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What to Know About the 2023 Flu Season 

Immunologist Jenna Guthmiller, PhD, talks timing, vaccines, testing, and more. 

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Written by Greg Glasgow on November 7, 2023

Flu season has arrived, bringing with it questions about vaccines, symptoms, testing, and more. During the 2022–23 flu season, it’s estimated that there were as many as 670,000 hospitalizations and 58,000 deaths from the virus in the U.S. 

For a better understanding of this year’s flu season and how to prepare for it, we spoke with vaccine researcher Jenna Guthmiller, PhD, an assistant professor in the Department of Immunology and Microbiology in the University of Colorado School of Medicine

Q&A Header

Is 2023 turning out to be an early or late flu season?

It's a little bit delayed. We've had some unusual seasonality lately because of COVID. The flu really diminished during COVID because people were taking precautions such as wearing masks, social distancing, and not going out in public. Because of that, flu dropped a lot. And the seasonality of it hasn’t returned to normal yet. For example, last year and the year prior to that, the flu season started incredibly early. Whereas right now, if we look at the number of cases that are being reported to the Centers for Disease Control, it’s still quite low. We may be shifting back to what it was like pre-COVID, when infections started to rise significantly in December, persist through January and February, then start to taper off in March and April. 

How many different strains of flu vaccine are there?

The current flu vaccines are quadrivalent, which means they protect against four different strains. These include two influenza A viruses, the H1N1 and H3N2 subtypes, which are cocirculating. The other major influenza type that causes severe morbidity and mortality is influenza B, so we incorporate two influenza B strains into the vaccine each year as well. 

So when you get a flu shot, you're getting vaccinated against four different strains?

Yes. We know, for instance, that immunity against H1N1 does not provide protection against H3N2, because they're so different from one another. It's important to have multiple types in the vaccine to provide the best protection. 

How do scientists discover which variants are going to be the most prevalent each year? Are they all variable from year to year?

The vaccine can be reformulated every year. Generally, around the world, there are H1N1, H3N2, and influenza B viruses that are constantly circulating and constantly evolving. Because of that, we do studies to understand: Does immunity from last year's vaccine provide protection against what is currently circulating in the southern hemisphere? If it does protect, then we don't reformulate the vaccine against that particular type of the virus. But if the virus that is currently circulating evades immunity against last year's flu vaccine, then we update the vaccine. 

Can you get the flu from the flu shot?

Absolutely not. There are two types of vaccines you can get. The first is an inactivated flu vaccine — it’s a live virus that's grown, but then it's treated to be inactive so it can no longer live. The vast majority of people receive an inactivated influenza vaccine. The other type of vaccine that people, largely children, receive is the live attenuated vaccine. This is a live virus, but it’s adapted to inefficiently infect humans. It can persist upward of a day to two days within the nasal tract, but it is typically cleared in healthy children with no to minor symptoms. Often the symptoms people experience following vaccination that feel like the flu is just your immune response trying to mount an effective immune response. 

Can you get the COVID-19 vaccine and the flu vaccine at the same time?

Yes, you absolutely can.  

If you have the flu, and you’re not vaccinated for it, can you get vaccinated while you’re still sick? Or should you wait until you're better before you get the vaccine?

There's no point in getting the vaccine if you actively have an infection, because it's too late for it to do anything. What a vaccine is intended to do is to inform your immune system that, ‘Hey, this is something you could be exposed to in the next year,’ so it can jack up the antibodies and mount a response. But if you're already infected, it's not going to do much, because antibodies take weeks to develop

Does having the flu make you immune to getting it again?

Generally, the immunity you generate from an infection is fairly good; it's just not as pleasant as getting a vaccine. If you want to make sure you don't get the next flu virus, it doesn't hurt to get the vaccine within three months after getting infected. One reason for doing that is that when you get infected, you’re only seeing one strain, such as H1N1, H3N2, or one of the B viruses. When you get vaccinated, you see all of them. You're boosting your immune response against all influenza, as opposed to just one type. 

Is there a benefit in going to the doctor to get tested for the flu?

We don’t have at-home tests for the flu like we do for COVID-19. One benefit of getting tested is so you can know if you have something you can spread to other people. The second benefit is that if you catch the infection early enough, there are antivirals that can be prescribed. Those give you the potential to beat it before it gets too bad, or at least minimize infection and spread.  

What's the best way to protect yourself from getting the flu?

Vaccination is the obvious one, then just being cautious of where you're going and what you're doing. If you're going somewhere you know there could be sick people, such as a clinic, be cautious and wear a mask.

Topics: Community, Vaccine

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Jenna Guthmiller, PhD