Fast food places don't assemble cold ingredients with hot ingredients until ordered. It's a big part of the reason why burgers with lettuce tend to cost more at those places.

Taco Bell still has lettuce in their stores. They only pulled certain lettuce from certain sources and warned customers that not every store might have lettuce available for a few days, but any shortage didn't last long and they have been able to serve food with lettuce in most places the entire time.

If you're that concerned about this outbreak, you should probably catch up on the information of how these things work.