Washington, D.C. ? Seasonal influenza is spreading widely throughout the United States, with nearly half the cases caused by strains of the virus that are not directly covered by this year’s flu vaccine.

Whether the winter will end up being worse than usual remains to be seen. Flu mortality in adults has been higher than in the last two years, but deaths in children – an important marker of severity – have been rare.

Nevertheless, this winter is likely to be one of the few times that public health experts lose the bet they make each year when they devise the formula for the flu vaccine – eight months before the virus starts circulating in the fall. Experts must decide on the formulation then because of the time it takes to produce mass quantities of the vaccine.