For example, U.S. Global Jets ETF (JETS) has more than doubled since March 2020 low but remains well off its 2019 highs. There are new indications from travel-data firm Cirium that show the recovery in airlines could be reversing, this could be bad news for traders who have piled into JETS. 

Bloomberg, citing a new Cirium report, said the number of single-aisle aircraft in service has plunged since the beginning of the year amid a flare-up in COVID-19 cases across the country and fresh restrictions. 

Cirium's data showed single-aisle aircraft in service fell below the 8.8k mark on Feb. 5, a drop of 15% compared with Jan. 3 figures. 

"Wide-body usage slumped 14% from a Dec. 19 peak, though bigger aircraft remain comparatively far less popular, with just half the number in service compared with pre-pandemic levels as long-haul travel remains largely out of bounds," Bloomberg said, quoting the report.