“It’s kind of cause for alarm,” Dr. Megan Berman, associate professor of internal medicine at the Sealy Institute for Vaccine Sciences at the University of Texas Medical Branch Health, tells Fortune. 

Experts say a flu shot is your best defense against the virus, but some people are opting to skip the vaccine, lamenting that they will get the infection anyway. 

So can you get the flu after receiving the shot? 

You can still get the flu, but it may not be as severe 

The flu vaccine usually reduces the risk of getting sick with the flu each year by between 40% and 60%, depending on the strain that circulates and how well the vaccine matches that strain. 

While the vaccine cannot guarantee you won’t get sick, it helps protect against severe illness that can lead to hospitalization and death, similar to the COVID shot. Data found this year’s flu shot reduces hospitalization risk by nearly 50%, per the U.S. Centers for Disease Control and Prevention (CDC). People with chronic conditions, like lung or heart disease, are at a higher risk of getting severely sick from the flu, so the vaccine is crucial in preventing hospitalization in this group. 

“When we talk about severe disease, we’re talking about people getting in the hospital, exacerbating underlying chronic illness, or dying from infection,” Berman says. 

In the 2019–2020 flu season, the estimates, the vaccine prevented 7.5 million flu cases, over 100,000 hospitalizations, and over 6,000 deaths. 

You can get the flu before antibodies kick in

It takes two weeks for the body to generate an immune response after the jab. If you get the flu within that two-week window post-vaccine, it’s likely because your body hasn’t guarded itself against the virus yet. Also, you may have already had the flu when you received the vaccine. 

You may have another virus with similar symptoms 

Flu and COVID share a host of symptoms, including cough, sore throat, fever, chills, and more. If you have flu-like symptoms and you’ve been vaccinated, it may be a different virus. Testing will give a clear answer, Berman says. She commonly sees patients who assume they had either COVID or the flu based on their symptoms but seldom have proof. She always tells them that it’s better to get tested instead of assuming a particular virus caused the symptoms—it can lead many to think they are immune from an infection they never had, she says. 

“We have the capability to test, and a lot of people were just assuming they have something without actually knowing,” she says. “There are a lot of these viruses. They mimic one another.” 

There are also different treatments for COVID versus the flu (Paxlovid and Tamiflu respectively). 

You might not have as strong of an immune response 

Those 65 and older, who make up the bulk of hospitalizations from the flu every year, don’t have as strong an immune response to the flu vaccine as those younger. The CDC recommends other vaccines for this population, some being higher-dose, according to the CDC: Fluzone High-Dose Quadrivalent vaccine, Flublok Quadrivalent recombinant flu vaccine and Fluad Quadrivalent adjuvanted flu vaccine. 

For people receiving immunosuppressive therapy, like for cancer treatment, or for those who have an autoimmune disorder, immune response to the vaccine may also be less effective. If you’re over 18, you can get the Flublok vaccine, which can work better for this group. 

If the pharmacy doesn’t have the particular flu vaccine you’re looking for, get the general vaccine that’s available. 

“Any vaccine is better than no vaccine,” Berman says. 

The flu shot cannot give you the flu 

Some worry whether the flu vaccine can actually give you the flu itself. However, the vaccine is made from an inactivated or killed part of the virus, so it’s not possible for the virus to replicate and cause illness. 

Your body may have side effects right after the shot like a fever, soreness, headache, nausea, or muscle aches, but it’s a sign of the body creating an immune response. The side effects subside within a few days. 

As people travel for the holiday season and gather indoors when it’s cold, it’s recommended to do your part and stay protected even if you’re not high-risk, Berman says.