What’s Different About Messenger RNA (mRNA) Vaccines for COVID-19?

Memorial Sloan Kettering infectious disease specialist Tobias Hohl

Infectious disease specialist Tobias Hohl explains that messenger RNA vaccines prompt the body’s own cells to make a protein fragment found in the COVID-19 virus — which stimulates a protective immune response.

Two vaccines for the COVID-19 virus (SARS-CoV2) that have been issued Emergency Use Authorization by the US Food and Drug Administration have a unique design. The vaccines, produced by the pharmaceutical companies Pfizer and Moderna, use messenger ribonucleic acid (mRNA) to stimulate an immune response that can protect against future infection. Memorial Sloan Kettering infectious disease specialist Tobias Hohl discusses the technology of mRNA vaccines and how they may change the outlook of the pandemic. 

How do COVID-19 messenger RNA (mRNA) vaccines work?

Traditional vaccines put a weakened or inactivated germ into our bodies. Messenger RNA (mRNA) vaccines, like the Pfizer and Moderna COVID-19 vaccines, teach cells how to make a protein that triggers an immune response if someone gets infected. When the vaccine is injected into the upper arm, the mRNA enters cells near the site of the injection and tells the cells to start making the same protein that is found in the COVID-19 virus. The immune system recognizes this protein and begins producing antibodies that can fight the virus if the vaccinated person is later infected.

None of the vaccines interact with or alter your DNA in any way, and therefore cannot cause cancer. MRNA is not the same as DNA and cannot be combined with DNA to change your genetic code. The mRNA is fragile, so after it delivers  the instructions to your cells, it breaks down and disappears from the body (in about 72 hours). The mRNA never even goes into the nucleus of the cell — the part that contains your DNA. Therefore, there is no truth to the myth that somehow the mRNA vaccine could inactivate the genes that suppress tumors.

The COVID-19 vaccine does not expose you to the virus that causes COVID-19.

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Have there been other mRNA vaccines?

These are the first messenger RNA vaccines to be produced and tested in large-scale phase III human trials. The advantage of mRNA technology compared with conventional approaches is that it allows for faster development and scale-up of production. Vaccine development has traditionally been measured on the timeframe of a decade. It’s an amazing scientific accomplishment to be where we are right now. A year ago, most people had not even heard of this disease, and now healthcare workers are starting to receive a vaccine for it.

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Why do mRNA vaccines need to be stored at such low temperatures?

RNA breaks down easily and quickly unless it is kept at low temperatures. In the body, this isn’t a problem because RNA does not need to exist long in order to carry out its function. But vaccines may need to be kept stable for days or even weeks before they are administered. This has not been nearly as much of a concern with conventional vaccines. Weakened or inactive versions of a virus can remain stable longer without requiring low temperatures.

In the clinical trials, the Pfizer vaccine was kept at minus 70 degrees Celsius (minus 94 degrees Fahrenheit), which requires storage in dry ice. The Moderna vaccine was kept at minus 20 degrees Celsius (minus four degrees Fahrenheit), which is more like a regular freezer. Both vaccines continue to require cold storage — any approval of the vaccines is based on the data generated during the trials, and so the same conditions need to be met.

I personally will have no hesitation in receiving either of these vaccines. I'm going to recommend to my family members and colleagues to take them when they are available.
Tobias M. Hohl infectious disease specialist
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Why are the vaccines given in two doses spaced weeks apart?

The two-dose approach is common for many childhood vaccines. The idea is that the first shot primes the immune system, helping it recognize the virus, and the second shot strengthens the immune response. Pfizer’s second shot is given three weeks after the first one; Moderna’s is spaced four weeks later. It is very important to have both doses of the same vaccine and follow the same procedures that made the clinical trials so successful.

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How can people feel confident that these mRNA vaccines are effective and safe?

We now have data from tens of thousands of people, and they show that these vaccines are more than 90% effective in terms of preventing infections.

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Some people have mild to moderate side effects, but they don’t last long — about one to three days. The most common side effects include soreness at the injection site, fatigue (feeling tired), headache, aches, and fever. They are more common after the second dose, and you might need extra rest. Severe side effects are rare and treatable. Experts at the US Food and Drug Administration and the Centers for Disease Control and Prevention are still monitoring trial participants — and will continue to monitor them — as well as members of the general public who get the vaccine.

It’s very important for people to understand that these data are reviewed by groups independent from the pharmaceutical companies that make the drugs. These are experts who have no stake in the vaccine development or commercialization. I personally will have no hesitation in receiving either of these vaccines. I’m going to recommend to my family members and colleagues to take them.

The vaccines offer intermediate to long-term solutions to this pandemic. But we can’t give up the short-term measures that we know are effective, such as wearing masks, social distancing, and washing hands. There is light at the end of the tunnel.


January 12, 2021


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