A long line was seen outside the convention center, where those who drive to the location are required to park their vehicle and enter the building - unlike at drive-up vaccination sites like the one at Dodger Stadium.
The number of people that the Ontario clinic can accommodate continued to fluctuate. As late as Wednesday evening, only 200 appointments were available. Hours before the clinic opened, another 100 slots became available and were quickly reserved.
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Residents were urged to continually check the San Bernardino County website for updates on registration.
The vaccination clinic at the convention center marks the fourth of its kind in the county and the second one in Ontario. The others are located in Hesperia and San Bernardino.
Some have asked why availability of appointments at the location seemed to be announced only at the last minute. Officials pointed to the amount of vaccine doses that the county has been receiving from the state, noting that the number change from week to week.
One week in December, the county received almost 25,000 doses. Two weeks ago, only 15,000 doses were delivered.
California officials said San Bernardino County should expect nearly 22,000 doses each of the next two weeks.