United Airlines Holdings (NASDAQ:UAL) said Wednesday that it will require all passengers to complete a health self-assessment prior to boarding a flight, part of the airline's effort to halt the spread of COVID-19 and make travelers feel more confident it is safe to fly.

United and other airlines have been hit hard by the pandemic, with Delta Air Lines, for example, warning earlier in the day that it expects second quarter revenue to be down 90% year over year. The airlines have boosted their cash reserves to ride out the pandemic, but need travel demand, and revenue, to return in the months to come to avoid a liquidity crisis.

A United Airlines plane exits the hanger at night.

Image source: United Airlines.

United's "Ready-to-Fly" checklist asks all travelers if they have been diagnosed with COVID-19 in the last 21 days and asks if they have experienced any one of a number of symptoms, including fever, cough, shortness of breath, or sore throat in the past 14 days.

The questionnaire, which United developed in consultation with the Cleveland Clinic, also asks if a passenger has been denied boarding by another airline in the last 14 days or has had close contact with someone who tested positive for COVID-19 in the last 14 days.

"The health and safety of our customers and employees is our highest priority, and we have been working closely with trusted medical experts and partners to institute new practices and procedures to further protect those who work and travel with us," Pat Baylis, United's corporate medical director, said in a statement. "United's 'Ready-to-Fly' wellness checklist sets clear guidelines on health requirements for our customers and helps minimize the risk of exposure during the travel experience."

United travelers will be required to review the checklist at check-in for their flight via the airline's app, its website, or at an airport kiosk or when meeting a gate agent. The airline is also requiring that passengers wear a face covering while on board their flight.