Park Nicollet Clinic is notifying more than 3,400 patients, including about 1,700 children, that they may need to be revaccinated because the shots they received weren't kept at the proper temperature.

The clinic says that no one's in any danger, but that the vaccines may have lost their effectiveness because they were exposed to freezing temperatures.

In this case, the mishap affected polio booster shots, hepatitis A and B shots and Prevnar, a vaccine for infants and toddlers, given at nine Park Nicollet clinics since December.

But health officials say the problem may be much more common.

Since they started spot-checks this spring, Minnesota Health Department nurses have found the same cold-storage problems at five or six other clinics, according to epidemiologist Kristen Ehresmann, who oversees the department's immunization program.

The same thing has been happening around the country, according to the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta.

"It's definitely an emerging issue that we're dealing with," Ehresmann said.

Traditionally, the big concern with vaccines has been keeping them from getting too warm and spoiling. Now, health officials are drawing attention to the opposite problem: cold damage.

On Tuesday, Park Nicollet sent letters offering free shots and apologies to thousands of patients who received their vaccines on days when the storage rooms fell below freezing. The vaccines are supposed to be kept within a range of 35 to 46 degrees.

"Probably everybody has been immunized fine," said Dr. Richard Freese, an internist and senior vice president of Park Nicollet. "It's just not a medical certainty. That's why we're going back and doing the extra thing to make sure everybody gets it."

Park Nicollet is spending an estimated $300,000 to correct the problem, including extending office hours, operating phone lines and providing replacement shots.

Medical officials stress that there's no danger in getting a second vaccine. "It might cause a sore arm, but you cannot be overvaccinated," said Dr. Hal Martin, medical director of Park Nicollet's travel clinic, where half the affected patients were treated.

The only risk, he said, is that some people may have thought they were protected against disease, but weren't. But so far, there's no sign that anyone who had the questionable vaccines became ill. "My biggest concern is that this is going to cause more fear . . . than it should," Martin said.

Many patients infants

The case first came to light in March, when a Health Department nurse found that the storage temperatures at a Park Nicollet Clinic in Minnetonka had periodically dipped below freezing, according to officials. The clinic was notified in April. At that point, officials say, they voluntarily checked the logs at their 24 other clinics, and found the same pattern at eight of them: Bloomington, Minneapolis, Eden Prairie, Plymouth, Shakopee, Airport and the pediatrics and travel clinics at St. Louis Park.

Many of the patients are children under age 2, who received Prevnar, a pneumococcal vaccine that can help prevent meningitis and other blood infections.

The rest were mainly adults traveling to parts of the world where they might be exposed to diseases such as polio or hepatitis through contaminated food or blood, Martin said.

The Health Department recommended revaccination, citing the guidelines issued in February by a federal immunization advisory panel: "Vaccines that have been mishandled (eg. . . . exposed to freezing temperatures) should not be administered." But the decision was left to the clinic, Ehresmann said.

Temperatures vary

Experts say the temperatures in the storage units can vary by several degrees from day to day, much like refrigerators at home, especially if they're opened and closed frequently. "These [vaccines] are very susceptible to freezing and thawing and freezing and thawing," said Patrick Schlievert, a microbiology professor and vaccine expert at the University of Minnesota. "One time may not hurt it, but two times may inactivate it."

No one knows how much damage, if any, was caused to the vaccines, but Park Nicollet officials said they decided to play it safe. "Most of these are children," Freese said. "We're really sorry that they're going to get poked again." The clinics also have taken steps to prevent the temperatures from dropping again.

Ehresmann praised Park Nicollet's actions, saying, "they've taken it very seriously and taken a very cautious approach," she said.

Meanwhile, Health Department officials confirm that Park Nicollet isn't alone. State epidemiologist Dr. Harry Hull, in a letter to Park Nicollet in May, said "other clinic chains in Minnesota are currently facing or have already dealt with similar situations." But the department declined to identify them, saying it's up to the clinics to decide whether to make it public.

In addition, isolated reports of similar cases in other states have been reported to the CDC in Atlanta, officials say. Ehresmann said she's heard of similar cases in at least four other states this spring.

"The message I want to convey overwhelmingly is . . . this is part of our routine review of clinics, and we're working with them," Ehresmann said. "I don't want parents to feel alarmed or a great deal of concern about this."

-- Maura Lerner is at mlerner@startribune.com .