Mexican President Andres Manuel Lopez Obrador first felt the onset of COVID-19 on Sunday and was tested after returning to the capital on a commercial flight from an event in central Mexico, his spokesperson said on Tuesday.

The 67-year old President said on Sunday evening he was infected with the coronavirus, just hours after taking the Aeromexico flight, raising concern about the extent of potential exposures.

Spokesperson Jesus Ramirez said that passengers on the flight were being contacted and that journalists travelling with the President were recommended to isolate.

Lopez Obrador had a fever on Sunday and was still experiencing some mild symptoms by Tuesday, including a minor headache, Deputy Health Minister Hugo Lopez-Gatell said in an evening news conference.

“The President is doing well… he does not have other symptoms,” Lopez-Gatell said, adding that Lopez Obrador was in good spirits.

The pandemic is worsening in Mexico. Hospitals in the capital are close to saturation, and residents are scrambling to find supplies of medical oxygen. With more than 152 000 deaths officially registered, it has the fourth-highest toll worldwide and fatalities are expected to overtake India within days.

People who met with Lopez Obrador’s entourage during a three-day tour at the weekend included corporate bosses and his former chief-of-staff, Alfonso Romo.

Lopez Obrador had felt chills at the end of the event in the central city of San Luis Potosi, but believed it was no more than a cold, Ramirez told Reuters. The symptoms were stronger once he was back in Mexico City, Ramirez said.

Ramirez told a national radio station earlier in the week that Lopez Obrador felt congested on Saturday while in the industrial city of Monterrey, and took a test there. However, he told Reuters on Tuesday that information was incorrect.

Mexico has a tiered system that avoids mandatory lockdowns. Authorities in Mexico City, which is in the strictest red tier, say people should not travel or leave home unless necessary but does not prohibit such activities.

Mexican health officials said they had identified 11 people by Tuesday who may have come into close contact with the president and that local officials from the areas visited by Lopez Obrador at the weekend were also working to identify other contacts.

Several top officials, including Deputy Health Minister Hugo Lopez-Gatell, the public face of Mexico’s coronavirus strategy, said they were self-isolating and had tested negative. Others, including Foreign Minister Marcelo Ebrard, also announced they had tested negative for the virus but are expected to take new tests this week.

Lopez Obrador was still working, Interior Minister Olga Sanchez said on Tuesday.

“I can tell you he’s strong and is even intervening in some of the most important decisions,” she said while subbing for Lopez Obrador at a regular government news conference.

His typically active Twitter account was quiet, sharing only a link to the morning news conference by the afternoon.

Lopez Obrador has been holding virtual meetings, including a Monday call with Russian President Vladimir Putin.

In the event that Lopez Obrador, who has a history of high blood pressure and heart problems, cannot carry out his duties while in office, the Mexican constitution states that Sanchez would assume the presidency in his absence.

Congress would then vote on an interim successor within sixty days who would remain in office until the end of Lopez Obrador’s term in 2024.