Flu season in the US is heating up, driven by new subclade K variant

By Brenda Goodman, CNN

(CNN) — The latest data on respiratory illness in the United States shows that shoppers and merry-makers are spreading more than just holiday cheer: They’re also passing around germs. In many cases, it’s a new virus variant that’s been causing early and busy flu seasons in Asia, Australia and Europe.

The US is on the cusp of finding out what this flu variant, called subclade K, will do. For the week ending December 6 — the first full week after the Thanksgiving holiday — the proportion of doctor’s visits for symptoms including fever plus a cough or sore throat rose to 3.2%, according to the US Centers for Disease Control and Prevention.

That’s above a national baseline that infectious disease experts call the epidemic threshold. It’s a signal that flu season is officially underway, said Dr. Caitlin Rivers, who directs the Center for Outbreak Response Innovation at Johns Hopkins Bloomberg School of Public Health.

There are two ways to gauge the start of flu season, said Rivers, who has been closely tracking cases. By the calendar, it’s week 40, around the first week in October – but that date doesn’t necessarily mean flu is spreading widely.

Going by flu activity, “the 3.1% threshold marks the onset of flu season,” she said.

According to the latest CDC update, at least 14 public health jurisdictions are showing moderate to high flu activity, mostly concentrated in the Northeast. These include New York City, where activity is very high; New York and New Jersey, where it’s high; and Connecticut, Massachusetts and Rhode Island, which are in the moderate category. Flu activity is also high in Louisiana and Colorado. Other states and territories showing upticks are Georgia, South Carolina, Texas, Puerto Rico and Idaho.

This week also saw the season’s first reported death of a child from the flu in the United States.

“We know that from our surveillance, influenza activity is increasing in the US right now, and therefore that the time to get vaccinated for this season is right now,” said Dr. Tim Uyeki, chief medical officer of the CDC’s influenza division, said on a call with doctors this week.

Early signs of a rough flu season

There are signs that this season could be a bad one.

In New York state, flu cases began rising about two weeks earlier than usual, said Dr. James McDonald, commissioner of the New York State Department of Health, which keeps its own flu data, separate from the CDC’s. Flu hospitalizations have doubled each week for the past two weeks, he said.

“It’s here, and it announced itself quite loudly,” McDonald said.

Plotted on a graph, McDonald says, the curve is J-shaped, indicating a rapid rise in severe cases.

“It’s not common where you see a curve that goes like that,” he added. “What that means is, you’re seeing more people end up in a hospital quicker compared to last year. And we’re seeing more people get diagnosed with flu than last year.”

The state Department of Health sent an alert to hospitals on Monday, flagging the quick rise in cases and urging them to subtype specimens and take proper infection control measures.

The majority of flu viruses collected and analyzed by American laboratories during this respiratory virus season have been subclade K, a variant of a flu strain called H3N2, which itself is a variety of influenza A.

“Seasons during which influenza A(H3) causes the majority of influenza infections are typically characterized by more severe illness, particularly among older adults, resulting in more persons seeking care,” the New York alert said.

Other countries including Japan, China, the UK and Canada have also reported busy flu seasons.

In Australia, where subclade K was first detected, there were nearly half a million confirmed flu cases, smashing the record set just the year before.

Scientists often look to Australia, where flu season hits between April and October — winter in the Southern Hemisphere — for clues as to what could be coming to countries in the Northern Hemisphere.

Still, there’s an old joke public health veterans tell about the unpredictability of flu season: “If you’ve seen one flu season, you’ve seen one flu season.” Some experts caution that the US may not see the kind of activity as in other countries.

“We had a really bad flu season last year, and whether that means we will have a bad flu season this year, it’s hard to say,” said Dr. Jennifer Nuzzo, who directs the Pandemic Center at Brown University.

“It’s not typical that you have two back-to-back bad flu seasons,” Nuzzo said. “I have wondered if perhaps we might be somewhat more protected than some of these other countries because of what happened in the US last year.”

But with a new variant in the mix, the old patterns may not hold.

“I think it’s going to be a pretty bad flu season,” said Dr. Angela Rasmussen, a virologist at the University of Saskatchewan. Canada too has seen an early and brisk start to its flu season.

“I was supposed to actually be on a call right now, but the person who was supposed to be on the call with got the flu. School is closed, and their whole family has it.”

Vaccines vs. a new flu variant

This new variant wasn’t included in this year’s flu shots because it was identified after scientists had chosen the four strains to include this year.

The flu shots contain related strains, though, and they seem to be working pretty well against this variant.

A recent look at the efficacy of flu vaccines in the United Kingdom by scientists with the UK’s Health Security Agency found that even though antibodies generated by this year’s shots didn’t neutralize subclade K viruses as effectively as other strains, the vaccines still cut the odds of an emergency department visit or hospitalization for the H3N2 strains, which were dominated by subclade K, by almost 75% in children. The effectiveness was lower for adults, including those over 65. This years vaccines cut the odds of needing to go to the ER or hospital for the flu by about 30% to 40%.

Although that’s hopeful, it doesn’t mean the US will see the same results. Because of differences in how the vaccines are made, protection may not be as strong.

“I hope that what the UK is seeing in the early season data is what we can expect, but they mostly use cell-based vaccine, and we mostly use egg-based vaccine. A little worried that we’re not going to see that level of performance,” Rivers said.

Even with the flu shots’ limitations, any protection is better than no protection, said Dr. Alex Greninger, who is head of infectious disease diagnostics in the University of Washington’s Department of Laboratory Medicine.

It takes about seven days for vaccines to take full effect in the body. With flu season well underway, Greninger said, it’s best not to wait if you haven’t gotten this year’s vaccine yet.

“If you get vaccinated today, that thing will be home by Christmas in terms of your immune system,” Greninger said.

The protection from flu shots is especially important for kids, who’ve had less exposure to influenza than adults and therefore have less natural immunity against it.

“It’s a bigger deal to have gotten your vaccine than to have not gotten your vaccine when you’re a kid,” Greninger said.

Flu vaccination data collected by the CDC shows that just 38% of kids have gotten a flu shot this year, lower than at the same point for the previous six seasons.

The US set a record in child deaths from the flu last season. At least 280 kids died of influenza last fall and winter, the most deaths in a non-pandemic year since data was first collected in 2004. In cases where vaccination status was known, nearly 9 in 10 were unvaccinated.

How to stay safe from flu

Flu shots don’t keep people from getting infected. But getting vaccinated is the best way to cut the odds that you’ll need to see a doctor or be hospitalized if you do get the flu.

They also don’t completely prevent transmission — passing the flu virus to someone else if you are sick — but they do reduce the chances of that happening.

Because flu shots aren’t perfect, it’s a good idea to take more than one precaution to avoid getting sick.

Rivers, from Johns Hopkins, says she gets a flu shot every year and then watches the data. When infections start to go up, “by the holidays, for sure, I am wearing a mask in crowded places, like the airport,” she said. She also tries to avoid high-risk settings “like those indoor trampoline places my kids love.”

If you’re in a place where you can’t mask, such as eating dinner around a holiday table, opening a window and bringing in fresh air or using an air purifier can help keep people from getting sick.

“Yes, there’s a cost. You know, we’re all thinking about our heating bill these days, but it’s an intervention that I think is always useful. It’s particularly useful when it’s not practical or it’s not desirable to wear a mask,” Rivers said.

The flu hits hard. Symptoms including a high fever, cough, sore throat, body aches and chills tend to appear all at once rather than gradually.

If you get sick, it’s a good idea to get tested since there are antiviral medications that can reduce the length and severity of illness if you start them within the first 48 hours of the illness.

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