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A Life-Saving Education: The Body’s Response to a Vaccine

Pfizer Vaccines
Pfizer UK

25th October 2021

Despite the global scientific expertise that goes into developing effective vaccines, immunisation is a surprisingly straightforward concept.


When an individual is vaccinated, the vaccine formula works to prepare the immune system to recognise a pathogen so that it is ready to fight that pathogen if it encounters it in the future.1 It’s like a fire drill for ‘the real thing’ - an infectious disease.

As the world undergoes a vaccination drive to help to control the spread of COVID-19, many people want to know more about how vaccines train the immune system.


A sequence of events

Traditional vaccines contain either: live weakened pathogens, such as in the measles or chicken pox vaccines; dead pathogens, such as in some flu and polio vaccines; or only parts of a pathogen, which are called antigens.2 When included within a vaccine, these components can trigger an immune response resulting in the body producing antibodies to help protect against the specific infectious disease that the vaccine was designed for.3

The newer mRNA vaccine technology works differently. With mRNA vaccines, the antigenic protein is not used in the vaccine to create an immune response. Instead, this new technology harnesses the natural process that cells use to make proteins. The vaccine formula contains genetic instructions that encode for the antigenic protein. These ‘instructions’ steer the body’s cells to produce the antigen once inside them.4 Once the mRNA strand in the vaccine is inside the body’s cells, the cells use the genetic information to produce the antigen which is then displayed on the cell surface where it is recognised by the immune system.4 




“After a vaccination—and once the antigen is recognised 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 defence, the innate immune response, is triggered almost immediately,” he adds.

After that, the immune response begins to ramp up defences 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 recognise it in the future,” says Gruber.

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



Initially, T-helper cells emit chemical messages that summon the key arms of the immune response:5

  • Cytotoxic T-cells recognise and kill virally-infected cells, an important early stage of the eradicating of the virus.6
  • B-cells produce virus-specific antibodies, which after binding to the foreign antigen, can inactivate the virus and mark the pathogens for destruction, mainly by making it easier for cells of the immune system to ingest them.7


The persistence of memory

Another important element of immunisation is that it helps produce memory B- and T-cells that are specific to the virus.8 Like football substitutes at the side of the pitch, these immune cells can be quickly deployed in the future to produce antibodies which stop the virus from getting ‘one up’ on 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. 



This tailored immune response does not develop immediately. “For someone who has not been previously exposed to the pathogen, it can take typically a couple of weeks to mount sufficient antibodies to provide protection,” says Gruber. That’s why, in some cases, people need to receive one or more additional doses of a vaccine, often known as a “booster”, to build or maintain a strong immune response.

For someone who’s been immunised, the next time they come across the pathogen, the immune response is faster. “That’s the basis of vaccination. Before the pathogen can replicate in the body, you've got antibodies 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 feeling tired. This doesn’t mean you’re infected with a disease.9 Rather, your body acts as if it’s fighting a mild form of the virus or other pathogen and produces a related immune response. Fever, for example, is one of the body’s main protective responses to fight a pathogen.10 There are of course other rarer side effects, such as allergic reactions which usually occur within minutes and should be dealt with immediately.11



By training our body to fight a specific infectious disease—without being exposed to the potentially dangerous form of the virus or pathogen—vaccines are a powerful tool in helping to provide immunity to a disease without getting the disease.3

And by receiving a vaccine, you could also be helping to prevent your family, community, and the population at large from getting the 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.



  1. WHO. How do vaccines work? – December 2020. Available at: [Last accessed April 2021]
  2. University of Oxford. Vaccine Knowledge Project, Types of vaccine – January 2021. Available at: [Last accessed April 2021]
  3. British Society for Immunology. How are vaccines developed? - February 2020. Available at: accessed September 2021] 
  4. PHG Foundation. RNA vaccines: an introduction – October 2018. Available at: [Last accessed April 2021]
  5. Britannica, The Editors of Encyclopaedia. 'Helper T cell' - June 2020. Available at: [Last accessed August 2021]
  6. British Society for Immunology. Immune responses to viruses – February 2020. Available at: [Last accessed April 2021]
  7. Alberts B, Johnson A, Lewis J, et al. (2002) 'The Adaptive Immune System'.  Molecular Biology of the Cell 4. Chapter 24. Available at: [Last accessed July 2021] 
  8. Centers for Disease Control and Prevention. The Immune System – The Body’s Defense Against Infection – August 2018. Available at: [Last accessed April 2021]
  9. U.S. Department of Health & Human Services. Vaccine Side Effects – March 2021. Available at: [Last accessed April 2021]
  10. Medical News Today. Why fever can be your friend in times of illness – May 2018 [Last accessed April 2021
  11. NHS. Why Vaccination is safe and important – 2021. Available at: [Last accessed July 2021]
PP-VAC-GBR-1771 / Oct 2021