RIVERSIDE, Calif. (CNS) — Staffers with the Riverside University Health System are working to contact more than 300 people who may have received doses of the Prfizer COVID-19 vaccine that were improperly stored and possibly had reduced or lost potency, according to the Palm Springs Desert Sun.

The agency said Friday 324 Riverside County residents received Pfizer vaccine doses at two community health center sites that were stored in a freezer longer than the manufacturer recommends, the Desert Sun reported.

The sites were at community health center sites in Jurupa Valley and Riverside, the newspaper said.

The vaccines were administered between Oct. 8 and Nov. 23 at the Jurupa Valley site, 8876 Mission Blvd., and between Oct. 23 and Nov. 23 at Riverside Neighborhood, 7140 Indiana Ave.

Staff members discovered the error on Tuesday.

RUHS said the doses do not pose a danger but may be less effective.

The Centers for Disease Control and Prevention recommends that patients who receive incorrectly stored vaccines get a replacement dose as soon as possible.

People who received a vaccine at the Jurupa Valley or Riverside sites can also call 800-945-6171 between 8:30 a.m. and 5 p.m., Monday through Friday.