Americans are urged to get flu shots every year, but there may be another factor that could sway them toward immunization: The country's rising employment rate.

Because the U.S. is close to full employment, workplaces are more crowded, allowing the flu virus to spread more easily, according to an economist. In fact, each 1-percentage point increase in the employment rate correlates with a 16% bump in flu-related doctors' visits, Erik Nesson, an associate professor of economics at Ball State University in Indiana, said.

"It seems to be a place where higher economic activity is detrimental to people's health," Nesson said in an interview Wednesday.

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His study, published last year in the Economics & Human Biology journal, relies on the employment rate, which shows the percentage of working-age adults who hold jobs. By comparison, the jobless rate has a narrower focus, showing the percentage of workers who are out of a job but actively looking.

The employment rate has been rising steadily since reaching a post-recession low in 2011 of about 66.3%. In November, it reached 71.7%, almost on par with its pre-recession peak of slightly more than 72%, according to the latest available data from the Federal Reserve Bank of St. Louis.

The increase in employment combined with an already deadly 2019-2020 flu season should prompt employers to re-examine their sick day policies, Nesson said.

"Our study suggests that companies should be cognizant that if someone comes to work when sick, even if it saves the company in one aspect, it might cost them money in another area, in that someone who comes to work sick could get other people sick," he said. "Companies should take this into account when designing whatever sick day policies they have."

Low-wage workers in retail and food industry jobs often don't receive paid sick days, which critics say encourages workers to show up to work while sick, spreading illnesses to co-workers and customers. While some states have instituted laws that mandate paid sick days, the federal government doesn't require employers to offer it.

Flu activity remains high in the U.S., according to the Centers for Disease Control and Prevention. It estimates there have been at least 9.7 million flu illnesses, 87,000 hospitalizations and 4,800 deaths from flu so far this season. It estimates there's a 15% chance the flu will peak in late January and 25% chance it will peak in February.

Service industries such as health care and retail are especially likely to spread the virus because workers in those industries interact with many members of the public. "If our economy continues to shift to more service-oriented employment, the results presented here suggest there is greater potential for flu spread in the future," Nesson added.