Don't panic: The CDC and other public health experts say you do not need to revaccinate your child.

RODALE NEWS, EMMAUS, PA—A leading vaccine manufacturer is recalling 800,000 doses of pediatric H1N1 vaccine doses after quality testing last week found the vaccines' potency levels fell below pre-set limits. The company, Sanofi-Pasteur, the U.S. Centers for Disease Control and Prevention, and public health experts stress there are no safety concerns related to the recalled vaccines. Most of the recalled lots of vaccines have already been administered. "Absolutely none of this has to do with safety concerns. No adverse reactions have been reported," explains William Schaffner, MD, president-elect of the National Foundation for Infectious Diseases and chair of the department of preventive medicine at the Vanderbilt Medical Center in Nashville. "It's not a safety issue, and it shows that the FDA is working constantly to do surveillance, along with the company, of the vaccines."

THE DETAILS: The now-recalled vaccines, for use in children 6 months to 35 months old, met all standards when tested before the doses were released to the federal government. However, subsequent company testing from samples of the four affected lots last week found that the vaccines had lost potency, meaning the strength of the main ingredient concentration, the antigen, was diminished. "One of the fortunate things is that we put so much potency into each vaccine, we exceed the minimum by far," explains Dr. Schaffner. The FDA standard for potency is set high, in part because vaccines can lose potency, but when it dips below that high-bar level, a recall is mandatory. In this particular case, the potency diminished by one-sixth, Dr. Schaffner says. But that still leaves the vaccine strong enough to provide protection.

It's not immediately clear what caused a dip in potency, or exactly where these affected lots have been distributed.

WHAT IT MEANS: While news of a mass vaccine recall is likely to cause parental worry, this one is nothing to worry about. That's because children who received a recalled dose will still build full immunity, Dr. Schaffner explains. "There's still plenty of potency in these injections to get a routine immune response." CDC is also assuring parents that they do not need to take their children to the doctor's office for a replacement shot. It's important to note, however, that children 9 and younger do need an H1N1 booster shot about a month after their first shot to build full immunity. So parents should stay on the vaccination schedule recommended by their pediatrician.