A year after having no fans at the US Open, organizers announced earlier in 2021 that 100 percent capacity would be achieved come August.

That’s not changed in the lead-up to the New York major, but one significant entry requirement has. Previously informed that masks would not be required to attend, fans 12 and older must now show proof of vaccination status in order to access the USTA Billie Jean King National Tennis Center in a switch that comes three days before the main draw begins.

“Today, the USTA was informed that the New York City Mayor’s office will be mandating proof of COVID-19 vaccination for entrance to Arthur Ashe Stadium during the US Open. Given the continuing evolution of the Delta variant and in keeping with our intention to put the health and safety of our fans first, the USTA will extend the Mayor’s requirement to all US Open ticket holders 12-years-old and older,” a statement released Friday read.

What does that mean? Anyone attending, from Arthur Ashe Stadium seat buyers to grounds pass holders, must show they’ve been inoculated with at least one dose of a COVID-19 vaccine. A CDC vaccination card—photos and photocopies included—are among the acceptable ways of meeting this requirement.

“As the USTA developed its plans for the 2021 US Open, we have consistently followed the guidelines established by the CDC, the New York City Mayor’s Office, and the New York City Department of Health. The Mayor’s recent introduction of the Key to NYC Pass led to additional discussions between the USTA and NYC.”

First-round action begins Monday. Naomi Osaka and Dominic Thiem are the defending singles champions, though the Austrian is missing in action after ending his season last week to recover from a wrist injury.