SAN MATEO — More than 1,500 pediatric patients will have to repeat their vaccinations after a faulty refrigerator compromised 10 different vaccines stored at a medical office affiliated with Stanford Children’s Health, officials said Friday.

Dr. Mark Showen, the lead physician who oversees five other pediatricians at the Peninsula Pediatric Medical Group in San Mateo, said the safest thing to do is to offer new vaccinations — including shots every four weeks for some — to everybody who could possibly be affected, from babies to 18 year olds.

He said letters are being mailed Saturday to the patients’ families to help them arrange an appointment, and if a patient or their family has not responded by a certain date, they will be re-contacted.

The vaccines will be free of charge, and the cost will not be billed to insurers.

“Cost is not the issue — we want to make sure everyone is properly vaccinated,” said Showen. And so far, he said, there is no evidence patients were harmed.

“It’s inconvenient and unfortunate to have to have the extra doses of vaccines, but we would rather be on the side of safety,” said Showen, 60, adding he has not run into this problem in his 30 years of practice.

Showen said the discovery of the faulty refrigerator was reported around noon on Sept. 21, after an employee noticed below-freezing temperatures in one of two refrigerators where the office’s vaccines are kept.

“I was horrified,” Showen recalled upon learning the news. He said a different employee assigned to check the refrigerator’s temperature apparently had failed to consistently do so as far back as Jan. 14, when the new refrigerator was installed. Over the past nine months, the temperature vacillated from freezing to normal, he said.

Robert Dicks, a Stanford Children’s Health spokesman, said it has taken almost two weeks to determine which vaccines and how many patients were affected.

“We have finished the investigation, and letters are going out tomorrow to all those affected,” said Dicks, adding that the letters had already been addressed for mailing before a worried mother contacted the Mercury News on Friday about the issue.

While 10 vaccines, including those for whooping cough and polio, were stored in the refrigerator, three vaccines, including one for measles, mumps and rubella, and two others for chickenpox were not stored in the refrigerator.

Dicks and Showen said as a result of this incident, Stanford Children’s Health is installing new refrigerators at the medical group’s office that will monitor temperatures around the clock and send alerts to the staff around the clock if there is any problem.

But the San Mateo mother who first contacted the Mercury News on Friday said she’s unlikely to take her baby daughter back to the medical group.

The mother, who asked not to be identified, said she had taken her baby into the office this week and was told during the appointment about the compromised vaccines.

“The doctor said I would be receiving a letter in the mail, but why did they wait two weeks?” said the mother. “My question for the doctor is: what do I do with my baby now?”

A similar incident occurred in May when the Palo Alto Medical Foundation alerted more than 4,000 pediatric patients that they might need to repeat their vaccination because a faulty refrigerator at its Los Gatos office had kept vaccines a few degrees too cold. Those vaccines were administered from February 2014 through March 2015.

Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.