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Not Cow, But Horse

An early smallpox vaccine used horsepox to inoculate people against disease, according to a sequencing analysis in the New England Journal of Medicine.

Edward Jenner developed a vaccine against smallpox in 1789 and it's been thought that he used cowpox in the vaccine, Stat News adds. As the story goes, Jenner noticed that milkmaids, who were in close contact with cows, didn't develop smallpox, though they sometimes caught the less severe cowpox. To develop his vaccine, he scraped out pus from lesions on a milkmaid and inoculated a boy with it. That boy, though then exposed to smallpox, didn't become ill.

In the 1930s, researchers realized that the virus being used to protect against smallpox was not cowpox or smallpox itself, but a separate virus, vaccinia virus, which they suspected was a lab virus without a natural counterpart.

Researchers from the Robert Koch Institute in Berlin and their colleagues have now sequenced the contents of a smallpox vaccine that was manufactured in the US in 1902. As they report in NEJM, phylogenetic analysis of that sequence found the vaccine contents to be most similar to horsepox. The researchers also note that some of the deletions present in the 1902 vaccine strain are present in vaccinia virus.

Author Clarissa Damaso from Federal University of Rio de Janeiro tells Stat News that the analysis indicates that horsepox was sometimes used to protect against smallpox.