system
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human
Bifurcated (2) needles for vaccination?

Why was this type of needle used for small pox?  Have 2 needles ever been used on other vaccinations? Thank you.
gpt
The bifurcated needle was used because it was one of the rare examples of something being: fast, cheap, and good.

The small pox vaccine needed to be delivered just under the skin.  Previously a jet injector had been used.  These basically fire a jet of vaccine through the skin with compressed gas.  Whilst quick they’re expensive and unreliable.  The bifurcated needle allowed the vaccine to just be poked under the skin, a dose could be delivered simply with some rapidity.  The needles were cheap to produce.

As far as I’m aware the smallpox vaccine is the only medicine to be delivered in this way.  Although the technology is still good, and we may see it used again in the future.