Hundreds of patients with a Northern California health care provider may have received an incorrect COVID-19 vaccine dose, the provider announced this week.

Those who received Pfizer COVID-19 vaccinations at the Kaiser Permanente Walnut Creek Medical Center between Oct. 25 and Dec. 10 last year — roughly 3,900 people — “may have received a slightly less than recommended dose of vaccine.”

In a statement to Nexstar, Kaiser Permanente said affected patients may have received between 0.01 to 0.04 mL less than the recommended 0.30-mL dose. Officials say that once they became aware of the issue, they consulted with infectious diseases and vaccine science experts as well guidelines from the Centers for Disease Control and Prevention.

“All experts agreed the difference between the recommended dose and the dose an individual may have received was not significant and not likely to reduce their protection against COVID-19,” the statement reads. “Nevertheless, Kaiser Permanente is offering to provide a repeat dose of Pfizer vaccine to any affected individual who wishes to receive it, offering special hours and locations.”

The issue was caused by “some staff misunderstanding instructions” and drawing slightly less than the recommended dose. Kaiser Permanente says staff has been retrained and the provider is “continuously monitoring so this does not happen again.”

This isn’t the first case of incorrect doses being given.

In November, over a dozen kids were given the wrong COVID-19 vaccine dosage at another Northern California vaccine clinic. The health care provider would not say whether the doses were larger or smaller than recommended.

Around the same time, some children in Virginia were also given the incorrect vaccine dosage. According to the Virginia Department of Health, kids who received their vaccine at a specific pharmacy over a two-day period may have received a smaller-than-recommended dosage.