• Among 140 infected students who were interviewed, 89 said they recently traveled outside of Chicago during spring break, the CDC said.
• The agency said 57 of them attended an unmasked indoor social gathering, such as a dinner or party.
• Destinations included seven different countries and 23 U.S. states, most commonly California, Colorado, Florida and New York, the CDC said.

New Covid-19 cases increased rapidly at the University of Chicago this year after students, most of whom were not fully vaccinated, returned from spring break, according to a study published Thursday by the Centers for Disease Control and Prevention.

Despite several prevention strategies at the university, such as weekly testing for students living on campus, 158 Covid cases were diagnosed among undergraduate students between March 15 and May 3, according to the new study, published in the agency's Morbidity and Mortality Weekly Report.

Among 140 infected students who were interviewed, 89 said they recently traveled outside of Chicago during spring break, and 57 attended an unmasked indoor social gathering, such as a dinner or party, the agency said.

Destinations included seven different countries and 23 U.S. states, most commonly California, Colorado, Florida and New York, the CDC said. The most common reason for travel was vacation, according to the agency.

In response to the cluster of cases, the university implemented a stay-at-home order for all students living on campus, prohibited gatherings and shifted to remote learning, the agency said. The stay-at-home order was lifted two weeks later after additional testing found few new cases.

Notably, the outbreak occurred immediately before eligibility for vaccination expanded in Chicago. Undergraduate-aged people in the city were largely ineligible for the shots before April 19, when local health officials moved to "phase 2" of their vaccination program, according to the CDC. Among the students who were infected, only three were fully vaccinated, according to the agency.

The CDC said the campus outbreak illustrates the importance of masking indoors and avoiding travel, particularly for those who are unvaccinated, amid the spread of the fast-moving delta variant.

"These results demonstrate the potential for COVID-19 outbreaks on university campuses after widespread student travel during breaks, at the beginning of new school terms, and when students participate in indoor social gatherings," the agency wrote in the study.

The new report comes a day before the start of Labor Day weekend, when many families across the U.S. travel and more schools prepare to begin their fall semester.

The CDC's director, Dr. Rochelle Walensky, this week advised unvaccinated people against traveling over the holiday as the nation continues to battle back a surge in Covid cases and hospitalizations fueled by the highly contagious delta variant.

While the daily average of hospital admissions has fallen in recent days, U.S. officials and health experts say Labor Day could be a threat to the nation's progress going forward.

"Labor Day weekend does pose a major risk as people swarm back to work and to school," said Lawrence Gostin, director of the World Health Organization's Collaborating Center on National and Global Health Law.

Among the students interviewed for the CDC study, 93 were unvaccinated and 43 were partially vaccinated, meaning they had only received one dose of either Pfizer's or Moderna's two-shot vaccine.

The agency noted the study had limitations, including some students who refused to be interviewed and others who provided potentially false information, such as denying travel when other students indicated that they had.