Flu cases this year have doubled those from last year, validating experts' fears that this season could be particularly bad.

Between Oct. 1 and Nov. 25, 5,070 clinical lab tests were positive for the flu, up from 2,510 in the same period last year, according to data from the Centers for Disease Control and Prevention. The number of Americans who have contracted the illness is likely even greater, an agency spokeswoman cautioned, since most people don't go to the doctor and get tested.

This year's flu vaccine may not be as helpful as in years past. The one used in Australia was only 10 percent effective, according to research published in October in the journal Eurosurveillance.

Researchers warned the implications for the Northern Hemisphere may not be the same, although the vaccine Australia used has the same composition as the one the U.S. is using. As of Nov. 24, about 148.2 million doses of the vaccine have already been administered in the U.S., according to the CDC.

A spokeswoman for Sanofi Pasteur, a division of Sanofi that produces about 40 percent of the influenza vaccines distributed worldwide, cautioned the research only analyzed Australia. These findings may not reflect what will happen in the U.S., she said, because there is no guarantee the same strains that predominated in Australia will be the same that predominate here.

The effectiveness of flu shots can vary each year. The average number between 2006 and 2017 was 46 percent, according to data from the CDC. As for the outliers, 19 percent (in 2014-15) and 60 percent (in 2010-11) were the lowest and highest rates, respectively, of the bunch.