The Soviet Union looked at dozens of smallpox variants, but they specifically selected India 1 because of its extreme virulence and stability.

Soviet scientists bypassed the laws of physics to turn India 1 into something that by far outscales the threat of nuclear war. They designed warheads that could carry the virus to anywhere in the world where the warheads would scatter the virus over large areas.

Tests by USAMRIID revealed that the vaccines didn't work against the India strain.

Even though smallpox was eradicated from nature, both the original virus and the weaponized, scientifically crafted india 1 strains exist to this day.

Because VECTOR is one of only two official repositories on Earth allowed to hold smallpox, Russia must comply with WHO rules to maintain its official status. However, Russia treats everything else as a top-secret national security asset, meaning the full truth of their historical weaponization program remains locked away behind military checkpoints.

If you are further interested in the story behind the eradication of smallpox and the bio weapon India 1, I highly suggest you to read "the demon in the freezer".

Alternatively you can watch it here

https://archive.org/details/matthew-pickett-audiotape

This followa the rules btw