For parents of children who have received their first H1N1 shots, the frustrations may not be over.

Clinics are warning they may not have enough vaccine on hand to give booster shots exactly on schedule -- 28 days after the first dose.

Federal officials have said children 9 and younger need two doses of the H1N1 vaccine.

But this week, Park Nicollet Clinic began notifying the parents of 4,200 children that their booster-shot appointments have to be postponed because of the vaccine shortage. "We will reschedule them," said spokesman Jeremiah Whitten. But at the moment, it's not clear when.

Parents are getting a similar message at Allina clinics.

"We've been telling patients 'go ahead and make that four-week appointment,' [but] please understand that you may not have it available," said Lynn Berg, safety director for the Allina Medical Clinic.

Health experts say there is no reason to panic about delays between the first and second shots. The recommended interval "is a minimum, not a maximum," according to Dr. Anthony Fiore of the U.S. Centers for Disease Control and Prevention (CDC).

At the same time, he said, "one shot is better than none."

Clinics, however, are in an awkward spot because they have been told by the CDC that they cannot hold vaccine supplies in reserve, said Kris Ehresmann, director of infectious disease prevention at the Minnesota Department of Health.

"If you've got 500 doses, you don't give it to 250 kids and then save those 250 doses and wait for them to come back in a month," she said.

Park Nicollet issued a statement Thursday saying it has 9,000 high-risk patients who are still waiting to be vaccinated. The plan is to vaccinate them first, and then start calling children back for booster shots as more vaccine becomes available.

"I don't think [parents] should be concerned at all," Ehresmann said. "Their children will eventually be able to get that vaccine; it's just a question of will it be at week four or will it be at week six."

Originally, experts had predicted everyone would need two doses of the H1N1, or swine flu, vaccine. But in recent months, research showed one dose was enough for all but those younger than 10, Ehresmann said.

She said one dose probably will protect children from the most serious disease. "The rationale for the two doses is to make sure these kids have full protection," she said.

But with the vaccine supply in flux, that has been a challenge, especially with so many other high-risk patients still waiting for their shots.

HealthPartners clinics, for example, are planning to call young patients to schedule their H1N1 booster shots "as we receive additional vaccine," said spokesman Joe Dangor.

Officials at Fairview clinics, meanwhile, say they have adequate supplies for now to provide booster shots to those who need it.

But many clinics can't be sure when their next shipment of flu vaccine will arrive, and that is making planning difficult.

"There will be vaccine coming," said Ehresmann, "but we can't guarantee a date."

Maura Lerner • 612-673-7384