GlaxoSmithKline has advised doctors in Canada to stop using a batch of its swine flu vaccine, amid reports of severe side-effects in some patients.
The batch of some 170,000 doses was put on hold because of the reported higher than usual number of patients having anaphylactic reactions.
This may include breathing problems, raised heart rate and skin rashes.
The pharmaceutical company said it was investigating the reports, which could lead to the withdrawal of the batch.
The reports say one in 20,000 people suffered adverse reactions to the batch of GlaxoSmithKline's Aperanix vaccine.
This is five times the expected number. None of the patients reportedly suffered long-term ill-effects.
In a statement issued on Tuesday, the company said that following discussions with Canadian health authorities, it was advising a batch of the Arepanrix vaccine with the lot number A80CA007A to be put on hold as a precautionary measure.
GSK said it was taking this action "as a result of a higher than expected rate of serious allergic reactions related to this lot number compared to other lots".
The company said no such reactions had been seen with other lots and that they were unaffected by the voluntary hold.
To date some 15 million doses of Arepanrix have been distributed across Canada and overall the frequency of severe allergic reaction following immunisation was less than one event per 100,000, GSK's statement said.
"This rate does not exceed the rates typically reported for other vaccines," GSK said.