"The reports are is that it probably lost between 10 to 12 percent of its potency," said Dr. Peter Katona.
A 10 to 12 percent loss of effectiveness is enough to bring these vaccines below required federal guidelines.
Experts say this type of reduction is unusual over a short period of time and why it occurred is unclear.
The recalled vaccines were formulated for young children, ages 6 months to nearly 3 years old.
The 800,000 pre-filled shots that are being called into question were manufactured by Sanofi Pasteur. They've been distributed across the county and CDC officials say many of them have already been given out.
Despite the drop in potency, UCLA infectious disease expert Dr. Peter Katona says these vaccines can still protect a child against the H1N1 virus.
"Even though the potency has been lost it doesn't mean that the vaccine is ineffective," said Dr. Katona. "So I would not worry with children who got vaccinated with this particular lot which you wouldn't know as a parent anyway."
The Centers for Disease Control is saying the same thing. The agency does not recommend kids get re-vaccinated even if they got two doses from the same lot.
Dr. Katona says the public should feel reassured that vaccines are being tested and re-tested for safety and effectiveness.
"Every vaccine loses some potency over time, but we have safeguards for that," said Dr. Katona. "We check and we double check so this is much more of an unusual occurrence than the norm."
Dr. Katona says it could be any number of things during production that could have led to the drop in effectiveness. He believes it's an isolated incident and has no bearing on the effectiveness of other vaccines.
