The number of doses of H1N1 recommended for children varies by age. (CBC)

Healthy children between three and nine years old may need only one shot of H1N1 vaccine, federal health officials said Thursday.

The updated recommendation reflects the results of clinical trials from Europe that suggest a single half-dose of the H1N1 vaccine containing an adjuvant, or booster, offers sufficient protection for healthy children, Health Canada said.

"For healthy children between three and nine years old, this means that one shot may be enough to provide protection from the H1N1 flu virus," said Dr. David Butler-Jones, Canada's chief public health officer.

"This is good news for parents of healthy children and for Canadians. It frees up resources that can be devoted to immunizing priority at-risk populations, which means that the H1N1 vaccine will be available for the general public sooner," he added in a release.

The three recommendations are:

• Healthy children between three and nine years of age should only receive a single half-dose of the H1N1 vaccine, and they do not need to return for a second vaccine for now. This recommendation may be updated as more information becomes available.
• Children between six months of age and three years old should receive two half-doses of adjuvanted H1N1 flu vaccine, given at least 21 days apart. This guideline is unchanged.
• Children with chronic health conditions between three and nine years of age should receive their first half-dose of the pandemic vaccine as soon as possible. They should also receive a second half-dose at least 21 days later.

An adjuvant is added to a vaccine to stimulate a stronger immune response. Meanwhile, Health Canada on Thursday approved the non-adjuvanted H1N1 vaccine made in Canada by GlaxoSmithKline. The non-adjuvanted vaccine that is being used this week came from Australia.

The updated recommendations and studies have been reviewed by the Canadian Paediatric Society and provinces and territories.

The vaccine dosage recommendations may be adjusted as the results of more research and clinical trials on the vaccine become available.

The guidelines also said the H1N1 vaccine may be given at the same time as seasonal flu and other vaccines. If so, injections should be given in separate limbs.