How Do Vaccines Work, Exactly?

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Credited for causing drastic reductions in dangerous diseases like measles and polio, vaccines are widely heralded as one of the greatest public health achievements in modern history. But how do they work? How can a simple shot protect us from getting sick?

Vaccination trains your body's immune system to identify and fight specific diseases. It's is a lot like prepping your army before a war begins.

You ready your soldiers and teach them to detect and take out the enemy before they ever see a battle field. It sounds simple, but it's actually a highly complex and coordinated effort by the body's natural defenses. 

The Immune System

To grasp how vaccines work, it’s helpful to take a step back and look at the human body’s immune system. When pathogens like viruses and bacteria get inside our bodies, they go on the offensive. Left unchecked, they can multiply and spread, often resulting in us getting sick.

The human body has several lines of defense to help protect itself against diseases and fight off infections. Some parts of the immune system shield against or attack anything that’s not already a part of the human body, while others are much more targeted. Our skin, for example, is the first line of defense against germs. It’s, in essence, our body armor, dedicated to keeping germs from getting inside.

Cuts or scrapes can weaken that armor, allowing invaders to find a way in, and natural openings—like our nostrils or mouth—can be gateways, too. Chemicals like saliva in the mouth or gastric juices in the stomach can break down or kill bacteria, and fevers are the body’s way of turning the temperature up in the room in an attempt to kill or weaken invaders that only survive in cooler environments.

When an infection occurs, the body also starts making different kinds of white blood cells. These cells act like soldiers, coordinating attacks on the invader by seeking out specific targets known as antigens.

Antigens

An antigen is a piece or byproduct of a pathogen—like a protein found on the surface of a virus, for example—that the immune system looks for in the event of an infection. White blood cells and antibodies sniff out specific antigens and latch on, setting in motion an attack to take down the microbes and keep them from multiplying. When the battle is won, and the infection has cleared, our immune system’s cells remember what to look for in case it comes in contact with the pathogen again. Knowing what antigens the immune system detects and responds to is key to developing an effective vaccine.

Vaccination

Vaccines work a lot like a wild infection. In fact, to our body’s defenses, they look exactly the same. Vaccines are made up of antigens that are the same as or closely resemble antigens found on wild pathogens. When these vaccine antigens enter the body, they set off the same kind of alarms to create the same kind of white blood cells and antibodies needed to seek and destroy an invader.

 The body remembers what to look out for, so it can mobilize much faster if it ever comes across the invader again. Unlike a wild infection, however, vaccines won’t try to get you sick. They provide the benefits of an infection—that is, immunity—but with significantly less risk, and that's because of how they are made. 

Types of Vaccines

All use antigens to help stimulate an immune response, but not all vaccines are made the same way. Which antigens and how many varies, depending on the vaccine type and the disease it's meant to protect against.

  • Live, Attenuated Vaccines: These vaccines use a whole, live virus that has been “attenuated,” or weakened, in a way that makes it virtually harmless to people with healthy immune systems. Because it’s live, it can replicate and spread throughout the body just like a wild virus would. It’s the closest thing to a natural infection, and therefore is extremely effective at prompting a strong immune response. That being said, people with weakened immune systems—like transplant recipients or those undergoing cancer treatment—shouldn’t get these types of vaccines because even though they’ve been weakened, the body might not be able to fight them off. Examples include the MMR (measles, mumps, and rubella) and varicella (or “chickenpox”) vaccines.
  • Inactivated Vaccines: Similar to live vaccines, inactivated vaccines use the whole virus, only they aren’t alive. They are inactivated—or “killed”—in the lab. Because they can’t replicate and spread throughout the body, more doses are often required to get the same kind of protection spurred by live vaccines, and sometimes booster doses are needed to maintain immunity. Examples include the polio vaccine and many flu vaccine formulations.
  • Subunit Vaccines: Subunit vaccines use only select antigens, such as a piece of the germ or a bit of protein, to spark an immune response. Because they don’t use the whole virus or bacteria, side effects aren’t as common as with live or inactivated vaccines, but multiple doses are often needed to be effective. Examples include the pertussis (or “whooping cough”) component of the DTaP and Tdap vaccines.
  • Conjugate Vaccines: These vaccines are designed to protect against a group of bacteria that have a sort of sugar-like coating around them. During a wild infection, this layer hides the antigens from our immune system, so conjugate vaccines bind antigens to the coating so that the body’s defenses will know what to look for and are better at seeking and destroying the bacteria in the event of an infection. Examples include the meningococcal conjugate vaccine, which can help protect against a bacterium that can cause meningitis.
  • Toxoid Vaccines: Sometimes it’s not the bacterium or virus you need protection against, but rather a toxin that the pathogen makes when it’s inside the body. These types of vaccines use a weakened version of the toxin—called a toxoid—to help the body learn to recognize and fight off these toxins before they can cause harm. Examples include the tetanus component of the DTaP and Tdap vaccines.

