After returning to New York, Nick Fury quickly arranged for Steve Rogers to be placed in the S.H.I.E.L.D. New York Branch hospital, using the most advanced thawing equipment to awaken this hero from over seventy years ago.

After that, Natasha had nothing more to do with it.

However, with Natasha's clearance, borrowing Steve's shield wasn't an issue.

Yang Han found that replicating the properties of this weapon took significantly less time than replicating genes; it took less than two hours to copy the properties of Vibranium.