Question

I was browsing Wikipedia and learned that vaccinia virus, the basis of the smallpox (variola virus) vaccine, was originally thought to be derived from cowpox but was later discovered to be a separate species in its own right. Genome sequencing shows that it is most closely related to the distinct horsepox virus†.

My question is whether vaccinia virus was already a separate species when it was first taken and propagated for smallpox vaccines or if was originally a strain of horsepox (or some other unknown pox virus) that only became a separate species after passaging over many years.

† I have consulted the article in Wikipedia and the ICTV 9th Report'... for background information to this question, but they do not provide a definitive answer.

Answer 1

The vaccinia virus used in smallpox vaccines was likely a distinct species similar to horsepox when Edward Jenner developed the vaccine, though its exact origin is unclear. The process called 'passaging' over time may have further evolved the virus into its current form, which is well-suited for vaccination.

Step-by-step explanation:

Origins of Vaccinia Virus in Smallpox Vaccination

The vaccinia virus, which is used for smallpox vaccination, was once considered to be derived directly from cowpox virus. However, it is now understood to be a separate species, closely related to horsepox virus. When Edward Jenner first developed the vaccine, it's likely that what he called cowpox was a virus already distinct from both cowpox and horsepox, though possibly more closely related to the latter. The exact origin of the vaccinia virus used in vaccines is not clearly documented, and it may have been derived from a now-extinct virus. Some theories suggest it could be a laboratory adaptation or hybridization of several poxviruses including cowpox and horsepox.

The process of passaging, which involves repeatedly cultivating the virus in living cells or organisms, may have led to the evolution of the vaccinia virus into its current form. This means that while the vaccinia virus was already likely distinct when first used by Jenner, its characteristics and classification could have been shaped by the history of its cultivation and use in vaccine production.

It's crucial to remember that speciation in viruses, which are often quite prone to mutation and recombination, can be a complex and sometimes rapid process. Thus, the passage of time and the conditions of vaccine production may have contributed to the divergence between vaccinia, cowpox, and horsepox, solidifying vaccinia as a unique entity well-suited for vaccine use.