As the drug is after all not as potent as it should be to effectively fight against the virus, the US Food and Drug Administration on Tuesday announced that AstraZeneca's MedImmune division is voluntarily recalling some of its H1N1 vaccine.

According to Norman Baylor, Director of the Office of Vaccines Research and Review at the FDA, the recall includes a total of 4.7 million doses of the firm's nasal spray drug, but as of now, only some 3,000 are left in the warehouse.

The remaining doses were given out over October and November, when the drug still boasted complete potency, Mr. Baylor said.

The FDA, on the other hand, has stressed that people who have already been vaccinated by MedImmune's spray do not need any other vaccination.

"The doses were well within potency specifications at the time of distribution", said Tor Constantino, a spokesman for the unit of AstraZeneca, while also sharing that the recall covers unused doses of 13 specific lots sent out in 2009.

The cause of the fall in potency, as shared by the company, is being investigated.