Hundreds of thousands of H1N1 flu shots for children have been recalled because tests indicate the vaccine doses lost some strength, government health officials said Tuesday.

The shots, made by Sanofi Pasteur, were distributed across the country last month and most have already been used, according to the Centers for Disease Control and Prevention. The 800,000 pre-filled syringes that were recalled are for young children, ages 6 months to nearly 3 years.

Dr. Anne Schuchat of the CDC stressed that parents don’t need to do anything or to worry if their child got one — or even two — of the recalled shots. The vaccine is safe, she said.

Tests done before the shots were shipped showed the vaccines were strong enough. But tests done weeks later indicated the strength had fallen slightly below required levels.