Delivery Mechanisms

Vaccines are designed to be administered in highly specific ways to ensure maximum effectiveness and to minimize harm. Some vaccines, for example, are meant to be injected in the muscles at a 90-degree angle, while others should be given at a 45-degree angle in the fatty tissue between the muscle in the skin. For adults, that could mean receiving the shot in the arm, whereas babies often get the injections in their thigh muscles. Some vaccines aren't meant to be injected at all; instead, they should be administered via the nose or orally, and so on.  

How, when, and where a vaccine is administered is determined by extensive research, experience, and theoretical risks. A vaccine against a diarrheal disease, like rotavirus, might be given orally, for example, so that it can more closely mimic a natural infection. Vaccines given incorrectly could result in them being less effective or more likely to result in unnecessary side effects. 

It should be noted, however, that no vaccine is ever given intravenously—that is, directly into the bloodstream. 

Vaccine Testing 

Despite vaccine stories we may see on social media or myths we may hear from friends, vaccines are incredibly safe and effective at protecting against diseases. Throughout the development process, there are multiple tests vaccine candidates must pass before they ever make it to your doctor's office or local pharmacy. Prior to being licensed by the Food and Drug Administration in the United States, manufacturers have to prove that the vaccine is both effective and safe in humans. This often takes years and means being first tested in thousands of volunteers. Even after the vaccine is approved, it continues to be monitored for safety and effectiveness by researchers. 

After the vaccine is officially licensed, the research is then reviewed by the Advisory Committee on Immunization Practices—a volunteer panel of public health and medical experts—to determine whether it's appropriate to recommend that the vaccine be given. These recommendations are updated on an annual basis and take into consideration a wide range of data, including how safe and effective the vaccine has been shown to be. If at any point the benefits of the vaccine outweigh the risks, the panel rescinds its recommendation, and the vaccine is typically pulled from the market. Thankfully, this is very rare. 

The process is extremely rigorous. That's because unlike many medications, vaccines aren't typically designed to treat someone who is already sick. They are designed to protect your health by preventing diseases in the first place. As a result, vaccines are held to a higher standard of safety than many other medical products on the market, including nutritional supplements.  

Herd Immunity

Vaccination might be an individual activity, but its benefits—and ultimately, its success—is communal. The more individuals vaccinated in a given community, the fewer people who are susceptible to infections and therefore spreading the diseases. Many germs need humans to survive. But if enough people in a community are vaccinated, those germs have nowhere to go, and, therefore, they die off. This is how we, as a species, eradicated smallpox—not by getting single individuals vaccinated necessarily, but by ensuring whole communities were. 

Some individuals don't—or can't—create an immune response even after they've received a vaccine. Others are too young or too sick to get vaccinated in the first place. These individuals can't protect themselves from certain infections, but that doesn't mean vaccination can't help protect them. By making sure everyone who can be vaccinated safely does get vaccinated, a community can form a sort of barrier against disease that keeps the vulnerable among them safe. 

Harm Mitigation 

Even though a person is vaccinated, it doesn't mean that they are immune or fully protected in the event of an outbreak. Though some come very close, not all vaccines are 100 percent effective. That's because medicine is not one size fits all. 

Vaccination helps prep the body with the appropriate white blood cells and antibodies, but it doesn't necessarily guarantee lifelong immunity. These defenses can fade or be less effective overtime without the help of booster doses. The good news, however, is that because soldiers are already in place, if you do get sick with a disease you've been vaccinated against, your illness will likely be shorter and less severe than if you had not been vaccinated at all. 

Sources: 

Centers for Disease Control and Prevention. Epidemiology and Prevention of Vaccine-Preventable Diseases. Hamborsky J, Kroger A, Wolfe S, eds. 13th ed. Washington D.C. Public Health Foundation, 2015.

Centers for Disease Control and Prevention. Vaccine testing and the approval process

Centers for Disease Control and Prevention. Understanding how vaccines work

The History of Vaccines. Herd Immunity. The College of Physicians of Philadelphia. 

Vaccines.gov. Types of vaccines. U.S. Department of Health and Human Services.

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