Businesses soon could be suffering a double whammy of a flu season, with many employees missing work not only because they need to take care of sick kids or parents, but also because they're ailing themselves.
The number of reported flu cases nationally began spiking in recent weeks, much like 2012-13's particularly bad flu season, in which workers lost an estimated $8.5 billion in wages, according to government and privately compiled data.
For the first time since the 2009 pandemic, the predominant flu strain is the H1N1 virus rather than one of the traditional forms of flu, which tend to hit children and the elderly the hardest.
"We are seeing an increase in H1N1," said Dr. Michael Jhung, medical epidemiologist at the federal Centers for Disease Control. "This virus in the past affected young and middle-age adults more than older adults. It stands to reason that many more of these people would be in the workforce."
Another worrisome sign for businesses is that the spike, which began in December, looks much more like last winter's monster flu season than the tame 2011-12 season.
A chart compiled by AthenaResearch, which tracks flu diagnoses at primary care physicians affiliated with Athenahealth's network, shows that this year's spike began just weeks after last winter.
It also has the same steep slope as last season, albeit with a lower rate of reported cases so far. In contrast, the gradual increase in flu cases from 2011 to 2012 looks like a small hill that peaked in March 2012.
According to Josh Gray, vice president at AthenaResearch, this year's trend "looks like last year with a two-week lag built in."
(Read more: Mum's the word on Obamacare demographics)
"We're still in a vertical ramp phase, and it's not at all over," Gray said. "I do think businesses should be prepared for some more absenteeism."
More lost time ahead
If the trend continues to look like last year and approaches the same number of diagnoses, businesses could face a second straight year of major loss of workdays.
Top ten flu states * Weeks ending December 21 and December 28, 2013, PCP providers.
|Place||State||Current flu rate *|
In a survey released in October, Walgreens said data suggested that during the 2012-13 season, the worst in more than a decade, adults missed 230 million workdays because of flu, with affected employees missing three days on average.
Employers that season had an estimated $30.4 billion in flu-related costs from lost productivity.
Walgreens' Flu Impact Report pointed out that 2010-11 was a more typical flu season. It had just 100 million lost workdays related to the flu, or an average of one day per affected worker, and flu-related losses to employers of $10.5 billion.
(Read more: Obamacare enrollment tops 2 million, lots more to go)
The possibility of lost wages or aggravating employers by taking sick days is on the minds of many workers confronted by flu in their families.
Dr. Sally Ginsburg, a pediatrician who practices in western Massachusetts, said that more than half of the parents she talks to after their kids have been diagnosed with the flu "are not happy about it," because they will have to miss work to take care of them.
On average, she said, parents miss "four or five days" because of sick kids.
Parents ask, "How long is it going to last, can I take them to day care?" she added. "We hear, 'I just got a new job and I can't take anytime off from work, and what are you going to do about it?'
"Then there are other parents who say, 'I have to go out of town, and I'll have to cancel my visit or meeting,' " Ginsburg said. "Nobody likes missing work … and they worry about their bosses."
Almost every parent who misses work because of a sick child asks for a doctor's note to excuse their absence from the workplace, she said.
"Unless they're big executives, they need a note for work," Ginsburg said, adding that parents tell her their bosses demand such documentation.
The best defense
To avoid getting the flu, some workers and others get vaccinated, although at a much lower rate than what federal officials recommend. Only an estimated 40 to 45 percent of people get vaccinated, compared with the CDC's 70 percent goal.
Even among health-care workers, the rate of immunization has lagged what is needed to substantially reduce flu acquired in a hospital or other health-care setting, according to the Association for Professionals in Infection Control and Epidemiology.
Last January, Walgreens was swamped with customers getting flu shots in response to the wide spread of illness, which had already reached its peak in total cases the month before.
"Company-wide for the season we did about 7 million flu shots, but nearly 1.5 million came in the month of January," said Walgreens spokesman James Cohn. "January was crazy."
But those shots may have come too late for many people.
"The CDC recommends getting it as soon as the vaccine becomes available," Cohn said. "It takes two weeks to build up immunity from a flu shot."
This season, Walgreens has seen a big jump in the number of people getting shots early.
Through the end of November, the company's drugstores had administered 6.1 million vaccinations, versus 4.6 million in the same period last year.
"The incentive to get a flu shot is more at the top of mind for some people," said Cohn. Walgreens plans to release updated vaccination data Monday.
The CDC estimated that vaccinations prevented about 6.5 million flu cases and about 80,000 hospitalizations last year.
But Dr. Leonard Friedland, director of vaccines for drugmaker GlaxoSmithKline's North America division, said that while the H1N1 virus "could disproportionately affect young and middle-age adults … it's those age groups that typically don't think they have to be vaccinated.
"Vaccinations are still available, and that's an important message for your readers," he said. "The best tool to prevent influenza is vaccination."
Friedland noted that new insurance that kicked in Jan. 1 under the Affordable Care Act will enable several million people to obtain vaccinations with no out-of-pocket cost.
But if younger and middle-age adults don't get vaccinated, they could exacerbate the rate of illness.
"If they are going to work and they are sick, they are spreading it to people of the same age," he said.
Vaccines are including a wider variety of flu strains in the hope of providing greater immunity.
This year, for the first time, GSK distributed two forms of so-called quadrivalent flu vaccines, which contain four different strains of flu compared with traditional vaccines containing three strains.
Because it contains an additional "B" strain, Friedland said, quadrivalents "takes the guesswork out of what's required" to fight a strain that becomes prevalent for a given year.
(Read more: Expanding Medicaid increases ER trips)
Friedland said there were an estimated 138 million doses of flu vaccine available nationwide, with GSK providing about 25 million of them.
About 12 million of GSK's vaccine doses this year are the the quadrivalent version. Next season, he said, "we expect to produce almost 35 million vaccine doses for the U.S., nearly all of which will be quadrivalent.
—By CNBC's Dan Mangan. Follow him on Twitter @_DanMangan.