Saw this article discussed on Rick Steves forum. Makes me glad we have trips scheduled for late summer/fall and that we had Tauck air reserve flights for us. If the dam breaks and Europe starts allowing American tourists in, getting a flight could be a food fight. One thing the Tauck agent told me when I booked ours was that if we saw better connections they could change ours if prior to final payment.
https://www.bloomberg.com/news/articles/2021-03-10/how-to-think-about-booking-europe-summer-travel-2021-during-covid