Clinics offering H1N1 shots have been busy across Canada this week. (CBC)

Canadian children will have to line up twice for H1N1 shots in the short term.

Canadian public health officials continue to recommend children between the ages of six months and 10 years receive two half-doses of the H1N1 vaccine at least 21 days apart.

But Dr. David Butler-Jones, Canada's chief public health officer, told reporters Friday he's optimistic a single half-dose will be found to be enough.

Public health experts worldwide have been studying whether one or two injections should be given to immunize young children against swine flu, which makes a difference in their estimates of how much vaccine is needed.

Butler-Jones said he will review the latest evidence.

"We'll be able to hopefully say very shortly, 'parents, you don't need to come back once you've had the first dose,' " Butler-Jones said.

It takes a single dose of the H1N1 vaccine to immunize adults and children over the age of 10, the World Health Organization said earlier on Friday.

The WHO's group of immunization experts, known as the Strategic Advisory Group of Experts on immunization, or SAGE, looked at the research to date on safety and effectiveness of the H1N1 vaccines made by 25 companies worldwide during a three-day meeting in Geneva this week.

"The experts reviewed early results from the monitoring of people who have received pandemic vaccines and found no indication of unusual adverse reactions," the Geneva-based WHO said in a statement.

"Some adverse events following vaccination have been notified, but these are well within the range of those seen with seasonal vaccines, which have an excellent safety profile."

No booster needed for children

The expert panel recommended more studies on the effectiveness of the vaccine in children up to age 10.

WHO also said 10 per cent of swine flu hospitalizations have been among pregnant women in their second or third trimester.

Next week, the first 200,000 doses of adjuvant-free vaccine for pregnant women are expected to be given in Canada.

An adjuvant is a substance used to strengthen a vaccine's active ingredient and boost immune response.

Adjuvant fine in pregnancy

The latest WHO recommendations suggest that pregnant women do not have to wait for vaccine without an adjuvant. The vaccine with the adjuvant that Canada and other countries are offering is fine for use in pregnant women, WHO said.

"After having looked at the safety profile of these pandemic influenza vaccines, both in clinical trials and in deployment, [SAGE] have concluded that the safety profile was good and recommended that pregnant women can be immunized with any of the licensed vaccines," Dr. Marie-Paule Kieny, WHO's director of vaccine research, told a news conference in Geneva.

The latest immunology research also suggests that the adjuvanted vaccine produces a better immune response and may offer better coverage if the H1N1 virus starts to change or drift, the UN health agency said.

It is safe to give both the H1N1 and seasonal flu shots together in most cases, WHO added.

Butler-Jones said Canada will continue to offer an adjuvant-free H1N1 vaccine for pregnant women.

There is no concern with the adjuvant, but given that there is less clinical information about its use in pregnancy, federal health officials consider it an important option for pregnant women, Butler-Jones said.

In its weekly pandemic update Friday, WHO also said one to 10 per cent of patients who developed a serious illness needed treatment in hospital. Of this group, 10 to 25 per cent needed treatment in an intensive care unit and two to nine per cent died.

With files from The Associated Press