Delta Air Lines expects to add about 1,000 flights in August but not many more for the remainder of the year, Chief Executive Ed Bastian said in a memo on Thursday that mentioned a resurgence of COVID-19 in parts of the country.

Airlines including Delta have added back some summer capacity after modest improvements in domestic-travel demand in May and June, but analysts said a spike in cases in some states could dampen the recovery.

Bastian said Delta continues to see slow-but-steady increase in passenger volumes two weeks into the summer-travel season, but warned that the airline expects overall demand this summer to be only 25% of last summer’s revenue.

“While it’s encouraging to see flights returning ... we likely remain at least two years away from a return to normal,” Bastian said.

To prevent the spread of the novel coronavirus on airplanes, U.S. airlines have recently cracked down on passengers who do not follow requirements to wear face coverings. Bastian said Delta has already banned some passengers from future travel on Delta for refusing to wear masks on board.

Delta on Thursday restarted service between Seattle and Shanghai via Seoul. It will fly that route twice a week. Next month, it will start a weekly flight between Detroit and Shanghai, also via Seoul.

The airline, which is the dominant carrier at Minneapolis-St. Paul International Airport, hasn’t indicated when it will restart trips to Asia and Europe from the Twin Cities.

Last week, the company began daily testing of employees at MSP for COVID-19. Bastian announced at Delta’s shareholder meeting, held last week, that about 500 of the airline’s 90,000 employees globally had contracted the illness, and that 10 had died from it.