A public health crisis faces the US as the influenza season begins, after US drugmaker Chiron was blocked from shipping half the expected US flu vaccines by UK regulators.

Chiron on Tuesday said human error at a late-stage in vaccine manufacturing at its UK plant in Liverpool had caused UK regulators to suspend its license to make Fluvirin vaccines for three months. The company said the suspension by the Medicines and Healthcare Products Regulatory Agency effectively blocked its ability to ship all of this year's vaccines - about 50m doses.