Despite the complex technology and global scientific expertise that go into building a safe and effective vaccine, when it comes down to it, vaccination is a simple concept.

Vaccination trains the immune system to recognize a pathogen and to be prepared to help fight it if it encounters this enemy in the future. It’s like a boot camp to learn to fight an infectious disease.

As the world awaits a vaccine to help control the spread of COVID-19, many people may be wondering how this immune system “training” works.

A cascade of events

In traditional vaccines, the vaccine formula contains a version of a virus, bacteria or other pathogen. It can be live, but weakened, such as in the measles or chicken pox vaccines; dead, such as in the flu or polio vaccines; or contain only portions of the pathogen—called antigens. Antigens are proteins that are foreign to the human. When part of a vaccine, they are capable of sparking the immune system to develop antibodies against it.

With RNA vaccine technology, the antigenic protein is not used in the vaccine to create an immune response. Instead, RNA vaccine technology harnesses the natural process that cells use to make proteins. The vaccine formula contains genetic instructions that encode for the antigenic protein. Once inside the body’s cells, these instructions guide the cell’s internal machinery to create the antigen. The cells then take small bits of the antigen and display it on their surface receptors, issuing a warning alarm for the immune system. Immune cells known as “antigen presenting cells” are specialists at triggering immune responses, but other types of cells can also present antigens.

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“After a vaccination—and once the antigen is recognized as foreign by surrounding cells—it sets off a cascade of events in motion that may help provide protection against disease,” says Bill Gruber, Senior Vice President of Vaccine Clinical Research and Development at Pfizer. “The body’s first line of defense, the innate immune response, is triggered almost immediately,” he adds.

Then, the body’s adaptive immune response begins to ramp up defenses that are specific to the pathogen. “It moves from an initial response that says, ‘Hey, there is danger,’ to one that now starts looking at the specifics of the pathogen to kill it off and to recognize it in the future,” says Gruber.

In the below example, we depict the body’s expected immune response to a virus or a vaccine that “mimics” the virus.

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Initially, T-helper cells secrete chemical messages that summon the key arms of the immune response:

  • Cytotoxic T-cells recognize and kill virally-infected cells, an important part of the early elimination of the virus.
  • B-cells produce virus-specific antibodies, which can neutralize the virus and mark it for destruction. If we’re exposed to the virus in the future, these antibodies can potentially protect us in two ways: They prevent the virus from entering our cells and they mark viruses and virus-infected cells for other immune cells to come in and destroy them. The level of these antibodies circulating in the body declines over time.

The persistence of memory

Another critical component of vaccination is that it helps produce memory B- and T-cells that are specific to the virus. Like an army reserve force, these immune cells can be quickly activated in the future to produce antibodies to stop the virus from invading your body. And unlike antibodies, these cells persist. “Once you stimulate memory cells for a particular antigen, they can remain with you for life,” says Gruber. For some pathogens, however, memory cells can be short-lived.

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But developing this tailored immune response is not immediate. “For someone who has not been previously exposed to the pathogen, it can take typically a couple of weeks to mount sufficient antibody to provide protection,” says Gruber. And sometimes people need to receive one or more additional doses of a vaccine, commonly known as a “booster,” to build a strong immune response.

For someone who’s been given a vaccine, the next time they encounter the pathogen, the immune response is sped up. “That’s the basis of vaccination. Before the pathogen can replicate in the body, you've got antibody generated to prevent that from happening.”

Finally, when people receive a vaccine, some may experience some mild symptoms for a day or two, such as a fever, chills, or fatigue. This does not mean that you’re infected with a virus or other pathogen. Rather, your body acts as if it’s fighting a mild form of the germ and produces a related immune response. Fever, for example, is one of the body’s protective responses to fight a pathogen. “Fortunately, these events for vaccines are typically mild or moderate and infrequent,” says Gruber.

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By training our body to fight a specific infection—without being exposed to the potentially dangerous pathogen—vaccines are a powerful tool to safely develop immunity to a disease.
And by receiving a vaccine, you’re also helping to protect your family, community, and the population at large from the spread of an infection. This is the concept of herd immunity. “The more people who engage responsibly as a community to get vaccinated, the better the protection is,” says Gruber.