American Airlines is the first U.S. global network carrier to warn that rising COVID-19 case numbers are contributing to a softening of demand, but the company has not refined its capacity or financial guidance for the current quarter.

Delta Air Lines’ decision to up its Airbus A321neo order book by 30 aircraft reflects a larger trend among U.S. carriers that are working to ensure they have optimal fleet flexibility to drive cost and efficiency advantages as the recovery from the COVID-19 pandemic could turn more uncertain.

Porter Airlines is the latest North American carrier to mandate vaccines or testing for its employees, joining United Airlines, Frontier Airlines and Hawaiian Airlines, which have already issued vaccine requirements.