AILSA CHANG, HOST:

If you haven't done it yet, doctors and pediatricians are united in this plea - get a flu shot. A mix of COVID-19 and seasonal influenza could make for a particularly messy winter. And for now, of course, there is only a vaccine for flu. NPR's Allison Aubrey joins us to discuss.

Hey, Allison.

ALLISON AUBREY, BYLINE: Hey there, Ailsa.

CHANG: Hey. So when is the best time to get a flu shot?

AUBREY: Well, now, really.

CHANG: (Laughter).

AUBREY: Now through the end of October is a good time. You'll be inoculated before flu season sets in. And if you do it now, there won't be lines or hassles. Now, some people have heard that it's better to wait until later in the season so the immunity lasts a bit longer through spring. And experts tell me that there is some evidence that immunity can wane a bit, especially in older people, but it's marginal, Ailsa. And it's not true for every individual, so there's really no need to put it off. Go do it.

CHANG: OK.

AUBREY: I spoke to Michael Ison. He's an infectious disease physician at Northwestern University Feinberg School of Medicine.

MICHAEL ISON: This year in particular it's going to be especially important for everyone to get their flu shot. If you get sick with what you think may be the flu, you can't differentiate that based on symptoms from COVID-19. And so that'll mean those people are going to be seeing their doctor, asking for tests in a system that's going to be burdened with the additional burden of COVID-19 this fall and winter.

AUBREY: You definitely don't want the flu. The vaccine can really help prevent it, especially as the coronavirus continues to circulate widely. You definitely don't want to get both.

CHANG: No way. So it is possible then to get both flu and COVID this winter.

AUBREY: You know, there were cases of patients in the early months of the COVID pandemic that had both flu and coronavirus. And in general, Ison says people that have two viral infections at the same time tend to get sicker.

CHANG: Sure.

AUBREY: There were some instances of this documented in the U.S., in China, in Europe. And Ison says that, you know, these people appear to have more serious illness. He says the numbers are relatively low, particularly here in the U.S. because most of the coronavirus cases were documented after the flu season had started to wane. But yes, it is possible.

CHANG: Great. OK. Well, for all the science geeks out there, can you just tell us what is the makeup of the flu shot this year? Like, what strains are targeted?

AUBREY: Sure. That's right. The flu shot tends to change every year. It's based on the surveillance that scientists at the World Health Organization and elsewhere do to see which viruses are circulating. And based on this surveillance, they can make some predictions on what strains might be coming in the upcoming flu season, and they can match these expected strains. So I spoke to L.J. Tan. He's a scientist with the Immunization Action Coalition about this.

LJ TAN: So this upcoming season, the flu vaccine for this current flu season, the vaccines have actually changed a lot of the strains. There are three new strains in the quadrivalent vaccine. Both the type-A strains have been changed, and one of the type-B strains have been changed.

AUBREY: And he too emphasizes the importance of just getting a flu shot this year not only to protect yourself, Ailsa, but to protect everybody in your community.

CHANG: Absolutely. I will totally do this. So do we have any sense of how bad the flu season is going to be?

AUBREY: You know, it's hard to predict what will happen here in the U.S., but we can look to the Southern Hemisphere where the winter is ending. Some countries including Chile, Brazil, Australia appear to have had a very, very light flu season. Some of the data is preliminary, but L.J. Tan points to Australia, where there was this huge, huge drop-off in flu, more than 90%. Now, it could just be because people aren't travelling, and they're practicing social distancing. It could also be because there were a lot of people who got vaccinated. So, you know, two messages here - get the flu shot, and continue to remain vigilant with social distancing and masking.

CHANG: That as NPR's Allison Aubrey.

Thanks so much, Allison.

AUBREY: Thank you, Ailsa. Transcript provided by NPR, Copyright NPR.