This year's flu season has officially crossed the epidemic threshold, according to the Centers for Disease Control and Prevention. Fifteen children have died across the country from influenza, as the number of states reporting a "high" level of influenza activity jumped from 13 to 22 in one week.

The determination follows an earlier warning from the agency that this year's flu season could be a severe one.

Keep in mind, however, that epidemic-levels of flu activity in the U.S. are a typical part of the annual flu season. In other words, it's simply too early to determine just how severe this year's epidemic will be.

The CDC uses several methods to track and characterize the outbreak. "Right now, all of the CDC's influenza surveillance systems are showing elevated activity," The CDC's flu division said in an e-mailed statement. The influenza season reaches an epidemic level when the proportion of deaths attributed to pneumonia and influenza reaches a certain threshold: 6.8 percent. According to the CDC's latest available information on the flu season, the percentage is currently at the threshold.

The recent increases in flu activity, along with influenza-related hospitalizations and deaths, are so far a "typical pattern for the flu season."

Pediatric death tolls from previous outbreaks range from 35 to 171 deaths per season from influenza-related causes since 2004. That is, every year with the exception of 2009, when the annual flu season reached pandemic levels. During that season, 348 children died of influenza-related causes.

"It is a bit early to make any kind of characterization about pediatric deaths this season," Erin Burns,a  health communications specialist with the Influenza Division at the CDC  said in an e-mail, "but from looking at the curve going back  to 2011-2012, it doesn't seem like anything unusual is happening." However, the agency cautioned, pediatric death data "typically lag behind" other measures for a season's severity.

That doesn't mean that health officials aren't particularly worried about this flu season, even as assessments of the overall scope of this year's influenza epidemic will have to wait, experts say.

For one thing, the dominant version of the virus this year is H3N2 - which tends to lead to harder-hitting flu seasons. "H3N2 predominant seasons tend to have more hospitalizations and more deaths," CDC director Thomas Frieden warned in early December.

There's an even bigger potential problem: "the most concerning thing about the flu season this year there is a mismatch between the predominant strain that is circulating and what was put in the vaccine," Trish Perl, the head of the Johns Hopkins Medicine Office of Epidemiology and Infection Prevention told The Post.

The vaccines distributed this year have limited effectiveness against some "drifted" (i.e. antigenically different ) H3N2 viruses present in this year's outbreak. A "drift" is a problem for the effectiveness of vaccines, albeit one not quite as devastating as an abrupt "antigenic shift," which leaves much of the population without an immunity to the new form of the virus. Such a "shift" preceded the 2009 pandemic, for example.

Drifts happen each year, and vaccine makers must respond to them season to season. But the timing of this year's "drift" was particularly poor.  As we explained earlier in December, this year's "drift" in the H3N2 viruses wasn't detected until March,  too late to make it into this year's batch of vaccines.

Although the vaccines will still provide some protection against those drifted strains, "it's clearly not complete protection," Perl added, noting that "we could see a lot more illness because we have a lot less people who are immune."

This flu season could be a doozy. The CDC is warning that the flu vaccine isn’t as effective this year. The Washington Post’s Lenny Bernstein explains why. (Pamela Kirkland/The Washington Post)

Perl noted that experts will be watching the recent increase in flu-related hospitalizations, which can be one of "the first signs that you may have more severity" in an annual flu epidemic.

In order to get a sense of severity, officials compare a current season's weekly rates to those of previous seasons featuring the same predominant strain of the virus. Since 2014-2015 is a H3N2 predominant season, that means looking at 2012-2013's data, the last time the same strain was predominant.

Looking at the CDC's weekly data, influenza-related hospitalization rates for people 65 and older during the most recent week available this season was 38.2 per 100,000 people. That's higher than it was for the same week in 2012-2013, which was 28.4 per 100,000 people. The comparison "suggests this season may be on track to be more severe in terms of hospitalizations in older people, or it may just mean that we are having an earlier season than in 2012-2013," the CDC's flu division said in an e-mailed statement.

Still, some regions of the country are already experiencing what feels like a severe season, even if national trends are inconclusive. So the high-altitude assessment of this year's season might not match up to the experiences of health workers in areas with particularly high flu activity right now. "When you're in the middle of it, it feels like more" Perl notes.

For now, officials have a couple of messages for those worried about the flu. Namely: if you're not already vaccinated, do it. There are several strains of the flu circulating each year. So while this year's vaccine batch might have a limited effect on the predominant strain, it could still protect against other strains.

Perl particularly urged healthy individuals to get the vaccine this year, noting that what might feel like a "nuisance" of an illness for some is life threatening in more vulnerable populations, some of whom might also have a less effective response to the vaccines. "We forget that this is a bad disease and it kills people," she said, adding that by vaccinating healthy individuals, officials "are in essence protecting those that are vulnerable."

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