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It appears there may be a partial flu shot mismatch again this year, with early data from Canada, the United States and Britain suggesting the vaccine component meant to protect against influenza B is not a match for the B viruses causing the most disease.

Predicting which family of influenza B viruses will dominate in a coming year - and therefore should be covered by the flu shot - is a challenge that has defied the experts in five of the past seven flu seasons, at least as far as disease patterns in North America are concerned.

The ongoing problem has prompted exploration of the idea that future generations of flu shots should be reformulated to add a second B component to a vaccine that currently protects against one family of B viruses and the two influenza A subtypes, H3N2 and H1N1.

Scientists from the U.S. Centers for Disease Control are working on a cost-benefit analysis of such a move, and met yesterday to begin going over the pros and cons. They plan to present their findings next month to the Food and Drug Administration, which regulates vaccines in the United States.

"Historically it's obviously been difficult to predict which one's coming next," Joseph Bresee, the CDC's chief of influenza surveillance and prevention, says of the B viruses. "And so one way to hedge those bets is just put them both in the vaccine."

The flu shot for the current season has already been manufactured. So any recommended changes would affect only future batches of the vaccine, which must be reformulated almost every year because viruses mutate so rapidly.

Until about eight years ago, only one family or lineage of B viruses circulated globally at one time. But in 2001 a second emerged from Asia and the two - known as B/Yamagata and B/Victoria - have co-circulated since. Each year one or the other has been dominant, though there is no clear pattern of how or when they alternate.

Because the viruses are from distinct lineages, it is thought that a vaccine made to protect against viruses from one would offer little or no protection against viruses from the other. However, ongoing Canadian studies show that isn't always the case.

"All is not lost if there is that lineage-level mismatch, because in the seasons that we have monitored it, we have - at least so far - shown that there has been some cross-protection," says Danuta Skowronski, a flu expert with the British Columbia Centre for Disease Control.

Some years, that cross-protection can be reasonably high, Dr. Skowronski said, though in other years mismatched B vaccine appears to provide little protection.

This year's flu vaccine was designed to protect against B/Yamagata viruses. But in Canada, the United States and Britain so far this season, B/Victoria viruses have been responsible for the majority of confirmed flu B cases. It is still early in the season, though, and patterns could change.

"The activity is, I think, beginning and it's looking like ... the majority of B isolates so far have been mismatched," says Allison McGeer, an infectious-diseases expert at Toronto's Mount Sinai Hospital.