How vaccines work and what to expect when you get one

Vaccines help prevent serious illnesses and are an important tool for a lifetime of better health. Now, they also represent our best hope for getting back to experiencing moments like traveling for the holidays, visiting elder family members or new grandchildren, and even comforting those at the end of life.

But it’s critical that everyone understands how vaccines work and what to expect when getting one – especially with the development of the latest COVID-19 vaccines.

To understand how vaccines work, we first need to know how immunity works

Simply put, our immune system protects us from things that might make us ill. And when it’s introduced to something it doesn’t recognize as being part of our body systems, it responds by building an army of cells and antibodies to attack the potential threat.

This response can be triggered by things like:

  • breathing in particles,
  • having a scrape or puncture wound,
  • or by oral ingestion and so on.

Bottom line: Our immune system is a sophisticated defense system, but if this system stops running smoothly because it’s weak or doesn’t have enough time to respond adequately, we can get sick. And depending on the germ, we can get severely ill – or die – as we’ve seen with COVID-19 and older diseases like polio, meningitis and more. This is when vaccines can help.

Dr. Chris Andershock explains immunity in greater detail and offers tips to boost your immune system.

Here are 3 key facts about how vaccines work.

1. All vaccines help our immune system prevent dangerous disease, but they’re not all the same – because not all germs are the same.

Natural infection by many viruses can be serious and often deadly, but vaccines imitate infection so our immune system can learn how to better fight these germs:

  • Traditional vaccines mimic the way we develop immunity throughout our lives by using a piece of a dead or weakened germ that triggers our immune system to learn how to prevent infection. This immunity doesn’t happen overnight though – and some vaccines require more than one dose, so it’s possible to be exposed to a live virus before your immune system is fully prepared to fight it.
  • New messenger RNA (mRNA) vaccines for COVID-19 start from a different first step than traditional vaccines. Instead of injecting a harmless piece of the germ, they inject the instructions (mRNA) on how that piece is made, in order to trigger the same kind of immune response traditional vaccines do.

Bottom line: Each type of vaccine is specific to what it’s helping us build immunity against, and our immune systems need some time to create this protection after getting vaccinated.

2. The safety of vaccines (and their ingredients) are thoroughly studied before they reach the public.

The U.S. Food & Drug Administration (FDA) sets strict rules for the process of developing vaccines and testing them in volunteers through clinical trials.

Bottom line: For decades, vaccines and their ingredients have been studied extensively and given to millions of people – and like any drug or medication approved by the FDA, they’re continually monitored after distribution.

Read more about the development of the COVID-19 vaccine in this Q&A with Dr. Andrea Willis.

3. Complications from natural exposure to germs may be worse than possible side effects from vaccines.

It wasn’t long ago we were getting sick or dying from infections that are now prevented by vaccines. But vaccines are more effective overall when people who can get vaccinated, do so.

While true adverse reactions to vaccines are rare, they can happen. But more often than not, if people do experience side effects after vaccination, they’re usually mild. Like any medication, each vaccine can have different side effects, but it’s common to see:

  • A sore arm from the shot
  • Redness and swelling at the injection site,
  • or a low-grade fever or fatigue from the immune response to the vaccine.

Bottom line: It’s impossible to predict who will experience serious complications from an illness if we rely on natural infections to help us build immunity. But decades of widespread vaccination have put an end to some of those dangers by lessening the risk of illness after exposure.

Get the latest safety information from the CDC for each recommended vaccine.

Here are 3 things to expect when you get a vaccine

1. Shots can be unpleasant.

It’s common to be nervous about getting a shot. They can be painful for some people, especially if localized reactions like burning or stinging and soreness at the injection site occur.

Tip: Try an ice pack after your injection to ease any inflammation and light movement of your arm to help with soreness. 

2. You should prepare for uncomfortable side effects (which can vary by vaccine), but for good reasons.

Remember, you can’t actually get sick from vaccines because they don’t have a live germ, but it is common to experience side effects. This means your immune system is working to build immunity, but it can be uncomfortable and take a few days to go away. For example, COVID-19 vaccine side effects can include:

  • Pain or swelling on the arm where you got the shot,
  • and fever, chills, tiredness and headache.

Tip: Get to know possible side effects for each vaccine so you can manage them and be prepared if these reactions become more severe than expected.

3. You need to take care of yourself after you get vaccinated.

Your body needs a couple of weeks to build up its defense after vaccination. During that time, you may still become ill with the virus you were trying to prevent, in spite of getting the vaccine. You shouldn’t assume you’re protected immediately.

That means you need to take care of yourself and do things that promote health, which will also help you manage any experienced side effects:

  • Keep proper handwashing and hygiene standards.
  • Get plenty of rest.
  • Drink plenty of water.
  • Avoid excessive drinking.
  • Eat health-promoting foods.

Tip: Don’t hesitate too much, but it’s okay to plan your vaccination on a day you can take it easy and relax afterwards.

Need more advice?

Speak to a provider who knows your medical history if you have questions or concerns about vaccines based on your health status. Your friends and family may have good intentions, but they may not know your body like you and your doctor, so it’s important to speak to a provider who knows you well.

If you do decide to go online to learn more about vaccines, do seek reputable sources like the CDC, FDA or World Health Organization (WHO). BlueCross BlueShield of Tennessee members can also visit to get the latest on support for COVID-19.

Suzanne Corrington, M.D.

Dr. Corrington serves as medical director of care management services for BlueCross BlueShield of Tennessee where she directs teams and programs related to medical management and quality care initiatives. Before joining BlueCross in 2016, Dr. Corrington worked in emergency medicine, internal medicine for a private practice, and in clinic and hospital services.

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Filed under: Health Topics, Mind & Body


Dr. Corrington serves as medical director of care management services for BlueCross BlueShield of Tennessee where she directs teams and programs related to medical management and quality care initiatives. Before joining BlueCross in 2016, Dr. Corrington worked in emergency medicine, internal medicine for a private practice, and in clinic and hospital services.