The COVID-19 vaccine is using new technology that has never been used before in traditional vaccines. Here’s how an mRNA vaccine works.
Two COVID-19vaccines are now authorized in the United States.
The Moderna vaccine began arriving across the nation Monday, just three days after it was authorized for emergency use by the Food and Drug Administration. It comes on the heels of the Pfizer-BioNTech vaccine, which is being given to health care workers and nursing home residents. 
As Americans prepare to get vaccinated, many also have questions. What’s the difference between these two vaccines, how effective are they, and when do we all expect to get them? 
Heres what we know about the COVID-19 vaccines and what they might mean for the pandemic.
What are the leading COVID-19 vaccines?
Pfizer and the German biotechnology company BioNTech developed one of the COVID-19 vaccines that has been authorized by the FDA, BNT162b2.
Moderna, a Cambridge, Massachusetts-based biotechnology company, developed a COVID-19 vaccine, mRNA-1273, that was authorized Friday.
How effective are the candidate vaccines, and what does that mean?
All of the late-stage vaccine trials include at least 30,000 volunteers, half of whom receive the active vaccine and half the placebo.
Both vaccines require two doses. The Pfizer-BioNTech shots are being given 21 days apart. Modernas are given 28 days apart.
Beginning a week after the second dose, participants are watched to see whether they come down with COVID-19.
In each of the studies, after about 150 participants have developed COVID-19, it is statistically possible to determine the vaccine’s effectiveness.
Almost 200 trial participants developed symptomatic COVID-19 in the Moderna trial, only 11 of whom had received the active vaccine. Because the infection rate was so much higher in the placebo group, statistical analysis determined that the vaccine was 94% effective overall, according to safety and effectiveness data released Dec. 15. 
Pfizer/BioNTech reported on Nov. 18 that of 170 confirmed cases of COVID-19 among its trial participants, 162 were in the placebo group versus eight in the vaccine group. A safety and effectiveness report released Dec. 8 confirmed the findings. 
Are there any side effects to the vaccines?
In Modernas Phase 3 trials, the company said the most common side effects were fatigue, muscle soreness and aches, joint pain and headache, plus pain, redness or swelling at the injection site.
In Pfizer/BioNTech Phase 3 trials, many participants endured side effects for a day or two after getting their shots, particularly the second one. The most commonly reported side effect among vaccine recipients under age 55 was a sore arm, followed by fatigue (60% after the second shot); headache (52% after the second shot); other muscle aches (37%); and chills (35%). About 28% took pain medication after the first shot and 45% after the second shot.
A sore arm and feeling crummy for a day or two is a lot better than COVID, said Dr. William Schaffner, professor of health policy and of preventive medicine at the Vanderbilt University School of Medicine.
Physicians emphasize that the side effects are not just normal but also a sign that the body is reacting properly to the vaccine.
Side effects of the COVID-19 vaccines look similar to side effects of the influenza vaccine, which include soreness, redness, and/or swelling where the shot was given, headache, low-grade fever, nausea, muscle aches and fatigue, according to the Centers for Disease Control and Prevention. 
In trials of 44,000 and 30,000 respectively, Pfizer/BioNTech and Moderna saw very few problems more serious than a couple of days of feeling lousy.
Four people in the Pfizer/BioNTech trial and three people in the Moderna trial developed Bell’s Palsy, a neurological condition that leads to temporary drooping of one side of the face.
At least two people in Great Britain, where the vaccine was first approved and distributed, as well as several Americans have had powerful allergic reactions to the Pfizer-BioNTech vaccine so far, representing a tiny fraction of those vaccinated. The federal government is studying these reactions but has so far said the vaccine appears to be safe for anyone who has not previously had an allergic reaction to one of the vaccines’ ingredients.
What are the differences between the two vaccines?
Most of the U.S.-backed vaccines target the spike protein found on the surface of the virus that causes COVID-19, which allows the virus to attach itself to host cells and infect them.
Both of these vaccines work by presenting this spike protein to the immune system. The spike proteins arent dangerous because the rest of the virus isnt present; however, the body now sees the protein and designs immune “soldiers” to fight it.
The Moderna and Pfizer vaccines deliver strands of genetic material known as mRNA, which turns peoples cells into spike protein factories.
In this special edition episode of States of America, experts answer the biggest questions Americans have about the vaccine, side effects, how it’s getting to you and more.
This technology has never been used before in an approved vaccine, though it has been tested against other diseases. The mRNA technology was chosen this time because scientists knew it could be developed quickly.
Other COVID-19 vaccine candidates being supported by the U.S. government target the spike protein via a different carrier virus or tiny particle.
The vaccines require different types of storage.
The Pfizer/BioNTech vaccine must be kept super-cold at the temperature of dry ice until shortly before it is used. The Moderna vaccine needs to be frozen if stored for a long time, but it can be refrigerated for up to a month before being used.
When can I get a COVID-19 vaccine?
Since the Pfizer/BioNTech vaccine has been approved, health care workers and people in long-term facilities across the country have been lining up to get their scheduled vaccine.
A few high-profile politicians, such as Vice President Mike Pence and Florida Sen. Marco Rubio, were publicly vaccinated in hopes of instilling confidence in the vaccine.
A CDC advisory panel decided Sunday that police, firefighters, teachers and grocery workers will be among the next in line for a COVID-19 vaccine.
The committee vote recommended that Phase 1b include people 75 and older and front-line essential workers. They make up about 30 million people among these groups:

  • First responders such as firefighters, police
  • Teachers, support staff, day care workers
  • Food and agriculture workers
  • Manufacturing workers
  • Correction workers
  • U.S. Postal Service workers
  • Public transit workers
  • Grocery store workers

Because vaccine supplies are initially limited, Phase 1b isnt expected to begin until February.
Phase 1c will include people 65 to 74 and people 16 to 64 who have high-risk medical conditions, along with other essential workers. This would make up about 57 million people and would include:

  • Public health workers
  • Transportation and logistics workers
  • Food service workers
  • Construction workers
  • Finance workers
  • IT and communications workers
  • Energy workers
  • Media workers
  • Legal workers
  • Public safety engineers
  • Water and wastewater workers

Phase 2 would include all people 16 and over who were not in Phase 1 who are recommended for the vaccination. That means people 16 and over with high-risk medical conditions.
Coronavirus updates: Moderna vaccinations set to begin Monday; New York Gov. Cuomo urges feds to ban flights from UK
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told the USA TODAY Editorial Board Nov. 18 that he expected the general public to begin vaccinations as early as April.
Contributing: Karen Weintraub and Elizabeth Weise, USA TODAY. Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT. 
Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.
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