美国流感季今年来得更早：专家说别等了，现在就去打疫苗

Flu season is ramping up earlier than usual this year—though cases are still generally low.
Cases seem to be driven by a mutation that may make the virus more immune-evasive.
It’s OK to get your flu shot now, according to experts.

While you normally only hear about a surge of flu cases during the deep winter months, it’s still early fall and the U.S. is already seeing flu at higher-than-usual rates.

A recent report from the Centers for Disease Control and Prevention (CDC) notes that seasonal flu activity is “increasing” in many parts of the country, though it is still generally low. Based on the latest data, 3.6% of Americans tested for a respiratory issue were also positive for the flu.

“Flu season is happening much earlier than usual,” says Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo in New York. “The beginning of influenza season is always unpredictable—this is another example.”

What's driving this increase in cases and, more importantly, how can you stay healthy? Here's what you need to know.

Meet the experts: Asefeh Faraz Covelli, PhD, APRN, researcher and associate professor at The George Washington University School of Nursing and Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo in New York

Where are flu cases increasing?

The flu doesn't hit the country at the same level, at the same time. Right now, for example, the CDC points out that the highest flu activity is happening on the West Coast. California's test positivity rate for influenza currently sits at 8.8 percent, based on state data, while Alaska's public health department noted that flu cases started to "increase noticeably" in mid-August.

“There isn't one clear explanation for why it's happening earlier this year,” says Asefeh Faraz Covelli, PhD, APRN, researcher and associate professor at The George Washington University School of Nursing. “We need to keep watching the data to understand whether this early rise will translate into an earlier or more severe flu season overall.”

The earlier-than-usual season is largely driven by an H1N1 variant of the flu, with a D.3.1.1 HA subclade. That may be more immune-evasive than other strains, leading to the rise in early infections, Dr. Russo says.

But Covelli includes this important caveat: “An earlier start doesn't necessarily mean the flu is stronger.” Flu viruses change every year, and some of those changes may make it easier for the virus to spread in people who have some level of immunity already. "Researchers are watching the strains circulating this year, but it's too early to know whether these changes will make this flu season more severe," she says.

Flu symptoms to look out for

If you’re worried you have the flu, keep the following in mind. Flu symptoms don't really change from year to year, and these are the biggest ones to be aware of, per the CDC:

Fever
Cough
Sore throat
Runny or stuffy nose
Muscle or body aches
Headaches
Fatigue

When to get vaccinated

Public health agencies usually recommend getting vaccinated by Halloween. But given the season's early start, healthcare providers say you're fine to get the vaccine now. “If you haven't gotten your flu shot yet, I'd get it now rather than wait,” Covelli says. “September and October are generally the best time to get vaccinated, and it takes about two weeks for your body to build up protection.” Getting vaccinated now can help to protect you throughout the season, she says.

Also, while it's easy to assume an early start to flu season means it might also end sooner, that's not guaranteed. “Sometimes we have two waves due to influenza A and then influenza B," Dr. Russo says. "We will have to see.”