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What Happens After Vaccination: From Side Effects to COVID Infection

Medically reviewed by Regina Victoria Boyles, MD · Pediatrics

Written by Lorraine Bunag, R.N. · Updated Jan 28, 2023

What Happens After Vaccination: From Side Effects to COVID Infection

As vaccination rollout continues, questions about its effects abound. Why do some people experience side effects while I don’t? Does it mean it’s not working for me? How come some vaccinated people still get COVID-19? Does immunity decrease after the COVID vaccine, or does it mean the shot wasn’t effective? Let’s look into the answers here.

Side effects and why they happen

It’s common for people who received the shot to report mild side effects like sore injection site, fever, and fatigue. But why do they happen?

To understand why they occur, we need to briefly discuss what’s inside the vaccine. The vaccine contains an “instruction” manual for our body to make the SARS-CoV-2 spike protein, to which our immune system would react to.  

Once our body senses the spike protein, they start making the antibodies, which also works against the real SARS-CoV-2 virus. You see, our immune system cannot easily distinguish between the virus and the spike protein.

You experience side effects because your immune system responds to the SARS-CoV-2 spike protein. Reports say you might have a stronger reaction after the second dose because your immune response is more robust, having already made some antibodies from the first shot.   

No side effects, no problem – experts

If you didn’t experience side effects, you might wonder if you received a real vaccine. You might also feel like there’s something wrong with your immune system. And because you think your immune system didn’t respond, you might ask: does immunity decrease after the COVID vaccine?  

Experts assure us that all is well. Just because a person does not experience side effects, this does not mean that the vaccine is not working nor does it mean there’s something wrong with your immunity.

Health authorities emphasize that each person reacts to vaccines differently. Factors such as general health, age, gender, diet, and medications also contribute to your vaccination experience.

Does immunity decrease after the COVID vaccine?

It’s a difficult question to answer, seeing that many factors affect the level of someone’s immunity.

Does the immunity decrease specifically after receiving the COVID vaccine? Chances are, it doesn’t. Previous reports about non-COVID vaccines indicate that a shot does not overwhelm the immune system.

In fact, should an infant accidentally receive 11 vaccines at the same time, only 0.1% of their immune system would be “used up.” And “used up” is a loose term since we are capable of replenishing our immune cells.

To keep your immunity at the optimum level before and after vaccination, you need to take care of your general health. Get adequate rest and sleep, eat a healthy, balanced diet, and perform regular exercise. Also, steer clear of things that negatively affect your well-being like too much stress and smoking.

Why do some people get COVID after vaccination? 

Instances when vaccinated people still contract COVID-19 are called “vaccine breakthrough cases”. While these cases are rare, they are still possible. The US Centers for Disease Control explains that no vaccine prevents an illness 100% of the time.

Some people could get the infection and remain asymptomatic. Others might fall sick, need hospitalization, or even die.

Another possible reason why some people still get COVID-19 after vaccination is they got the virus right before or within 2 weeks of getting vaccinated. Remember that most vaccines need about 14 days after the second dose before you become protected.

Finally, let’s not forget that there are other variants of interest and concern. While initial studies show that current vaccines work against most variants, some might still cause an illness after vaccination.

Key Takeaways

Side effects do not directly tell you if the vaccine is working. You may or may not have them and the vaccine would still work. All vaccines work.

Does immunity decrease specifically after the COVID vaccine? We need more studies to answer this question. But reports about infant vaccines indicate that a jab doesn’t overwhelm the immune system. 

Remember that getting any brand of vaccine would most likely protect you against severe COVID-19.

Stay updated about COVID-19 here


Hello Health Group does not provide medical advice, diagnosis or treatment.

Medically reviewed by

Regina Victoria Boyles, MD


Written by Lorraine Bunag, R.N. · Updated Jan 28, 2023

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