Passengers board an American Airlines flight at Ronald Reagan Washington National Airport in Arlington, Virginia on April 11, 2021.

Andrew Caballero-Reynolds | AFP | Getty Images

The demand for travel has risen sharply since spring. Delta and American both saw positive outlook thanks to a jump booking last week. The Transportation Security Administration screened nearly 2.23 million people at U.S. airports on Sunday, most since February 28, 2020, weeks before the World Health Organization declared Covid a pandemic.

But concerns about the rapidly spreading Delta variant of Covid, now the dominant variety in the US, weighed on the market across the board on Monday. Travel stocks, which are sensitive to the number of cases and related restrictions, fell more sharply than other sectors.

The trend also raises questions about international travel. International and business travel were largely missed in the recent rebound in airline bookings, although executives said earlier this month that they have started to rebound. The United States still bans most non-US citizens from entering the European Union, the United Kingdom, India, and other nations, despite the travel industry’s repeated pushing for the Biden government to lift some of these rules.

The US Centers for Disease Control and Prevention Monday urged individuals to “avoid traveling to the UK” and raised their recommendation to “Level 4,” the agency’s highest. It was said that if people have to travel there they should be fully vaccinated.

Some pandemic rules are returning due to the increase in cases. The Los Angeles District reintroduced a mask mandate for indoor use last week, including for people who have been vaccinated, as the number of Covid-19 cases has increased there. The Southern Nevada Health District, which includes Las Vegas, also urges everyone to wear masks indoors as cases increase across the state.

According to a CNBC analysis of the data compiled by Johns Hopkins University, cases in the United States rose about 66% over the past week to a seven-day average of about 32,300 new cases per day.