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Starting with the same type 2 Sabin vaccine virus, they tweaked its genome in several places to make it less likely to revert.

If approved, the single-dose Sabin vaccine would most likely be the first in line for trial.

The Sabin vaccine was created in the 1940s and ’50s by passaging the virus through animal cells until scientists found a suitably weakened form.

When the Salk vaccine became available in 1955 — and again in the 1960s when it was replaced by the Sabin vaccine — Americans lined up in droves to receive it.

In time, the Sabin vaccine became the predominant form of polio immunization, and the Salk vaccine was largely abandoned.