A common misconception about vaccines stems from this thought: “I got vaccinated, but I still got sick! The vaccine didn’t work.”

I understand the frustration behind that statement, but at its heart is a fundamental misunderstanding of how vaccines work and what they’re designed to do.

Let me be clear from the start: Vaccines aren’t impenetrable shields that make you totally immune to an illness. But that doesn’t mean they’re ineffective. The key here is understanding what they’re supposed to do, and then evaluating how well they accomplish that goal.

Let’s start by taking a look at how vaccines work and what they’re actually good for.

How Vaccines Work: Mechanism and Goals

The goal of vaccination, whether for a virus, bacteria, or toxin, is to enable your immune system to recognize that illness when you’re exposed to it.

Your immune system has an excellent memory, so once it encounters a pathogen, it stores a “blueprint” of that bug and remembers how to fight it. If you’re exposed to that pathogen again, your body is able to use the stored memory to respond more quickly and efficiently than if it had never seen the germ before.

Ideally, your immune system will respond so effectively that you won’t get the illness at all. Other times, it will respond so that the severity of the illness is significantly less than it would have been otherwise.

So even if you’ve been vaccinated, it’s not uncommon to feel something when you’re exposed to a pathogen like influenza or COVID. But it will be a much more subtle and short-lived experience than if your immune system were encountering the pathogen for the first time.

How Vaccines Prepare Your Immune System

So exactly how do vaccines work to prepare your immune system for future attacks? They have to trick your body into believing an actual infection is occurring.

Depending on the specific disease you’re trying to gain immunity against, vaccines contain different ingredients to accomplish this, such as:

• A protein from the pathogen
• A dead version of that pathogen
• A synthetic, manufactured portion of the pathogen
• A live but weakened (attenuated) pathogen

The process of developing vaccines is rigorous and lengthy. It starts with research in labs, moves to testing in animals (usually lab rats), and ultimately involves trials with people to see how the exposure is tolerated over time.

Vaccines for viruses, which often mutate rapidly, are tricky. It’s important to make viable vaccines ready for the most up-to-date mutations of common viruses, like influenza and COVID, but the long production process means some educated guesswork is involved.

Breakthrough Infections: Why Vaccinated People Can Still Get Sick

Now, back to that statement of vaccine confusion: “This vaccine didn’t work for me; I still got sick!” Two separate things could be happening here, and it’s important to understand both.

Immune Response vs. Actual Illness

First, it’s impossible to get the illness a vaccine protects against from most vaccines.

For example, just the protein of a pathogen can’t infect you, nor can a dead bacterium or virus. The exception comes from live attenuated vaccines, in which a weakened version of the pathogen is included in the vaccine. Even then, however, the dose is carefully controlled.

When someone gets a vaccine and then feels ill shortly afterward, most of the time, they’re simply experiencing their immune system expressing itself due to the vaccine. It feels like a (very subdued) illness, with fatigue, body aches, and pains, but it’s actually just the immune system being stimulated. Similar to how your immune system protects you, that response is a sign your body and the vaccine are working together as intended.

True Breakthrough Infections

The second scenario is that you get vaccinated, time passes, and then you actually contract the illness you were vaccinated against. We call this “breakthrough infection,” but how does it happen?

Vaccines aren’t 100% foolproof, and several factors can lead to breakthrough infections:

Your immune system doesn’t respond as expected. Some people’s immune systems simply don’t mount the proper response to a vaccine, meaning they don’t store a helpful “memory” of the pathogen against future attacks.

The vaccine amount wasn’t enough for you. Certain people require larger amounts of vaccine to develop the same protective response.

You have overwhelming exposure. The vaccine might be working perfectly, but perhaps the amount of bacteria or virus you’re exposed to overwhelms your immune response. In these cases, however, even though you get sick, you would likely have gotten even more sick without the vaccine giving your immune system a head start.

Viral mutations. This is particularly relevant with quick-mutating viruses like influenza. Each year’s influenza vaccine differs because each year, the virus mutates.

Since it takes so long to formulate, test, and produce a new flu vaccine, the vaccines have to be based on history, specifically, which flu strains were circulating last year or in other parts of the world that hit flu season earlier than the U.S. We use this historical data to infer which mutation is most likely to cause illness in the upcoming flu season, and we target the new vaccine toward that strain.

The difficulty is that by the time the flu vaccine reaches you, the virus may have mutated since the vaccine was conceptualized one year prior. Even with mutations, though, you often still get partial protection, especially from severe illness.

Herd Immunity and Our Community Responsibility

One of the most important concepts regarding vaccines is something we call herd immunity.

If a certain threshold of the population, often somewhere around 85–95%, becomes immune to a certain illness, then the possibility of community spread significantly decreases.

In other words, you don’t need to vaccinate 100% of the community to get a significant protective response throughout that community.

This is important because a certain percentage of people can’t get vaccines, either due to allergic reactions in the past, intolerance, or because they’re immunocompromised and certain vaccines would be dangerous for them. There will also always be some people who simply don’t get vaccinated.

Now, you might think, “If 85–90% of people are getting vaccinated, why should I bother? I’ll still be protected.” But this thinking misses a crucial point about our responsibility to one another.

As a community, we can protect the vulnerable among us by being part of that immunity threshold. When you choose vaccination, you’re not just protecting yourself; you’re protecting those who cannot protect themselves.

Special Considerations for Immunocompromised Patients

For patients who are immunocompromised, there’s an absolute contraindication to live attenuated vaccines. These folks need to be extra careful about practicing healthy lifestyle choices and avoiding people with active infections, especially during seasons when particular illnesses are more likely to occur.

They also need a backup plan for when they’re exposed. For example, if someone in their home gets diagnosed with the flu, the immunocompromised person can take antiviral medications preventatively.

For bacterial infections like pneumococcal pneumonia, while you wouldn’t take antibiotics preventatively, you can be treated much more quickly at the onset of symptoms if your physician knows there’s been an exposure in your home.

If you’re immunocompromised, having these conversations with your doctor is essential.

How Vaccines Work and Why They Matter: Final Thoughts

Vaccines dramatically reduce risk; they don’t eliminate risk entirely. And honestly, that’s okay. We can’t have perfection, but we can have less illness and less severe illness when breakthrough infections do occur.

When you understand how vaccines work and what they’re intended to do, you realize they’re one of the most effective public health tools we have. They’ve saved countless lives and continue to protect our most vulnerable community members.

The single most important takeaway I want you to remember is this: Be responsible. Getting vaccinated isn’t just for yourself, but for those who can’t be vaccinated. Discuss your specific case with your doctor, including your personal health history, your family’s health, and any concerns you have.

Vaccines aren’t perfect, but they’re remarkably effective. And in a world where we can’t eliminate all risk, dramatically reducing it is something worth appreciating.