Canadian doctors are preparing for the worst case H1N1 flu pandemic scenario, devising rules about how to decide which patients should get access to life-saving treatments.

The discussions are taking place amid worries that there may not be enough vaccine to go around this fall.

The hope has been that a vaccine would be in place by November, so that it could be administered to all Canadians who want it, in an effort to stop H1N1 from sickening large segments of the population.

But vaccine manufacturers are reporting a frustratingly low yield from H1N1 viruses grown in fertilized chicken eggs, according to the World Health Organization.

The WHO says that yields have not been good so far - about half or less than what manufacturers get when making seasonal flu virus vaccines in eggs.

Dr. Donald Low, an infectious disease specialist at Toronto's Mount Sinai hospital tells CTV News Channel that growing flu viruses in chicken embryos is always time-consuming.

"For every vaccination shot, you need one egg that has been infected. So if these viruses are not growing well in eggs, maybe you need two eggs for a shot, or three eggs - which means there's going to be a limited supply of vaccine," he says.

The WHO's vaccine chief, Dr. Marie-Paule Kieny, says they are trying to fix the problem by making new seed strains that may help manufacturers increase yields in coming weeks.

The WHO previously estimated the world could have as many as 4.9 billion doses of H1N1 flu vaccine ready for the flu season -- but this assumed that production yields would be similar to seasonal vaccine. The agency is now reviewing its numbers in light of the latest yield results.

When the vaccine does become widely available, a WHO advisory panel is recommending that health-care workers be given first priority to receive it. By so doing, the workers would be better protected from infection and would be able to ensure that health-care systems can function should the virus increase demands on health care centres.

The advisory panel is known as SAGE, the Strategic Advisory Group of Experts on immunization, and is composed of experts who advise the WHO on vaccine issues.

"[SAGE] recommended first that healthcare workers should be immunized in all countries in order to retain a functional health system as the virus evolves," Kieny told reporters in a conference call.

"In terms of the other groups it depends on the strategy a country wants to follow."

SAGE also suggested countries might want to consider vaccinating pregnant women and healthy young adults first. Those groups seem to be among those hardest hit so far by the pandemic virus, while older adults have been less likely to fall ill.

Kieny said countries might also want to prioritize children, since they can transmit the virus rapidly at school and at home.

The WHO said countries should continue with their normal vaccination programs against seasonal flu, with seasonal vaccine production expected to be completed by the end of July.

Here in Canada, the federal and provincial governments are also developing guidelines to help doctors decide which H1N1 patients should receive treatment if resources were to become critically low.

Theresa Tam, director general of infection disease and emergency preparedness for the Public Health Agency of Canada, tells The Globe and Mail that if hospitals were to become overcrowded by flu patients, there may come a point where there is a need for triaging.

Some of the work to develop the rules around triage has already been done, said Dr. Tam. It is primarily the job of the provinces, but the federal government is assisting by providing information on the legal, ethical and clinical issues of triage.