- An estimated 4,300 people received less of the Pfizer vaccine than they should have, KTVU reported.
- Too little of the vaccine was administered due to a problem with new syringes, the media outlet said.
- California health officials have said patients will be informed "immediately" if they need a booster.
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An estimated 4,300 people were administered less than the recommended dose while getting a shot at the Oakland Coliseum, two unnamed medical workers told the media outlet.
The optimal vaccine dosage is 0.3-mL of Pfizer but thousands of people received around 0.2 mL, KTVU said.
Due to a problem with the syringes, too little of the Covid-19 vaccine was administered, the media outlet reported.
The mixup took place on Monday morning but was identified and resolved by 2 pm, state officials confirmed to KTVU.
Both agencies that run the mass vaccination site - the California Office of Emergency Services and the Federal Emergency Management Agency - were unaware of the issue until KTVU alerted them of it on Tuesday.
The State Department of Public Health, United States Health and Human Services (HHS), and Pfizer held emergency meetings on Tuesday to discuss the error, a California OES spokesperson told the media outlet.
Officials from the California Department of Health then visited the Oakland Coliseum out of "an abundance of caution," a spokesperson said.
Ali Bay, deputy director of communications for the health department, told KTVU on Friday that nobody who was vaccinated on March 1 "has been harmed or would be harmed if we conclude they received a slightly smaller dose of the vaccine."
Those vaccinated on that date are not being advised to seek out additional medical care but, Bay said, patients will be informed "immediately" should they need an emergency booster.
California's health department is now working with the HHS, CDC, and Pfizer to "ensure that vaccine best practices and quality assurance are adhered to at this site," Bay told KTVU.