Here's how much the flu season could cost this year

An aggressive flu strain that rapidly spread across the country during this past season could cost employers nearly $21 billion in lost productivity after more than 18,000 employers took sick days, according to a new study published by Challenger, Gray & Christmas Inc.

“If this strain sickens as many people as during the 2014-2015 outbreak, over 18 million workers could miss work due to illness, increasing our previous estimate by 64 percent,” Andrew Challenger, vice president of the global outplacement consultancy, said in a statement.

To arrive at the estimate, the firm said it looked at the number of illnesses for people over the age of 18 during the 2014-2015 flu season with the current estimate of an employment population ratio of 60.1 percent and the average hourly wage of $26.63.

With over 18 million estimated employed adults missing four eight-hour shifts, the cost to employers could reach $15,424,800,736.32,” the company said.

That number could grow even higher, however. Already, the Centers for Disease Control and Prevention warned that flu numbers this year could reach levels seen during 2009, when the swine flu swept across the country.

“If the flu season continues to be as severe as the 2009 swine flu pandemic, productivity losses could hit $21 billion,” the company added.

In order to avoid the flu, the CDC recommends that anyone over the age of six months gets an annual vaccination. Flu vaccines protect against three or four viruses that research suggests will be the most common.

Last year's flu season cost businesses nearly $10 billion, largely because of a particularly aggressive strain of the influenza that wasn't "very well-matched" with the vaccines being distributed.