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Today (December 9) is Smallpox Eradication Day.[1] 43 years ago, smallpox was confirmed to have been eradicated after killing hundreds of millions of people. This was a major achievement in global health.

So I'm link-posting Our World in Data’s data explorer on smallpox (and here’s the section on how decline & eradication was achieved).[2] 

This post shares a summary of the history of the eradication of smallpox and selected excerpts from the data explorer.

A summary of the history of smallpox eradication[3]

Smallpox was extremely deadly, probably killing 300 million people in the 20th century alone. The last known cases occurred in 1977, and smallpox is now the only human disease that has been completely eradicated.

So how was this accomplished?

Before we had a smallpox vaccine, we had the practice of variolation — deliberately exposing people to material from smallpox scabs or pus, in order to protect them against the disease (variolation traces back to 16th century China). While variolation made cases of smallpox much less severe, variolation infected the patient and could spread the disease to others, and the severity of the infection could not be easily controlled. So variolation did not lead to the elimination of smallpox from the population.

In the late 18th century, Edward Jenner demonstrated that exposure to cowpox — a much less severe disease that turns out to be related — protected people against smallpox. This, in turn, led to the invention of a vaccine against smallpox (the first vaccine ever). 

In the 19th and 20th centuries, further improvements were made to the smallpox vaccine, and many states were running programs to vaccinate significant portions of the population. By 1959, the World Health Organization (WHO) launched a global program to eradicate smallpox . This involved a coordinated effort to immunize large numbers of people, isolate infected individuals, and monitor the spread of the disease. The program used a technique known as ring vaccination, which involved vaccinating people who had been in contact with infected individuals, in order to create a protective "ring" around the infected person and prevent further spread of the disease.

Excerpts from the Our World in Data entry


Smallpox is the only human disease that has been successfully eradicated.

Smallpox, an infectious disease caused by the variola virus, was a major cause of mortality in the past, with historic records of outbreaks across the world. Its historic death tolls were so large that it is often likened to the Black Plague.

The eradication of smallpox is therefore a major success story for global health for several reasons: it was a disease that was endemic (and caused high mortality rates) across all continents; but was also crucial to advances in the field of immunology. The smallpox vaccine was the first successful vaccine to be developed.


How many died of smallpox?

In his review paper ‘The eradication of smallpox – An overview of the past, present, and future’ Donald Henderson reports that during the 20th century alone “an estimated 300 million people died of the disease.”

In his book Anderson suggests that in the last hundred years of its existence smallpox killed “at least half a billion people.” 500 million deaths over a century means 5 million annual deaths on average.


Eradication across the world

The last variola major infection was recorded in Bangladesh in October 1975, and the last variola minor infection occurred two years later in Merka, Somalia, on October 26th, 1977. During the following two years, WHO teams searched the African continent for further smallpox cases among those rash-like symptoms (which is a symptom of numerous other diseases). They found no further cases.


The world map shows the year in which each country recorded the last endemic case of smallpox. Europe, North America and Australia managed to eliminate smallpox relatively early, most by the 1940s (predating the WHO’s Intensified Smallpox Eradication Program, which was launched in 1966). Countries across Sub-Saharan Africa, Latin America and Asia eliminated smallpox several decades later in the 1960s and 70s.


How was global decline & eradication achieved? 

Discovery of variolation

Variolation (sometimes also inoculation), refers to the deliberate transmission of viral matter.

Before the year 1000, Indians and the Chinese had already observed that contraction of smallpox protected children against any future outbreaks of the disease. As a consequence they developed a procedure that involved the nasal inhalation of dried smallpox scabs by three-year-olds.


The disadvantage of variolation, however, was that during the course of the mild infection the person became a carrier of the disease and could infect other people. Additionally, it was difficult to control the severeness of the infection which sometimes developed into a full-blown smallpox case that could lead to the person’s death.

This meant that the practice usually reduced the severeness of an infection and the likelihood of deaths but that it would never lead to eliminating the virus. If anything, it helped to spread the virus in a population even further and thereby encouraged its survival.

Vaccine against smallpox

At the end of the 18th century British surgeon and physician Edward Jenner (1749-1823) pioneered the first ever vaccination against an infectious disease. He himself had been inoculated with smallpox at the age of 8 and later as a surgeon, variolation was part of his work.23 He observed that people who had suffered from cowpox would subsequently have a very mild, if at all visible reaction to the smallpox variolation. At the time unknowingly, he had discovered that the cowpox and variola viruses were members of the same orthopoxvirus family.

He hypothesized that variolation using the cowpox virus would protect children against smallpox as well. Since cowpox infections were much milder and never fatal, this would eliminate the problem of variolated children being carriers of smallpox and sometimes dying of the virus developing into a full-blown infection. On top of protection against the symptoms, it could reduce the stock of humans that the variola virus needed for survival and brought elimination and eventually eradication of smallpox into the realm of possibility.

In May 1796, Jenner inoculated a boy with cowpox, and then a few months later with the smallpox virus. When the boy did not develop any smallpox symptoms in response to being variolated, his hypothesis of the cowpox offering protection from smallpox was confirmed motivating his further research trials.

Initially, Jenner faced major barriers to spreading the word about his discovery. When he submitted a paper outlining his findings to the journal Philosophical Transactions edited by the Royal Society, it was rejected. They even advised him not to pursue his ideas any further, pointing to the detrimental impact on his career and reputation. Undeterred, he published his work with an increased number of trials at his own expense two years later (in 1798). He also went on to convince colleagues and supply them with vaccines in other British cities of his new procedure that became known as vaccination (derived from the Latin word for cow, vacca).

