U.S. Open spectators now must show proof of at least one dose of a COVID-19 vaccine to attend matches, a change made less than 72 hours before the tennis tournament starts.

The U.S. Tennis Association announced Friday that the New York City mayor’s office decided to require proof of vaccination to go into Arthur Ashe Stadium, the main arena at the National Tennis Center.

The USTA then opted to extend that rule to cover all ticket-holders who are 12 and older and enter the grounds during the two-week Grand Slam tournament that begins Monday. The event is returning to 100% capacity after all fans were banned from attending a year ago because of the coronavirus pandemic.

A CDC vaccination card — or a photo or photocopy — are among the acceptable ways of proving vaccination.

Earlier in the week, the USTA said spectators would not be required to wear masks or show proof of their vaccination status to attend matches at the U.S. Open.

“The goal is not to prevent all cases of COVID. The goal, really, is to be certain that we don’t have an outbreak of COVID that’s going to be unusual or that we would regret,” Dr. Brian Hainline, a USTA first vice president and member of its medical advisory group, said on a conference call with reporters on Wednesday.

Fueled by the highly contagious delta variant of the virus, new reported cases of COVID-19 in the U.S. have topped 150,000 a day, the highest level since late January.