At least 13 million cases of the flu have already been diagnosed so far this season, according to estimates from the Centers for Disease Control and Prevention (CDC), but experts are cautioning that those who have already battled the virus may be at risk of getting it for a second time as another strain gains traction across the U.S.

Recent updates on the CDC’s weekly flu surveillance said that “approximately equal numbers of B/Victoria and A(H1N1)pdm09 viruses have been reported.” And while you can’t catch the same strain of the flu twice, you can be sickened by one and then later contract another as we face what one expert calls a possible “double-barrel season.”

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“We have all these odd ingredients coming together so that we have, at this juncture, an awful lot of flu with the potential for being as bad as our last bad year, if not worse,” Dr. William Schaffner, an infectious disease specialist with Vanderbilt University Medical Center and the medical director at the National Foundation for Infectious Disease, told Prevention.

The CDC also noted that it’s unusual for the influenza B virus to emerge so early in the season, which hasn’t happened in the U.S. in the last 27 years. The B virus strains are also “more common among children and can cause complications, resulting in hospitalization and death.” So far this season, the agency has confirmed 39 pediatric influenza-associated deaths.

“This season has turned a lot of [what we know about flu] on its head,” Schaffner told HealthLine.com. “There’s a lot we know, and even more we don’t know about flu.”

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Still, Schaffner told the news outlet that for someone to be sickened twice in the same season would be a rare occurrence.

The CDC reports high flu activity across the U.S., and expects it to continue for weeks. And while 172.2 million doses of the flu vaccine have already been distributed, early analysis of cases reported in children in Louisiana suggests that the strain in the shot does not match the subgroup of influenza B currently circulating. Still, the CDC says the strains are close enough to offer at least some protection against the virus.

“If you’ve been vaccinated, and even if there is a mismatch, you are likely to have a less severe infection when you get it,” Schaffner told HealthLine.com.

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The CDC recommends everyone 6 months and older receive the flu vaccine each season, and those at risk of serious flu complications should seek immediate medical attention if suspected symptoms develop.