United Airlines said it would try to make the middle seat empty. This photo shows an almost full flight

(CNN) – After weeks of working in New York City hospitals on the front lines of the coronavirus pandemic, it was time for Dr. Ethan Weiss to go home.

But the University of California cardiologist San Francisco was shocked by what he saw when he boarded a United Airlines flight from New York to San Francisco on Saturday. Almost every seat is filled.

“This is the last time I will fly again for a very long time,” he wrote on Twitter thread over the weekend, adding that many passengers on the flight were “scared / shocked.”

Weiss said he was part of a group of 25 doctors and nurses who had worked in New York City hospitals for the past two to four weeks, and that United had flown them for free.

CNN had contacted Weiss to comment but had not heard from.

“I still have to be tested but this is crazy. 6 hours like this,” he tweeted to someone who commented on his thread.

A United spokesman declined to comment on how many people were on Weiss’s flight, saying that it “does have a higher than average load factor but still departs with empty seats.”

“Although we cannot guarantee that all customers will sit next to unoccupied seats, based on historically low travel requests and the implementation of various social distance measures that are possible outcomes,” the airline said in a statement to CNN.

The flight had “25 additional medical professionals flying free” and all passengers and employees were asked to wear face masks, the airline said in a statement.

The spokesman added that most airline flights operate at less than half the capacity and that passengers can reorder if they feel their safety is in danger.

On Sunday, Transportation Security Administration agents inspected 200,815 passengers – 8% of last year’s total, according to TSA.

That was up from 128,875 passengers on Sunday, April 26.

Although the airlines saw a significant reduction in the number of passengers, United and others also cut their schedules, potentially leading to more full flights than some passengers had expected.

Gregory Wallace, Pete Muntean and Alta Spells from CNN contributed to this report.