By 1802, the British Parliament did acknowledge his important contribution and awarded him £30,000. Meanwhile, vaccination had spread to most of Europe and New England.24

His 1798 publication Inquiry into the Variolae vaccinae known as the Cow Pox had been translated into German, French, Spanish, Dutch, Italian, and Latin within three years. US President Thomas Jefferson figured importantly in the widespread application of vaccination throughout the United States and in 1806, he thanked Edward Jenner in a letter for his discovery and famously predicted “Future generations will know by history only that the loathsome smallpox existed and by you has been extirpated.”25

The dramatic decline in smallpox fatalities in response to Jenner’s vaccine can be traced in the chart, which shows the number of deaths due to smallpox as a share of all deaths in London from 1629 to 1902. Before the introduction of a smallpox vaccine in 1796, on average 7.6% (1-in-13) of all deaths were caused by smallpox. Following introduction of the vaccine, we see a clear decline in smallpox deaths.

Smallpox Eradication Program

It was only with the establishment of the World Health Organization (WHO) in the aftermath of World War II that international quality standards for the production of smallpox vaccines were introduced. This shifted the fight against smallpox from a national to international agenda. It was also the first time that global data collection on the prevalence of smallpox was undertaken.

By 1959, the World Health Assembly, the governing body of the World Health Organization (WHO) had passed a resolution to eradicate smallpox globally. It was not until 1966, however, that the WHO provided the ‘Intensified Smallpox Eradication Program’ with funding to increase efforts for smallpox eradication.

By 1966, the number of infections of smallpox had already substantially been reduced by national governments’ efforts. Nonetheless, skepticism about the feasibility of eradication prevailed and the WHO lacked experience in administering projects that required both technical and material support, as well as coordination across countries. Furthermore, the funding provided to the Intensified Smallpox Eradication Programme was insufficient to meet global needs, resulting mostly in vaccine shortages.

Further still, continued globalization and growth of international air travel resulted in the continual re-introduction of the disease into countries that had previously managed to eliminate smallpox.

Overcoming the last mile problem: ring vaccination

Smallpox’s eradication was greatly spurred by making use of the fact that smallpox transmission occurs via air droplets. Initially, the WHO had pursued a strategy of mass vaccination which attempted to vaccinate as many people as possible, hoping that herd immunity (explained in our vaccine entry) would protect the whole population. Soon, however, vaccination efforts were targeted locally around smallpox cases as smallpox was transmitted by sick patients’ air droplets. This is known as the ring vaccination principle.

People who had been in direct contact with a smallpox patient over the last two weeks were quarantined and vaccinated. The downside of such an approach was that the virus could spread easily if it was re-introduced from overseas. This was the case in Bangladesh, for example, which had previously eliminated smallpox until 1972 when it was brought back from across its border with India.

Despite the risk of re-introductions, ring vaccination greatly reduced the cost of the eradication campaign. The number of administered vaccines dropped and smallpox was increasingly brought under control. Regional elimination came within reach.

One of the last strongholds of the variola virus was India. While 57.7 percent of global reported smallpox cases were reported in India in 1973, this increased to 86.1 percent in 1974. One major push in vaccination campaigns, however, successfully drove down the number of infections to zero in India in 1976.


The Our World in Data entry has more information about smallpox — the impact of eradication/variolation on life expectancyestimated numbers for lives saved by its eradicationsources and more context, etc. 

  1. ^

     Two days have a claim to this title: December 9 and May 8. We’re going with the former here. 

  2. ^

    On this date, we usually feature 500 Million, But Not A Single One More — a beautiful and classic post about the impact of smallpox eradication. But we recently featured it and have gone for a different great piece of content here.

  3. ^

    Written by me with the help of ChatGPT.

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Vaccination using cowpox seems to be the kind of technology that didn't require a lot of "prerequisites" in its way. I wonder how different history would've been if cowpox was discovered much earlier, and cowpox-vaccination became a widespread practice in at least some regions before the 1000s or so. 

Could smallpox eradictation be achieved on a national/regional level in a pre-industrial society? And how much would that change the course of history? 

Post summary (feel free to suggest edits!):
Smallpox was confirmed as eradicated on December 9th, 1979. Our World in Data has a great explorer on its history and how eradication was achieved.

Smallpox killed ~300 million people in the 20th century alone, and is the only human disease to have been completely eradicated. It also led to the first ever vaccine, after Edward Jenner demonstrated that exposure to cowpox - a related but less severe disease - protected against smallpox. In the 19th and 20th centuries, further improvements were made to the vaccine. In 1959, the WHO launched a global program to eradicate smallpox, including efforts to vaccinate (particularly those in contact with infected individuals - ‘ring vaccination’), isolate those infected, and monitor spread. They eventually contained the virus primarily to India (86% of cases were there in 1974), and with a final major vaccination campaign, dropped cases there to zero in 1976.

(If you'd like to see more summaries of top EA and LW forum posts, check out the Weekly Summaries series.)

Recommend the book "Sometimes Brilliant" about Larry Brilliant's life in this context! I read it with so much joy this year.

As mentioned in the article Effective Altruism as "nish kam karma yoga" [Larry Brilliant]

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