When Singapore announced the start of quarantine-free travel to certain countries including the U.S., U.K., France and Canada, Singapore Airlines Ltd.’s website crashed and ticket prices shot through the roof.

Expatriates scrambled to book trips home in time for Christmas, while cooped-up locals jumped at the chance to finally get off the tiny island. With domestic virus curbs extended recently after a surge of infections, the well-heeled are ever-more desperate to take flight.

Left behind by this exodus are more than half a million domestic and migrant workers, many of whom haven’t seen their families in years. Thousands live in cramped dormitories — some, according to recent claims, in unsafe and unsanitary conditions.