The Walgreens pharmacy in Sanford gave deactivated doses of the Pfizer COVID-19 vaccine to some members of the public in the past week, the Maine Center for Disease Control and Prevention said Saturday.

The vaccines are usually diluted with saline solution before injection, but health workers in Sanford accidentally used “sterile water,” which renders the dose ineffective. People who received the deactivated vaccine are being contacted to return for an active dose, Walgreens said.

It’s not yet clear how many people received the doses.

Maine CDC spokesman Robert Long confirmed on Saturday that Walgreens reported the error to the Maine Immunization Program. The Maine CDC determined there was no immediate public health risk, he said.

A spokesman for Walgreens said some patients received a COVID vaccine at the Sanford location “that was improperly diluted with sterile water for injection instead of the correct diluent, normal saline, used to prepare Pfizer COVID-19 vaccines. Immediate corrective actions have been taken. We are reaching out to the impacted patients and will administer a COVID-19 vaccination as soon as the these patients are available to return to the pharmacy for a corrected first and/or second dose within the appropriate time frame.”