(NEXSTAR) — Flu vaccines are available again and a new COVID booster has been released, which means there’s a good chance you’ll be getting a shot or two soon if you haven’t already. 

But, maybe you’re hesitant to get either, knowing you might feel unwell afterward. Maybe you develop a headache or a fever, or maybe you feel achy and tired. Meanwhile, others who get the vaccine feel fine. 

Are you just very unlucky, or does this mean the vaccine is working for you but not them?

Unfortunately, there is no scientific explanation as to why some people experience side effects and others don’t, Dr. Rachel Scheraga, a pulmonary and critical care specialist at Cleveland Clinic, tells Nexstar. However, she says it’s likely the immune response triggered by the vaccine is the cause of the symptoms you may experience.

How do vaccines work?

Vaccines imitate the infection they’re made for, to catch the attention of your body’s natural defenses, the CDC explains. Vaccines have an antigen, which prompts your white blood cells to make antibodies, which are proteins that “identify and neutralize foreign substances.”

After your body fights off the simulated infection, you’re considered immunized. It doesn’t mean you can’t get sick — immunity takes time to develop, meaning you could become sick within a couple of weeks of getting the vaccine. But, those who receive the vaccine are less likely to become seriously ill or die from an infection than those who are not vaccinated.

So why do I feel ill after the flu shot or a COVID booster?

Vaccines simulate an infection, causing a response from your immune system. 

“The goal of that simulation of the vaccine is that you do have an immunologic response, right? We want the antibodies to be amped up, we want your system to get ready, to be on the lookout to detect and slow severe disease,” Dr. Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, tells Nexstar. 

She goes on, explaining that feeling a bit under the weather after getting the flu vaccine or COVID booster is OK: “It’s likely your immune system is working hard to make sure the vaccine does its job, which is to help prevent severe illness.”

The CDC notes that you may experience soreness, redness, or swelling where you got either shot for a few days. 

While you cannot get the flu from the flu shot, you could have some flu-like symptoms afterward. That includes a headache, a fever, nausea, muscle aches, fatigue, and potentially fainting. 

Pfizer-BioNTech and Moderna said those who receive their newest COVID booster shots may experience a number of symptoms, including tiredness, headache, muscle and joint pains, chills, fever, nausea, diarrhea, vomiting, dizziness, irritability, swollen lymph nodes, and generally feeling unwell. 

If you felt unwell after getting a previous COVID-19 shot, expect to feel the same again this time. That’s because the new doses, updated to target currently dominant strains, “are made in the same manner as the original vaccines,” Dr. Kristin Englund, an infectious disease physician at Cleveland Clinic, previously told Nexstar.

Your symptoms from either vaccine should last hours to days, according to Dr. Scheraga. If you have a fever, headache, or respiratory symptoms for longer than a day or so, she recommends seeking medical attention because you may have a true infection.

If I feel fine after getting a vaccine, do I still have immunity?

Research has shown those who experienced more symptoms after getting a COVID booster had a “greater antibody response” when compared to people who either felt nothing or only felt sore at the injection site.

However, if you were symptom-free, that doesn’t mean the vaccine isn’t working. 

“It is recognized that individuals have varied symptoms after vaccine administration, however lack of symptoms has not been correlated with protection from the virus,” Dr. Scheraga said. “For example, about 90% of individuals who are vaccinated against COVID are protected, while about half of individuals develop symptoms.”

Another serious but rare side effect to be aware of is Guillian-Barré Syndrome. Seen in the face and sometimes other parts of the body, Dr. Althoff says GBS can occasionally cause temporary paralysis. It has largely been reported among people ages 50 and older, according to the CDC.

What else should I know?

If you still get the flu despite getting the vaccine, don’t be alarmed. The flu vaccine is altered slightly every year to combat the viruses predicted to be the most common during the flu season, not all of the possible strains. However, as Dr. Althoff notes, those who receive the flu vaccine oftentimes have fewer days that they are “feeling really ill.”

“It’s about making sure that you’re not becoming so seriously ill that you’re hospitalized. That is our first and foremost priority, that we can avoid severe illness,” she says, adding that the COVID vaccines and boosters have also been found to reduce a person’s risk of being hospitalized or becoming seriously ill. 

Health experts say you can get the flu vaccine and COVID booster at the same time, but you may want to do it when you know you’ll have some free time, in case you do experience side effects.