What Are mRNA Vaccines? How They Work
On August 19, 2026, Moderna and Merck announced positive results from a large trial of a personalized mRNA cancer vaccine for melanoma, a type of skin cancer. The finding offered a new reason to pay attention to a technology many people first heard about during the COVID-19 pandemic.
The phase 3 study enrolled 1,137 people whose high-risk melanoma had been surgically removed. It tested an investigational vaccine called intismeran autogene, also known as V940 or mRNA-4157, alongside the immunotherapy drug Keytruda (pembrolizumab).
According to the companies' trial announcement, the combination improved outcomes measuring time without recurrence or death, and time without distant spread or death, compared with Keytruda alone. Detailed results were not released with the announcement, overall survival follow-up was continuing, and the vaccine remained investigational.
That progress raises a broader question: what are mRNA vaccines, and how can the same technology be used against both a virus and cancer?
What are mRNA vaccines?
mRNA vaccines give cells temporary instructions to make a protein, or pieces of proteins, that help the immune system recognize a particular target. The letters mRNA stand for messenger ribonucleic acid.
Your cells already use messenger RNA to make proteins. Scientists can produce mRNA in a laboratory with instructions for a selected target, then package it so those instructions can reach cells.
A useful way to picture mRNA is as a working copy of a recipe. The cell reads the recipe and makes the protein. The immune system responds to that protein, which is called an antigen when it serves as an immune target.
Some other vaccines supply the target protein directly or use a weakened or inactivated germ. An mRNA vaccine supplies the instructions for making the target instead.
How do mRNA vaccines work?
The process has four main steps.
1. The instructions reach cells
Many mRNA vaccines package the RNA inside tiny particles made from lipids, or fats. These protect the fragile RNA and help deliver it into cells.
2. Cells make the selected protein
In the cytoplasm, the area outside the cell's nucleus, protein-making structures called ribosomes read the mRNA. They assemble the protein specified by its instructions.
3. The immune system learns the target
Proteins or protein fragments are shown to immune cells. Depending on the vaccine, this can train antibody responses, T-cell responses, or both. Antibodies help recognize specific targets; some T cells can identify and kill infected or cancerous cells.
4. The instructions break down
The mRNA is broken down through the body's normal processes. Immune memory can last beyond the RNA itself, although the strength and duration of protection vary.
With an mRNA COVID-19 vaccine, the instructions are for a version of the coronavirus spike protein. They do not contain the whole virus and cannot cause COVID-19. MedlinePlus explains this process using the example of the immune system learning to recognize that protein.
How is a personalized cancer vaccine different?
The delivery method can be similar, but the target changes. A virus has proteins that can distinguish it from human cells. Cancer cells arise from the body's own tissues, which makes choosing an immune target more challenging.
Researchers can examine a patient's tumor for mutations that create unusual protein targets called neoantigens. A personalized vaccine carries instructions for selected targets from that person's cancer. The goal is to help T cells recognize cancer cells carrying those targets. The National Cancer Institute describes how these vaccines are designed.
This also explains why two patients with the same cancer diagnosis may receive different vaccine formulations. The disease name alone does not describe every mutation within their tumors.
| Preventive infectious-disease vaccine | Personalized cancer treatment vaccine |
|---|---|
| Trains the immune system against a target from a pathogen, such as a virus. | Trains the immune system against targets selected from an individual's tumor. |
| Aims to reduce future illness or its severity. | Aims to help control an existing cancer or reduce its return after treatment. |
| Many people can receive the same formulation. | The formulation is tailored to the patient. |
A treatment given after surgery is called adjuvant therapy. Even after all visible cancer has been removed, tiny amounts may remain. An immune response aimed at those cells could help prevent a recurrence.
Do mRNA vaccines change your DNA?
No. Vaccine mRNA does not enter the cell's nucleus or alter your DNA. Its job is to provide instructions for protein production outside the nucleus.
The recipe analogy has a limit: receiving a temporary recipe does not rewrite the cell's genetic instruction library. The body also does not need to keep the vaccine's RNA indefinitely for the immune system to remember a target.
What does the melanoma result mean?
The melanoma study is evidence for a particular vaccine-and-drug combination in a defined group of patients. It does not establish that every cancer can be treated with an mRNA vaccine, or that a preventive COVID-19 shot can substitute for a personalized cancer treatment.
Understanding the technology helps put future headlines in context. Look for which cancer was studied, whether the vaccine was used with another treatment, and what outcome improved. Fewer recurrences and longer overall survival are related questions, but they are not interchangeable.
This article provides general information and does not replace individual medical advice.