc.co.uk/history/british/empire_seapower/smallpox_01.shtml]

In the late 1950s, the World Health Organisation decided the only way to protect the world from such events was to eradicate the disease. Smallpox was ideal for the attempt. [https://www.historyofvaccines.org/content/articles/disease-eradication]

1. It was easy to recognise. Patients develop a distinctive rash. Time from exposure to rash is short, so the disease usually can’t spread very far before someone notices it.
2. Only humans can transmit and catch smallpox. Many illnesses have an animal species they can also infect, so the disease can hide there and jump back to humans under the right conditions.
3. After surviving smallpox or being immunised, people are protected for a lifetime.

Ring vaccination

Which brings us to point 2. Carefully managed, 10% was enough.

The WHO strategy was to track down every contact of every smallpox case they found, and vaccinate them, thus putting a ring of immune people around the live disease. It’s as simple as that. With a disease that meets the three criteria above, you don’t need herd immunity across the entire population. You simply need to get rid of any case you find by ring-fencing it with people who can’t get smallpox, and therefore can’t pass it on.

Quarantine is essentially the same strategy: you take away any chance smallpox has to jump to a new human host, and the disease dies (at least in that vicinity) when those being treated either recover or die. Except that quarantine tends to  be expensive, time consuming, and leaky. Vaccines work better.

In 1977, the last patient in the world to catch smallpox outside of the lab was diagnosed in Somalia.

Ali Maow Maalin, a 23-year-old hospital cook in Merca, had never been successfully vaccinated. After his diagnosis, an intensive tracing and vaccination campaign led to 54,777 people being vaccinated in the next two weeks. The disease was cornered, with no vulnerable hosts nearby to spread to.