The Austin, Texas Public Health Department is investigating a cluster of positive COVID-19 cases involving a large group returning from Cabo San Lucas, Mexico.

According to a news release, 70 University of Texas student chartered a plane for a spring break trip. Some of the group returned on separate commercial flights. Currently, 28 students from the trip have tested positive for COVID-19 and dozens more are under public health investigation. Four of the confirmed cases did not present any symptoms, the release says.

Austin Public Health and UT Health Austin and University Health Services have made contact with every spring breaker onboard the plane using flight manifests from the Center for Disease Control and Prevention (CDC). The 28 confirmed cases are self-isolating. Others are under quarantine while being monitored and tested. The Department of State Health Services has been notified.

While Mexico at the time of their travel was not under a federal travel advisory, the release notes, Austin-Travis County residents should follow CDC’s travel recommendations indicating travelers avoid all non-essential international travel. A leisure vacation of any kind is not considered essential.