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A 115-year-old vaccine vial has provided an important clue in the search for an answer to one of medicine’s enduring mysteries: What went into the world’s first vaccine?

Medical legend has it that Edward Jenner — the father of vaccination — used cowpox virus to protect against the dreaded smallpox.


But a new report, published Wednesday, shows a virus closely related to the horsepox virus was used in a 1902 smallpox vaccine, providing fresh ammunition to those who believe the history books have it wrong.

“People ask me if we have found the missing link. I don’t know if there is one missing link,” said Dr. José Esparza, one of the authors and a collector who discovered the vial in a box of old vaccine containers among his vaccine memorabilia.

Esparza and others have been fascinated by the mystery related to the source of the first-ever vaccine — tested in 1796 — as well as the source of the virus used in modern smallpox vaccines. The current vaccine virus is called vaccinia and, oddly, no one knows from whence it came.


Historians know a great deal about the origins of the first vaccine in general.

Jenner, a physician and scientist, noticed that milkmaids generally didn’t develop smallpox, a disfiguring and sometimes deadly disease. He guessed it was because they sometimes caught cowpox, a related disease that only caused mild illness in people.

To test his theory, Jenner scraped pus from what were thought to be cowpox lesions on the hands of a milkmaid into the arm of an 8-year-old boy, and later exposed the boy to smallpox pus. Jenner was right; the boy was protected.

For decades the practice of using scabs or pus from lesions on cows or on recently vaccinated people was how smallpox vaccination took place.

But it was never clear if Jenner’s original source material was cowpox. He himself suggested it might have been horsepox, which can infect cows.

With the evolution of science and the advanced tools now used to conduct it, it has become clear that vaccinia — the virus used in modern smallpox vaccines — is neither cowpox nor horsepox. Whether it is a virus that formerly infected some species of animals — rodents, maybe — or is something that evolved in laboratories through the deliberate mingling of pox viruses isn’t clear.

The new report, published in the New England Journal of Medicine, suggests that horsepox viruses were used to vaccinate against smallpox at some point and in fact may have been the original tool used to protect against the dreaded disease.

A semi-retired virologist who formerly worked for the World Health Organization and the Bill and Melinda Gates Foundation, Esparza realized that he had an old smallpox vaccine vial a couple of years ago. It was produced in 1902 by a Philadelphia company, H.K. Mulford.

The original wooden and glass containers that held the capillaries with the Mulford 1902 smallpox vaccine. The New England Journal of Medicine

He asked scientists at the Robert Koch Institute in Berlin if they could extract enough genetic material from the vial to figure out what vaccine virus Mulford used in its vaccine.

That they could speaks to the miracles of modern science. “This is material that has been at room temperature for more than 100 years,” Esparza noted.

The German scientists then compared the genetic sequence they retrieved from the Mulford vial to that of other known pox viruses. The closest hit was to horsepox; the sequences were 99.7 percent similar.

Clarissa Damaso is also an author of this paper. Damaso, an associate professor of virology and molecular biology at the Federal University of Rio de Janeiro in Brazil, has studied the history of smallpox vaccination, and in August she published a detailed review of how the vaccine was shared across the world in the 18th century.

(Vaccinated orphans and prisoners were shipped around the world to bring vaccine — in the form of pus and scabs — to the Americas and beyond. Eventually cows became the preferred vehicle; it was easier to sustain material by serially vaccinating a herd. “When you have a cow, you don’t ask the cow,” Damaso said. “You don’t need permission.”)

Damaso said this new work shows that horsepox was at least sometimes used as the source material for smallpox vaccine.

David Evans agreed. Evans, who is not involved in this work, is a University of Alberta virologist who made headlines earlier this year when it was revealed his lab had recreated horsepox — which hasn’t been seen for decades and may be extinct.

“The evidence is pretty strong that whatever virus was adapted by the [smallpox] vaccinators to become vaccinia shares a lot of resemblance to the one example of a horsepox. So it would be not unreasonable to guess that it” — Jenner’s vaccine — “too was a horsepox virus,” Evans said.

Figuring out the origins of the Jenner’s vaccine and tracing how the source virus evolved to become vaccinia won’t change modern science, scientists involved in the detective work admit.

After all, smallpox was declared eradicated decades ago; the last recorded infection occurred in 1977.

But Damaso and Esparza are keen to continue the pursuit; Esparza said the team is looking for other old smallpox vaccine vials to test.

“It is the nature of humans to solve mysteries,” he said.

  • The early history of vaccination is very interesting. The word “vaccine” is derived from the Latin word for cow, “vacca”. Vaccination pre-dates the germ theory, (Pasteur, 1860’s), and antiseptic use (Lister, 1865). There are woodcuts (as vaccination also pre-dates photography, 1840 or so), of early vaccine production where calves are restrained in open air stalls with no screens to keep out flies or other insects. The calves’ abdomens are shaved, a crosshatch pattern is cut into the hide and material from either human or animal pustules was introduced. After festering, the material was collected and strained through what is basically cheese cloth and declared “purified calf lymph”. Administration was performed sans any sanitary practices. The instruments for puncturing the skin in preparation for applying the paste were very crude and would be impossible to sterilize even today in a modern autoclave, due to the porous construction materials and design. Contemporary guides describe a simply wipe down of the tool with a handkerchief. When one reads the historical summary in the Lancet article linked above- and the incredible practice of ‘humanized’ , arm to arm transfer of infection, where the pus from an active sore on the donor is directly pressed into an intentional cut on the receiver’s arm, along with whatever other advantageous material was piggy backing along- it is remarkable that the practice survived long enough to be modernized.
    The final genotyping of the virus is a fascinating issue- for both its origin, and its ability for cross-protection when compared to other vaccine viruses, when slight variations in the strains can significantly impact efficacy.

  • I’ve been interested in this mystery for years. It is indeed one of the great mysteries of science. Such an important invention, but we don’t know where it came from. Hypotheses I have heard is that it maybe a hybridization of pox viruses if that’s even possible, that it may have gone extinct in the wild which seems unlikely, or that there is some animal out there which harbors wild Vaccinia that has not been discovered yet. Maybe there’s a pox virus among badgers or hedgehogs or something that hasn’t been found.

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