Let's start with what DNA is. DNA has all the recipes it takes to make the proteins that build an individual and make them work. Proteins include everything from muscles, to some hormones to the enzymes that make all of our chemical reactions work. In beings with nuclei, the DNA stays in the nucleus. Messenger RNA copies just one recipe and takes it out of the nucleus to where the little "workbenches" in the cell (ribosomes) read the instructions and make the proteins. Many copies of the original can be made. It's like keeping a valuable cookbook safe, making many copies of just the recipe you need, and giving those to many cooks.

Viruses are not alive, but they have either DNA or RNA. They can't reproduce themselves. What they do is invade a cell, inactivate the cell's DNA, and turn them into virus factories by making the cell use the recipe on their RNA or DNA.

Most vaccines work by introducing a weakened virus or a part of a virus so that our immune system is ready for it. It's sort of like showing your immune system a "wanted" poster.

The RNA vaccines are different. They contain a fragment of a recipe. Then your own cells make tons of the fragment of the protein, giving your immune system a good look at it. It's like giving your immune system an electronic copy of a wanted poster so that it can make tons of copies.

When they make mRNA vaccines they use a piece of the virus that isn't likely to change, even if there's a mutation (change) in the RNA or DNA.

Why do you think viruses are not considered to be alive?

All vaccines work by exposing your body to a part of a virus that is not infectious in order to build up immunity to the actual virus. Many vaccines in the past have either used whole virus particles that have been weakened or killed to do this, or used injections of viral proteins or parts of viral proteins.

The mRNA vaccines inject viral mRNA into you instead of actual virus or proteins. The mRNA is harmless, but contains instructions for producing the Covid-19 spike protein, an important protein on the outside of Covid-19 the virus needs to be infectious. Inside of your cell's cytoplasm the mRNA instructions will be used to produce Covid-19 spike proteins. These spike proteins are harmless on their own, but the body's immune cells will recognize them as foreign, and produce antibodies against the spike protein and build up what we call "immunological memory" against the spike protein. Then, if you are exposed to the actual Covid-19 virus, the body will recognize the spike protein due to the previous exposure and mount a strong, and fast, immune response to prevent serious infection.

Many vaccines will make you feel a little sick, this is because most of the symptoms of an illness are your body's own immune response to infection, so this sick feeling is just your body recognizing the foreign protein and mounting an immune response.

Vaccine is a type of drug that makes the immune system respond to a specific type of pathogen. During an immune response, the immune system produces antibodies to neutralize that pathogen. Traditionally, the pathogens in a vaccine are either inactivated or attenuated.

The pathogens in an inactivated vaccine are killed with either heat or chemicals; as a result they cannot infect but can induce an immune response. While you cannot get infected by an inactivated vaccine, dead pathogens cannot produce an as strong immune response as other methods. As a result you would need multiple booster shots to maintain antibody level.

The pathogens in an attenuated vaccine are alive but they cannot make the people as sick as the diseases itself; in other word, their virulence is attenuated. By producing a mild infection, attenuated vaccine can induce a much stronger immune response than inactivated vaccine. However, there is always that slim chance that the pathogens would regain their virulence.

In the past 40 years, many novel vaccines have appeared, including recombinant vaccine, adenovirus vector, and mRNA vaccine.

mRNA is a type of RNA. All lives on Earth use mRNA as a blueprint to produce proteins. The COVID-19 mRNA vaccine encapsulates the mRNA for the SARS-CoV-2 spike protein in a lipid nanoparticle. The spike protein is what the virus uses to enter the cell. The dendritic cell in the immune system can pick up the nanoparticles and produce the spike protein based on the mRNA. Dendritic cell can then present the spike protein for other cells in the immune system to make antibody against the spike protein. In such way a stronger immune response can be induced without involving the complete virus at all. This method of delivery mRNA via nanoparticles have been used by biologists since 1990 and the Pfizer COVID-19 vaccine is the first RNA vaccine to be approved for clinical use.

However, RNA is extremely fragile and the mRNA vaccine need to be stored at a minus 80 Celsius freezer until it is ready to use. This has posted a problem for the distribution of those vaccines.

mRNA stands for "messenger RNA": it is RNA that is transcribed from DNA in the nucleus of a cell, and it encodes the sequence of amino acids that is then used to make proteins.

I do not know much about immunology or vaccines, but I do know that the the idea of a vaccine is to trigger the host's immune system to create antibodies to a virus that the host has not yet encountered. My guess is that an mRNA vaccine contains an RNA molecule that is a portion of the virus, specifically, the portion that makes the virus' protein coat, but does not contain the other genes that make the virus infectious. By recreating the protein coat, the host's immune system will make antibodies to the protein coat, and thus, the virus itself.

mRNA is messenger RNA. That's the RNA message that gets translated into proteins.

The CDC has an answer to your question:
different vaccines

"mRNA vaccines are a new type of vaccine to protect against infectious diseases. To trigger an immune response, many vaccines put a weakened or inactivated germ into our bodies. Not mRNA vaccines. Instead, they teach our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response, which produces antibodies, is what protects us from getting infected if the real virus enters our bodies.

COVID-19 mRNA vaccines give instructions for our cells to make a harmless piece of what is called the “spike protein.” The spike protein is found on the surface of the virus that causes COVID-19.

mRNA Vaccines Are New, But Not Unknown. Researchers have been studying and working with mRNA vaccines for decades. Interest has grown in these vaccines because they can be developed in a laboratory using readily available materials. This means the process can be standardized and scaled up, making vaccine development faster than traditional methods of making vaccines.

Facts about COVID-19 mRNA Vaccines
They cannot give someone COVID-19.
• mRNA vaccines do not use the live virus that causes COVID-19.
They do not affect or interact with our DNA in any way. • mRNA never enters the nucleus of the cell, which is where our DNA (genetic material) is kept.
• The cell breaks down and gets rid of the mRNA soon after it is finished using the instructions."