How do vaccines work?

3 min read

Vaccines represent one of the greatest achievements in public health, saving millions of lives every year by preparing the immune system to fight infections without causing the disease [1]. They work by teaching the body to recognize and neutralize pathogens, such as viruses and bacteria, in a safe manner [2].

The immune system in simple language

Our body has natural barriers, such as the skin, mucous membranes, and nasal cilia, which reduce the risk of germs entering. When a pathogen invades, the immune system activates defenses, producing antibodies – like "soldiers" that recognize and fight the invader – and memory cells that "remember" how to defend themselves in the future [3]. Vaccines exploit exactly this mechanism, stimulating a specific immune response without causing the actual disease [2].

What happens in the body after we get a vaccine?

After administration, the vaccine introduces an antigen – a weakened or inactivated part, or instructions to produce a protein from the pathogen. This triggers an immune response: the immune system generates antibodies and memory cells, preparing the body to recognize and fight the real pathogen in a future exposure more quickly and effectively [1]. The vaccine does not cause the disease; it only teaches the body how to defend itself [2]. Some vaccines require multiple doses, with intervals of weeks or months, to develop long-lasting antibodies and strengthen immune memory [1].

The main types of vaccines and their differences

There are different technologies for creating vaccines, each with a different way of acting:

• Inactivated virus vaccines: Use the dead or inactivated pathogen. They present parts of the agent so that the immune system learns to recognize it without risk of infection [1].

• mRNA vaccines: Provide genetic instructions (messenger RNA) so that the body's cells temporarily produce a protein from the virus, such as the spike protein of SARS-CoV-2. This activates the immune response without using the live virus [4].

• Viral vector vaccines: Use a modified and harmless virus as a "delivery vehicle" to transport genetic instructions from the target pathogen. The vector enters the cells and causes them to produce the antigenic protein, triggering immunity [4].

Other types include attenuated (weakened) viruses, but the common principle is to activate the immune system to protect against infections [5].

Side effects: what to expect and how common are they?

Yes, vaccines can have side effects, which are generally mild and signal that the body is reacting and building defenses. The most common include pain at the injection site, low-grade fever, fatigue, and headache. Severe effects are very rare and are strictly monitored by surveillance systems [3]. The WHO estimates that vaccines save between 2 to 3 million lives annually, with benefits that far outweigh the risks [3].

How are vaccines tested before being approved?

Before reaching the public, vaccines undergo rigorous phases of clinical trials that assess safety, efficacy, and quality. These tests involve thousands of volunteers and are supervised by regulatory agencies, such as Anvisa in Brazil or international equivalents. Only after scientific proof do they receive approval for use [6]. Monitoring continues even after approval to detect any rare events.

What is herd immunity and why does it matter?

Herd immunity occurs when a large part of the population is vaccinated, reducing the circulation of the pathogen and indirectly protecting those who cannot be vaccinated, such as infants or people with certain health conditions. It matters because it prevents outbreaks and protects entire communities, especially in highly contagious diseases [1].

Conclusion

Vaccines activate the body's immune system to protect against infections and diseases safely and effectively [5]. They create immunological memory that helps prevent diseases, representing one of the most important tools of public health [7]. Staying up to date with the vaccination schedule and seeking information from reliable sources, such as the Ministry of Health, the WHO, and PAHO, is essential to protect yourself and others [8]. Getting vaccinated is an act of collective care.