How Vaccines Work

What is a Vaccine?

According to the World Health Organisation, a vaccine or a vaccination is a “simple, safe, and effective way of protecting you against harmful diseases, before you come into contact with them”. 

Vaccines reduce the risks of getting a disease by working with the body’s biological defenses to build protection prior to contact with the disease.

The human immune system is designed to remember. Once exposed to one or more doses of a vaccine, the body typically remains protected against a disease for years, decades or even a lifetime. Rather than treating a disease after it occurs, vaccines prevent us in the first instance from getting sick.

Vaccines work by acting as an imitation of an infection. The presence of a disease-causing organism in the body is used to engage the body's natural defenses. The active component of all vaccines is an antigen, any substance that causes the immune system to produce antibodies. 

In a vaccine, the antigen could be the weakened and/or inactivated version of a specific pathogen.

The Body’s Response

When a vaccine is administered into the body, the immune system fights off the mimic of a pathogen. The series of immune responses aimed at developing immunity without causing the disease itself allows the body to adapt to the disease, making it easier for the immune system to fight off the real pathogen if the body comes in contact with it.

Antigen-Presenting Cells (APCs) inside the body’s immune system consume the antigen and present its fragments on their surface. This is crucial for alerting other immune cells. 

Helper T-cells are activated by recognizing these fragments. They play an important role in orchestrating the immune response by signaling other immune cells to act.

B-Cells are then stimulated as the next step of the process to produce antibodies specific to the introduced antigen. These antibodies circulate in the bloodstream and help neutralize pathogens if the body encounters them in the future. After vaccination, some B-cells and T-cells become memory cells. These cells remain in the body for a long period of time and enable a faster and more effective immune response upon potential re-exposure to the same pathogen. This process is often referred to as "adaptive immunity" or "immunological memory".

Post-Vaccination Reactions / Side Effects

Following vaccination, some may experience mild symptoms such as fever or fatigue. Such side effects are generally due to the immune system's activation as it responds to the vaccine, indicating that the body is building protection against potential future infections. 

These side effects may be separated into Local and Systemic reactions. 

Local reactions are the most frequently reported side effects and include pain, redness, or swelling at the injection site. These are common and usually resolve within a few days. Itching or burning sensations may also occur but are typically mild. Larger local reactions such as swelling beyond the injection site peak within 24-48 hours and may appear more serious. 

Systemic reactions affect the body as a whole. This may include a fever, which is common and often starts within 24 hours of vaccination. Surveys have shown that about 15% of vaccine recipients experience mild fever directly after being vaccinated. Fatigue is also a common example of a systemic reaction to a vaccine. Many individuals report feeling tired or unwell after a vaccination. Headaches and muscle pain also occur in response to vaccines, affecting a significant number of recipients, according to studies. 

Bibliography

CDC (2024). Explaining How Vaccines Work. [online] Vaccines & Immunizations. Available at: https://www.cdc.gov/vaccines/basics/explaining-how-vaccines-work.html [Accessed 3 Feb. 2025].

Cedars-sinai.org. (2024). New COVID-19 Vaccines Hit the Market | Cedars-Sinai. [online] Available at: https://www.cedars-sinai.org/blog/new-covid-19-vaccines-hit-the-market.html [Accessed 3 Feb. 2025].

‌Gladstone.org. (2022). How Do Vaccines Work? [online] Available at: https://gladstone.org/news/how-do-vaccines-work [Accessed 3 Feb. 2025].‌

Hervé, C., Béatrice Laupèze, Giudice, G.D., Didierlaurent, A.M. and Tavares, F. (2019). The how’s and what’s of vaccine reactogenicity. npj Vaccines, [online] 4(1). doi:https://doi.org/10.1038/s41541-019-0132-6.‌‌

Who.int. (2024). item. [online] Available at: https://www.who.int/news-room/questions-and-answers/item/vaccines-and-immunization-what-is-vaccination [Accessed 3 Feb. 2025].