Think of the Earth as a vast pan and the seasonal influenza epidemic as the water inside it. Give that pan a nudge, and a wave hits the opposite side then bounces back towards you.
That analogy helps explain why we might not have a terrible flu season here this year.
Forecasts are by definition never entirely reliable but, when it comes to the flu, the experts say it’s best to not take any chances.
That’s particularly true this time of year as the United States and the rest of the Northern Hemisphere head into what might – or might not – be a bad flu season.
Top U.S. health officials say everyone should get a flu shot – and the sooner the better – whatever the predictions about the season’s severity and the vaccine’s effectiveness. That’s because getting vaccinated always gives some measure of immunity.
But, naturally, people want to know if the vaccine will work and what to expect in the coming months.
State and federal health officials say it’s impossible to predict how infectious, lethal and widespread the flu will be in any given year.
“I will never go on the record as predicting how severe flu season will be until I’m in the middle of it,” Ann Thomas, the Oregon Health Authority’s former public health physician, has said.
And the U.S. Centers for Disease Control and Prevention, the top public health agency in the country, puts it even more simply: “Influenza is unpredictable.”
And yet there is a method to influenza’s madness for Oregon Health & Science University infectious disease professor Marcel Curlin. Flu epidemics are incredibly complex, he acknowledged, and involve a multitude of factors, such as how immune people already are, virus structure, travel patterns and more.
“It’s a complex enough system that we can’t quite predict it with very high accuracy,” Curlin said.
But, he said, we know enough about how the flu works to know that what happens in the Southern Hemisphere is a good predictor of what happens here.
The vaccine used down south is similar, though not identical, to the one in the north. That’s why, he said, “it doesn’t take a genius” to predict that if the vaccine didn’t work well in Australia, the one in the United States won’t work well, either.
Influenza is a constantly mutating virus. That means people can’t prevent an infection with a one-time vaccine, and it means that, like the weather, experts’ predictions about a season’s severity become less and less reliable the farther out the prediction.
Influenza viruses bounce from the Southern Hemisphere during their winter – our summer – up to the Northern Hemisphere, when it’s our winter. While the virus will change by the time it gets here, its broad characteristics are less likely to change dramatically.
To illustrate the idea, Curlin used the Earth-as-a-pan example. The waves of water sloshing back and forth represent the flu.
“If you have a really big wave,” he said, “you will have a big wave coming back to you.”
The Oregon Clinic pulmonologist and influenza expert David Hotchkin gave a similar explanation. “We predict what flu we’re going to see based on what they’ve experienced, and then they predict what they’re going to see based on what we’ve experienced,” Hotchkin said.
Australia in 2017 had one of the worst flu seasons in at least eight years, according to a report published by the Australian government after the season ended. The United States followed up with one of its own worst seasons.
The key question, then, is how severe Australia’s 2019 season has been.
According to Australia’s most recent assessment, the clinical severity of this year’s epidemic, which is measured by the percent of patients who die or go to an intensive care hospital, was low. The vaccine Australians used matched well to the H1N1 subtype of the influenza A virus, but not to H3N2, which was the dominant subtype.
“Overall vaccine effectiveness appears good and as expected,” the Australian government’s Department of Health wrote in September.
Hotchkin said Australia had a “very weird” phenomenon this year, where the western side of Australia got hit hard and early, but the east side and New Zealand didn’t get hit as hard.
“Overall, it ended up not being more than is typically seen,” he said.
No matter how the flu season plays out, the more people get vaccinated, the fewer people will get sick or die. That, OHSU professor Curlin said, is why health authorities tell the public that the flu is unpredictable.
“The best outcome,” he said, “is if as many people as possible get vaccinated.”
— Fedor Zarkhin
desk: 503-294-7674|cell: 971-373-2905|@fedorzarkhin
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