A shipment of flu vaccines made in Liverpool has been delayed after some were found not to have met sterility standards.
The flu vaccines do not meet "sterilisation" requirements
Deliveries to the US of the Fluvirin (R) influenza virus vaccines will now be delayed until October.
Chiron Corporation was due to begin shipping the vaccines in August, but will now start transporting them in October.
The cause of the incident is being investigated.
In total, Chiron will produce almost 50m doses of Fluvirin to be shipped to the US.
Only a small number of doses were found to have fallen below sterility standards.
John Lambert, president of Chiron Vaccines, said: "Extra checks will ensure that the quality, safety and effectiveness of our product meet rigorous standards.
"We currently expect Fluvirin doses to be available in early October, in time to meet public health needs for this influenza season."