I

f you have been planning to get a flu shot but just haven’t gotten around to it yet, the time to act is now — especially if you hope to be healthy at Christmas.

Flu season in North America is off to an early start this year, and may be on track to peak over the holidays, influenza experts tell STAT.

“If it continues to go up like it has the last couple of weeks, yeah, we could have a fair amount of activity right at Christmas,” said Lynnette Brammer, an influenza epidemiologist at the Centers for Disease Control and Prevention.

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Influenza viruses are wildly unpredictable, so even experts like Brammer — who has been monitoring flu for 26 years — are loath to make too many prognostications.

But certain patterns come into focus when people start falling sick from influenza. Here are some signs the experts are seeing.

We’re already off to the races

Flu season isn’t like hunting season. There isn’t a fixed start date.

Influenza viruses circulate in low numbers all year round, but at a point in the winter they mount a major attack, triggering a spike in infections that generally lasts a few weeks in a particular location, maybe spanning eight or so weeks nationally. That’s flu season.

Often activity peaks in January or February. This year it will likely crest on the early side.

Each week, the CDC issues a flu report that includes a chart demarcated with a dotted black line. When the percentage of people nationally who are going to their doctor for flu-like symptoms crosses that black line, it’s taken as a sign that flu season has started.

FluView Chart
CDC

Brammer — who leads the team that compiles the report — said this week’s report will likely show we’ve crossed that line. “I would not be surprised,” she said.

The most recent report, which was posted Monday, showed that Louisiana and Oklahoma already have widespread flu activity and some nearby states are heating up too.

Older adults may face a double whammy

There are a number of different types of flu viruses, and they don’t all hit equally as hard. The type that is responsible for most of the illness, at least at the moment, is called H3N2.

H3 flu seasons are typically more severe; these viruses are especially hard on older adults, who often have other health ailments that make them vulnerable to severe illness when they contract the flu.

Last year H3N2 viruses dominated in North America, and experts had hoped one of the other viruses might take a turn this year. But they also feared the severe flu season Australia had during our summer — a lot of H3N2, but also some influenza B — might presage what was in store for us.

So far, the surveillance data is pointing to the Australia pattern, said Dr. Danuta Skowronski, an influenza epidemiologist at the British Columbia Center for Disease Control.

In most flu seasons there is a peak of influenza A — H3N2 and H1N1 are flu A viruses — and then, on the tail end of the epidemic, B viruses start to dominate. Though H3 is the dominant virus right now, Skowronski said, British Columbia has recorded more flu B infections this autumn than it has for a decade. “This early influenza B concurrence with H3 is really unusual,” she said.

What makes that especially troubling is that the influenza B viruses currently making the rounds — they are called B Yamagata — are also particularly hard on older adults.

“So I’m a little concerned that if … this profile persists deeper into the influenza season, it may be doubly hard on older adults with that combination of B Yamagata and H3,” Skowronski said.

“I don’t like the signals. I don’t like the pattern that is emerging,” she added. “It’s consistent and it’s persistent in a direction that I would rather not see.”

This year’s flu shot isn’t going to be ideal

The H3N2 component in this year’s flu shot is identical to the one that was in last year’s flu shot. And last year, it only lowered the risk of infection with H3 viruses by about 34 percent.

In Australia — where the same component was used — the protection was lower still. A commentary published Wednesday in the New England Journal of Medicine pegged the preliminary vaccine effectiveness estimate for the H3N2 component of the vaccine at 10 percent. The problem is due to a major mutation that developed in the vaccine viruses for the H3N2 component when those viruses were grown in eggs, the way most flu vaccine is produced. (We wrote about it here.)

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(At this point, you might be wondering: Why bother getting the shot? Well, it protects against several types of flu viruses, and the other components generally are more effective than the H3 part. Also, some protection is better than none.)

Public health officials in U.S. and Canada have warned doctors that some of their patients who got flu shots will nevertheless contract influenza. They’ve urged them to prescribe antiviral drugs if that happens to a patient who has chronic illnesses. Flu infection can be very severe and even fatal in someone with heart or lung disease or who has a weakened immune system.

The Association of Medical Microbiology and Infectious Disease Canada has gone a step further, recommending doctors who care for high-risk patients consider giving them a prescription for an antiviral in advance, so they can fill it quickly if they become ill. Antiviral drugs should be taken within a 48-hour window of the onset of flu symptoms.

But, but, but, but …

Remember, flu is unpredictable. The season is starting — that’s clear — but the distribution of circulating viruses could still change.

Brammer is puzzled by some of the things she’s seeing in the U.S. data. For instance, there hasn’t been that much illness yet among children, who are generally considered to be amplifiers of flu. They get sick and infect their parents and grandparents, who infect their contacts.

The data suggest kids who have been sick have been catching H1N1, the other flu A virus. H3N2, not so much. “I don’t know what that means. But it’s sort of intriguing,” Brammer said.

Another thing to remember is that an early start to the flu season does not mean the flu season will be awful. That will depend on the viruses that are circulating, how many people got vaccinated, and how well the vaccine protects.

“It does look a little bit earlier this year,” said Brammer. “And that just may mean that we start a little bit earlier and end a little bit earlier. It doesn’t necessarily correlate with severity or anything like that.”

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