(CNN) — Welcome to this year's flu season.

This year's flu strain has already begun spreading across the United States, according to new data from the US Centers for Disease Control and Prevention. There have been at least 880,000 cases of influenza, nearly 7,000 hospitalizations and, tragically, 360 deaths from the flu this fall, including one pediatric death. Not since 2009, during the height of the H1N1 swine flu pandemic, have there been this many cases of influenza so early in the season.

Despite these numbers, many people wonder if the flu is really that serious of an illness. What's the benefit of the vaccine, especially if some people may still get the flu despite being vaccinated? Could you get the flu from the vaccine? If you get the Covid vaccine, do you still need the flu vaccine?

To guide us through these questions and more, I spoke with CNN Medical Analyst Dr. Leana Wen, an emergency physician, public health expert and professor of health policy and management at the George Washington University Milken Institute School of Public Health. She is also the author of "Lifelines: A Doctor's Journey in the Fight for Public Health."

CNN: Is the flu a serious illness? What symptoms do people experience?

Dr. Leana Wen: It certainly can be serious. The CDC estimates that flu resulted between 9 million and 41 million illnesses, 140,000 to 710,000 hospitalizations, and 12,000 to 52,000 deaths annually across the US between 2010 and 2020.

Symptoms of the flu include fever, muscle aches, headaches, fatigue, coughing and a runny nose. A lot of people recover within several days, but some may still be feeling unwell as long as 10 days to two weeks after the onset of their symptoms. Some will develop complications, including sinus and ear infections, pneumonia, and inflammation of the brain. The flu can also exacerbate underlying medical conditions — for example, people with chronic lung and heart diseases can see their conditions worsen due to the flu.

Even generally healthy people can become very ill due to the flu. However, those particularly susceptible to severe outcomes include those 65 and older, young children under 2, pregnant people and people with underlying medical conditions.

CNN: What's the benefit of the vaccine, especially if some people may get the flu despite being vaccinated?

Wen: The flu vaccine does two things. First and most importantly, it reduces your chance of severe illness — that is, of being hospitalized or dying. Second, it can also reduce your likelihood of getting sick from the flu at all.

In a sense, this is not too different from the Covid-19 vaccine. The most important reason to get vaccinated against both the flu and the coronavirus is to prevent severe illness. New data released in the CDC's latest morbidity and mortality report shows this year's flu vaccine reduces the risk of hospitalization by about 50%. A 2018 study found that people vaccinated against the flu were 59% less likely to be admitted to the ICU due to influenza when compared with those who were unvaccinated.

The vaccine's effectiveness can vary depending on how well matched the vaccine is to circulating influenza strains. The CDC cites vaccine effectiveness against "medically attended illness" anywhere from 23% to 61% depending on the year and vaccine-to-strain match. It's true, then, that you could get the flu vaccine and still contract the flu. But the vaccine does reduce your chance thereof — and, crucially, it reduces the likelihood that you could end up very ill.

Another thing to consider is that there are a lot of other viruses that can cause flu-like symptoms. The flu vaccine helps protect against viral infections caused by influenza, but there are a lot of other causes of viral syndromes, including adenovirus, rhinovirus, parainfluenza and others. These other viruses spread easily, too, and there aren't vaccines against them. I often hear patients say they once got the flu the same year they had a flu vaccine, and that's why they don't want to get vaccinated again. But when I ask them whether they were actually diagnosed with the flu or just had flu-like symptoms, they would say the latter.

CNN: Should children and pregnant people also get the flu vaccine?

Wen: Absolutely. These are groups particularly vulnerable to severe outcomes, so it's very important they receive the flu vaccine.

One study found the flu vaccine reduces children's risk of severe life-threatening influenza by 75%. Another found it reduced flu-related emergency department visits in children by half.

Similar results are found in people who are pregnant. Not only does the flu vaccine protect the pregnant person, if the vaccine is given during pregnancy it also helps protect their baby from the flu for the first few months of its life. That's important, because the flu vaccine is not available to babies until they are 6 months or older.

CNN: Could you get the flu from the vaccine?

Wen: No. The flu vaccine is an inactivated vaccine, which means it does not contain the live virus and therefore cannot cause the flu. It is also a very well-tolerated vaccine, with the most common side effect being discomfort at the injection site that is gone after a day.

CNN: If you got the Covid-19 vaccine, do you still need the flu vaccine?

Wen: Yes. Different vaccines target different viruses. The Covid vaccine helps to protect against Covid, but does not protect against the flu, and vice versa. You can receive the Covid vaccine (or bivalent booster) at the same time as you receive the flu vaccine, just in a different injection site.

CNN: Some people have been waiting until later in the flu season to get the flu vaccine. Is this a good idea?

Wen: At this point, no, because it's now clear this flu season is starting earlier than usual. Cases are already high, and it takes about two weeks to reach optimal immune protection after vaccination. I'd encourage people who have not yet received the flu vaccine to get it now.

CNN: What should people know about treatments for the flu?

Wen: Most cases of the flu can be treated symptomatically, meaning patients get rest, hydration and treatment for symptoms that come up — such as fever-reducing medicines like acetaminophen or ibuprofen. There are also antiviral treatments available. These are really important for people at high risk for severe influenza complications and/or who are very ill. The earlier such treatments are started, the better. An oral medication, oseltamivir (Tamiflu), can also be given to non-high-risk patients, too, within 48 hours of the start of their illness.

I'd encourage everyone to have an influenza plan, the same way they should have a Covid plan. Ask your doctor in advance if you should receive Tamiflu or another antiviral treatment. Know how you can get testing and where you can access treatment, including after hours and on weekends.

CNN: How can people prevent catching the flu?

Wen: The flu is primarily spread through droplets — if an infected person coughs or sneezes, these droplets can land on someone else nearby. It's also possible that the droplets land on a surface, from which someone gets infected after touching it and then touching their nose, mouth or eyes.

We can help to reduce flu transmission by staying away from others while symptomatic. We should all cough or sneeze into our elbow or a tissue, and wash our hands frequently, including after touching high-contact surfaces. Individuals particularly vulnerable to severe outcomes should consider wearing a mask to reduce their chance of contracting viral illnesses like the flu. And, of course, get vaccinated!

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