A higher-than-expected rate of serious adverse events resulting from a batch of H1N1 vaccine in Canada should be no cause for public alarm, vaccine experts said – adding that these events may be a chance occurrence, and do not necessarily point to safety issues with the new shot.
ABC News Senior Health and Medical Editor Dr. Richard Besser said people in the U.S. need not worry -- the vaccine implicated in these reactions is not licensed for use in the United States.
After a string of serious adverse reactions, Canadian health providers were advised to pull the remaining doses from the batch in question out of use, pending an investigation.
"It is encouraging that the vaccine has been rapidly withdrawn from the market," Besser said. "It will be important now for the manufacturer to try to determine what led to these reactions."
After discovering that one lot of their vaccine, Arepanrix, had caused five times the expected number of adverse vaccine reactions, GlaxoSmithKline advised Canadian health authories on Nov. 18 to take remaining doses from this shipment out of use -- though by then, all but 20,000 of the 170,000-dose shipment had already been given out.
Though one serious adverse reaction for every 100,000 people vaccinated is to be expected with this vaccine, six people out of the 150,000 vaccinated from this lot needed to seek medical attention due to an adverse reaction such as an allergic reaction.
But the Canadian Health Ministry is reassuring the public there is no need for concern.
All six patients have fully recovered and subsequent testing has confirmed that "there's nothing wrong with this lot's vaccine quality," said Timothy Vail, spokesman for Canada's health minister.
As of Nov. 13, 7.5 million doses of this vaccine, which contains both an adjuvant (an ingredient to make it stronger) and the preservative thimerosal, has been distributed throughout Canada. So far, over 5 million citizens -- roughly 20 percent of the population -- have been vaccinated, Vail says.
Given that, overall, only 36 serious adverse reactions have occured during these 5 million vaccinations, Vail notes, "we're actually seeing lower than that [expected one per 100,000 rate] in our overall vaccination campaign."
Vail said there is a strong possibility that the response from this lot "is just a statistical anomaly" and there is no information yet that would suggest an issue with the vaccine itself.
But other experts were hesitant to say a nearly five-fold increase in serious adverse reactions might merely be a chance occurrence.
"The likelihood that this increase...is purely a chance finding is exceedingly low in my estimate," said Nicole Baumgarth, associate professor of comparative medicine at the University of California, Davis.
Dr. David Freedman, a professor of Geographic Medicine at the University of Alabama, Birmingham, agreed, saying, "I am sure this is real and not chance."
But until the investigation is complete, the cause behind a real increase in reactions can only be speculated, warned Dr. John Treanor, professor of immunology at the University of Rochester in upstate New York.