Canada continues to investigate an unusually high rate of a side effect known as anaphylaxis which resulted from a batch of the swine flu vaccine created by GlaxoSmithKline Plc. In one lot of 172,000 doses, anaphylaxis occured at a rate of 4 per 100,000.

Typically, just 1 per 100,000 will suffer from the reaction. Anaphylaxis is an accuate allergic reaction which includes swollen tongues, throats and respiratory distress.

GlaxoSmithKline is the sole provider of swine flu shots to Canada and has thus far shipped 7.5 million doses of the Arepanrix vaccine to the country. For all batches, the rate of side effects is 0.32 per 100,000, said health officials on Wednesday.

The higher than normal rate from one batch may be a statistical anomaly. Officials added that the reactions are not related to the adjuvant that is added to boost the vaccine's potency.

According to Glaxo spokeswoman Gwenan White, the company is working with Canadian officials.

To date, about 65 million people worldwide have been vaccinated with the swine flu vaccine. The World Health Organization said that so far shots for the pandemic H1N1 flu are as safe as those for seasonal flu. Nearly 80 million doses have been distributed in 40 countries with pandemic immunization campaigns underway.