This year's flu strain is especially aggressive and it could cost employers at least $15.4 billion in lost productivity, according to global outplacement and executive coaching firm Challenger, Gray & Christmas Inc.

The firm raised its estimate from $9.4 billion, but that was based on the comparable severity of the 2015-2016 flu season. Challenger, Gray & Christmas now believe that the 2018 strain will be as severe as the one three years ago. 

Nearly 40 million people got sick during the 2014-2015 flu season and if this strain is as aggressive more than 18 million workers could miss time because of the illness.

More than 30 million people over the age of 18 suffered from the flu during the 2014-2015 season and employed adults 18 and over lost, on average, four work days. 

While 2014-2015 was the most recent severe flu season, the 2009 swine flu epidemic takes the cake for time lost due to illness. Productivity losses that year totaled $21 billion, according to Challenger, Gray & Christmas.

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