Unlike any other vaccine, flu vaccines must be reformulated from scratch every year to keep pace with mutations of influenza viruses. The process is a long one, lasting close to a year.

In Canada, flu season usually runs from November to April. In the middle of this season, beginning in February, the World Health Organization studies the new strain mutations, known as antigenic drift, for next winter's vaccination.

The WHO's Global Influenza Surveillance Network, a partnership of National Influenza Centres in 83 countries, collects information and recommends a vaccine that targets the three most virulent strains in circulation.

With this analysis, countries can break down the information based on local needs. Health Canada is recommending that 2004-2005 vaccines contain antigens for these viruses:

• A: New Caledonia/20/99 (H1N1)-like
• A: Fujian/411/2002 (H3N2)-like
• and B: Shanghai/361/ 2002-like virus antigens.

Once the strains are chosen, the vaccine is then developed over the spring using fertilized chicken eggs -- a method developed more than 50 years ago. All flu vaccines therefore contain trace levels of egg protein and are generally not recommended for people with egg allergies.

The shell of a 10-day-old is cracked, and the flu virus injected into the fluid surrounding the chicken embryo. The egg is resealed, allowing the embryo to become infected. The resulting virus is then harvested two days later and purified.

Viral fragments from all three flu strains are then collected and combined into one. A single egg is needed to make one dose of a vaccine strain, requiring the use of tens of millions of eggs every year.

The strain is tested over the summer and by August, a new flu vaccine with the new viral strain is ready to be manufactured.

By September, the vaccine is ready for shipping. Mass vaccinations usually begin in October.

After vaccination, the vaccine kicks in about 10 to 15 days -- in plenty of time for the flu season peak which is usually at the end of January or early February.

The process is relatively laborious. It's also risky since the viral strain developed could be out of date once it reaches doctors' offices it there is a change in more than one of the three included influenza strains.

The WHO has stepped up its efforts on flu vaccines in recent years, in order to prevent and/or minimize an anticipated global flu pandemic.

Flu facts from Canada and around the world:

• Influenza viruses are defined by two protein components, known as antigens, on the surface of the virus. They are features called haemagglutinin (H) and neuraminidase (N).
• The influenza viruses that cause human disease are divided into two groups: A and B. Influenza A has two subtypes that are important for humans: A(H3N2) and A(H1N1).
• Three times in the last century, the influenza A viruses have undergone major genetic changes, mainly in their H-component, resulting in global pandemics. The most infamous was the Spanish Flu which is thought to have killed at least 40 million people in 1918-1919.
• An estimated 10-25 per cent of Canadians may get the flu each year. Among healthy adults the vaccine is 70-90 per cent effective.
• Approximately 10 million doses of flu vaccine were distributed in Canada during the 2002/2003 flu season.