More than 11,000 first-dose vaccine appointments at the Pennsylvania Convention Center this week are being voided, city health officials said, due to people again improperly using the digital scheduling system.

The city is still reviewing appointments but said it might be necessary to cancel as many as 30,000 appointments over the next three weeks.

The site began administering second doses this week, but people have been sharing registration links and a QR code given to people who got their first shot at the site, allowing thousands to make appointments for first doses.

“If you haven’t gotten your first dose,” said James Garrow, a health department spokesperson, “please don’t show up, because you’ll be turned away.”

Health department personnel first noticed the slew of first-dose appointments when there should be none on Monday, Garrow said. As first reported by WHYY, staff discovered a QR code designed to allow people who had received a first dose to schedule an appointment was being shared and used to make first-dose appointments. Those using the code first had to strip it of personal information to allow it to be used by others.

The city in response is canceling all first-dose appointments through the three weeks the Convention Center site is doing only second doses.

Starting April 11, the FEMA-run clinic will shift again to offering first doses for two more weeks but will carry only the Johnson & Johnson vaccine brand, which does not require a second dose.

A change of approach this week at the mass clinic has made it more equitable, though, health officials said. Six days of allowing people from some of the city’s most under-vaccinated neighborhoods to get vaccinated without appointments resulted in a more diverse swath of people receiving doses at a location that had overwhelmingly been used by the city’s white population.

Since allowing walk-ups, 18% of the people vaccinated at the FEMA-run clinic have been Black, city Health Commissioner Thomas Farley said, up from almost 13% in the first two weeks the clinic operated. White vaccine recipients at the Convention Center dropped from 58% to 37% in the last week, Farley said. Hispanic recipients rose from less than 11% to 15%, and Asian vaccine recipients rose from about 17% to 20%.

The rates of people of color being vaccinated are even higher among those who were vaccinated without an appointment, Farley said, a system that also doesn’t require a person to register online.

Through the first two weeks of the FEMA-run clinic, all appointments were made through a digital-registration portal. Public health experts have criticized digital scheduling systems as disadvantaging people who are less digitally savvy or are less likely to have internet access. Links that allowed people to register for appointments were also widely shared with people not eligible for vaccination. The result was a clinic that ended up serving a disproportionate number of white people.

The FEMA site is in its fourth week of an eight-week run, and is anticipated to fully vaccinate more than 200,000 by its conclusion. The city is in discussions to extend FEMA’s presence at the site but has not reached a resolution.

This week’s cancellations mark the second time improper sharing of a digital registration system led to a massive number of improper appointments. When the FEMA site opened at the beginning of March, unrestricted registration links were shared with thousands, allowing people not eligible to be vaccinated in Philadelphia to make appointments. The city tried to filter out those appointments made by people who live outside the city, but it admitted it would likely not be able to identify all the noneligible city residents who got slots.

» READ MORE: Philadelphia COVID-19 vaccine registration can finally keep out line jumpers, officials say

At some hospital-run vaccination clinics, the shot recipient is simply given a date and time to return for a second shot, and scheduling for the second dose is conducted entirely by personnel at the clinic. The FEMA site, though, offered people a card with a recommended day to return but also allowed them to scan a QR code that permitted them to schedule their own time, and even change the day if necessary, Garrow said.

The intention, he said, was to give people more flexibility. As has happened throughout the country, people quickly found a way to use the link to help friends and loved ones get access to the scarce vaccine, whether they are eligible or not.

“If you have a million people banging on a system, they’re going to find a way around it,” Garrow said.

People eligible for vaccination who had a first-dose appointment canceled should preregister with the city and they should be contacted to make another appointment.

The problems stemming from digital vaccine registration prompted the city to begin offering walk-up vaccinations, as well as appointments without registration to ensure doses don’t go unused.

“We definitely are starting to see the benefits of these walk-up clinics, and we’ll be exploring that further,” Garrow said.