“We’re seeing January-level transmissions currently,” Dr. Alex Greninger, a microbiologist at the University of Washington, referring to the West Coast specifically.

Here’s what you need to know.

What’s going on with this year’s flu?

This year’s flu season started unusually early, particularly on the West Coast where cases popped up in late August, some two months earlier than usual.

So far, the flu season is dominated by a strain of H1N1, the type that caused the 2009 flu pandemic.

But the latest strain has a specific mutation, called G155E, that is likely playing a role in the season’s early spread. Even those who have been exposed to H1N1 might not have as much natural immunity to this mutation.

“We’re seeing seasonality go out the window, specifically for H1N1,” Greninger said. “If there is a big enough immunological gap that opens it up, it doesn’t really need the help of winter.”

Who is most at risk for this mutated strain?

Jesse Bloom, an evolutionary biologist at Fred Hutch Cancer Center in Seattle, found that people aged 15 to 25 years old have less effective antibodies to this season’s flu, according to early findings that haven’t yet been peer reviewed.

Bloom said the findings, based on lab work, might show that adults with exposure to H1N1 from before the 2009 pandemic could have more immunity to this year’s flu.

“We suspect the reason is that influenza antibodies are shaped by lifetime exposure histories,” Bloom said. “The 15 to 25 age group, that’s sitting right on the age group of people that you would expect there’s a good chance their first exposure was in 2009 or shortly thereafter 2009,” which may not provide sufficient protection.

Will the flu shot still protect me?

The short answer: yes. But it won’t be clear until later this season exactly how effective the vaccine is.

This mutation clearly emerged after drugmakers updated seasonal vaccines, so existing shots appear to be partially mismatched to this year’s flu, Bloom said.

“I would probably expect it to be a slightly mid- to lower-vaccine-effectiveness year because we know that there’s this new mutation that came in and it’s good enough to spread in the population early,” Greninger said.

The efficacy of flu vaccines varies each year. Last season, the flu shot was 31% effective at preventing flu-related hospitalizations across all ages, according to early estimates from the CDC.

Current flu shots do include strains of H1N1 and the type of H3N2 that spread last season.

Experts say it is also possible other strains of influenza may emerge throughout the season, which might align more with what is in the shots.

Where is the flu spreading?

The flu normally starts in the Southeastern states, but this year it started on the West Coast—a break from the norm.

In California, more than 11% of flu tests are coming back positive—compared with around 1% this time last year, according to state health-department data. San Francisco reported its first flu-related death of the season in early October.

Doctors at Seattle Children’s Hospital in Washington are treating children with typical symptoms of flu in October—fever, dehydration and respiratory issues—at a volume more typical for around December each year, said Dr. Mersine Bryan, a pediatrician at the hospital.

What kinds of flu shots are available?

There are a number of flu shots available. Flu vaccines from companies like Sanofi, GSK and CSL Seqirus are produced by growing flu viruses in eggs—a process that has been used for decades. There are other approaches, too: Sanofi’s Flublok is made synthetically in labs. The company said one study found patients who received this vaccine had lower hospitalization rates than adults who received other flu shots.

Federal health officials recommend people ages 65 and older receive higher-dose flu vaccines, like Sanofi’s Flublok or Fluzone High-Dose, and CSL Seqirus’s Fluad. Older Americans are at higher risk of flu-related hospitalization and death.

A new option is available for some adults this year. The Food and Drug Administration approved Moderna’s mRNA-based flu vaccine called mFlusiva this year for adults ages 50 and older. These shots can be made quicker than egg-based shots, a prospect virologists say could allow the company to more swiftly update its vaccine to better align with strains spreading during the respiratory season.

It would take federal authorities deciding to update this season’s flu shots for Moderna to roll out a newer vaccine later this year.

Dr. Anna Durbin, director of the Johns Hopkins Center for Immunization Research, said the hope with the mRNA vaccine is that officials could meet later in the year if any flu-vaccine strains need updating.

“You can make the vaccine very quickly,” Durbin said of mRNA shots. “If we do see that it is potentially more effective, then that may drive FDA to have another meeting so we can have the best flu vaccine ready for the public for the flu season.”

What does this mean for the rest of the flu season?

It is too early to predict how these early trends might impact the rest of the flu season, particularly in terms of severity of the virus, hospitalizations and deaths.

It is possible this early start to the season might lead the U.S. to experience two big waves of the flu season—one early in the winter and another around the start of spring, experts said.

This explanatory article may be periodically updated.