TORONTO — The World Health Organization finalized its recommendation for the composition of next year's flu shot Thursday, making the rare move to recommend three new strains be included.

Replacing all three strains in the flu shot is rare and could complicate the process of making next winter's vaccine, experts agree.

"The manufacturers will have a challenge ahead of them. We all will," said Dr. Nancy Cox, head of the U.S. Centers for Disease Control's influenza division and one of the international experts who gathered at the WHO in Geneva this week for the vaccine strain selection meeting.

Cox said changing all three strains is "not unprecedented" but doesn't happen often.

"This year our data were just very consistent in pointing to the need to update H1, H3 and B," she said, referring to the three components in the vaccine.

Seasonal flu shots protect against three strains of viruses, H3N2 and H1N1, which are both influenza A viruses and one influenza B strain.

Because it takes months for vaccine manufacturers to grow the viruses in eggs and then process the yield into vaccine, experts involved in strain selection meet long in advance of flu season to determine what the vaccine should target.

At the gathering in Switzerland, they studied data about which viruses have been circulating where and which seem to be causing the most disease, using it to predict what viruses would be expected to launch the biggest assaults on human immune systems in the northern hemisphere in the winter of 2008-09.

A similar meeting is held each September for selection of the strains for the vaccine for the southern hemisphere's winter.

"We're looking for new variants, the spread of those variants and then whether the current vaccine protects against the spread of those variants," Cox said in an interview.

"We're trying to forecast what's going to be circulating almost a year from now. . . . (But) influenza has demonstrated that to us time and time again, that it's impossible to predict with certainty what's going to be circulating a year from now."

Still, often those predictions are good. The CDC's head of influenza epidemiology, Dr. Joe Bresee, recently reported that in 19 of the last 26 years, the strains included in the vaccine were well matched to those that were causing disease.

But some years the match isn't optimal and this year is one of them. The B strain and the H3N2 strain in the 2007-08 vaccine aren't well matched to the predominant circulating viruses, an unfortunate reality which may be contributing to a surge in H3N2 flu cases in the United States in recent weeks.

In Canada, most of the flu activity so far this year has been caused by H1N1 viruses. The H1N1 virus in the vaccine is an excellent match to circulating viruses, meaning Canadians who got a flu shot should get good protection this winter.

The recommended strains for next winter's vaccine are named A/Brisbane/59/2007, which is the H1N1 virus; A/Brisbane/10/2007, which is the H3N2 virus; and B/Florida/4/2006.

The last two are in the vaccine which is being prepared now for the southern hemisphere's upcoming flu season, so the vaccine manufacturers have some experience working with two of the three. But one of those two - the H3N2 virus - grew poorly and produced a low yield.

That wasn't such a big problem for the southern hemisphere product, because there is less demand for flu vaccine from southern hemisphere countries.

The demand for vaccine for the northern hemisphere is much higher and laboratories involved in making the vaccine seed strains and the manufacturers are working to try to find a better producing H3N2 virus, Cox explained.

Dr. Ryoko Krause, director for biologicals and vaccines with the International Federation of Pharmaceutical Manufacturers and Associations, said manufacturers are optimistic a better H3N2 seed virus will be available shortly.