The carrier said it would need time to determine how unvaccinated employees "might interact with our customers and their vaccinated co-workers."
The company says the bulk of its workers have already received their shots.
The airline said the plane will reduce flights between London and the New York area to just 3 1/2 hours.
The hospitality union Unite Here says the airline is trying to avoid a union contract after receiving billions in rescue money from the government.
The airline said safety remained its “highest priority” and that it was working closely with federal investigators.
The flight attendants union leader urged United to drop the probe because the GOP senator is a "public servant who has lied too many times."
United and the CDC say they're working to contact passengers on a recent Orlando-to-Los Angeles flight following the death of a man who'd been on board.
The airline lifted its capacity from 85% to 100% as of July 1 and also got criticized by CDC director Dr. Robert Redfield and Dr. Anthony Fauci earlier this week.
Senate Democrats to require airlines to give full cash refunds to passengers during the pandemic, even if it was the customer who canceled.
“We have weathered many storms since our first flight 36 years ago, but none has been as devastating as Covid-19," Virgin's CEO said.