Following a new emergency use authorization (EUA) from the US Food and Drug Administration, Americans now have two vaccine options in the fight against the COVID-19 pandemic: One from pharmaceutical company Pfizer and one from biotechnology company Moderna.
Moderna's COVID-19 vaccine was officially authorized for emergency use Friday evening after an FDA panel recommended for the vaccine's approval. Pfizer's vaccine was granted its EUA on December 11. That means Moderna's vaccine will now join Pfizer's in being administered to health care workers and residents of long-term care facilities in the US—the two high-risk groups recommended by the Centers for Disease Control and Prevention to receive the vaccine first.
It can be confusing to have two different vaccines—and possibly more to come—in use to protect against the same virus, so here's what you need to know about how the Pfizer and Moderna vaccines compare.
How do the COVID-19 vaccines work?
Both vaccines are made using a newer technology called messenger RNA (mRNA).
An mRNA vaccine works by encoding a portion of the spike protein found on the surface of SARS-CoV-2, the virus that causes COVID-19, the Centers for Disease Control and Prevention (CDC) explains.
The vaccines actually use pieces of the encoded protein to spark an immune response in your body. As a result, your antibodies to SARS-CoV-2—that is, proteins made by your immune system to help fend off future illnesses by the virus. Once your body creates that immune response, both the protein and mRNA are eliminated, while the antibodies stick around to protect you in the future.
These mRNA vaccines are different from conventional vaccines (like the flu vaccine), in that most conventional vaccines against viral disease are made from viruses grown in chicken eggs or other mammalian cells, according to Pfizer. No virus is needed to make a batch of an mRNA vaccine (though a small amount of the virus is used for gene sequencing and vaccine testing).
The body also responds to conventional vaccines versus mRNA vaccines in a slightly different way; with conventional vaccines, the antigen, or a piece of the virus is injected into the body, which then forms specific antibodies for the next time the body encounters that specific virus. In mRNA vaccines, however, the RNA provides instructions to the body's cells to produce antigens. Those cells then present the antigens to the body's immune system, prompting T-cell and antibody responses to fight the disease, per Pfizer.
How effective is each vaccine?
Each vaccine is the most effective after both doses. In an interview with The New York Times, William C. Gruber, MD, senior vice president of Pfizer Vaccine Clinical Research and Development said the Pfizer vaccine is 52% effective after the first dose, and about 95% effective after the second dose in adults ages 16 and up. The vaccine also has a high efficacy rate in people regardless of sex, age, and race.
According to information reviewed by the FDA, Moderna's vaccine is about 94.1% effective against COVID-19 in people ages 18 and older, in a trial of 30,000 people. And while there is a small difference in efficacy, it's not by much. "They both work," Derek Sant'Angelo, PhD, professor and associate director of basic science at the Rutgers Robert Wood Johnson Medical School, tells Health.
What are the side effects of each vaccine?
As with any vaccine, some minor side effects are to be expected. In a fact sheet provided by the FDA regarding Pfizer's COVID-19 vaccine, the following side effects are listed as a possibility:
- Injection site pain, swelling, or redness
- Muscle pain
- Joint pain
- Swollen lymph nodes
Pfizer also warns that there is a "remote chance" the vaccine could cause a severe allergic reaction that usually shows up within a few minutes up to an hour of getting a dose. Those with a history of severe allergic reactions should have a risk assessment conducted for the vaccine, and it's recommended that vaccines are administered in a setting where medical treatment is available.
In an FDA briefing document regarding Moderna's COVID-19 vaccine, the following symptoms are listed as potential side effects:
- Injection site pain, swelling, or redness
- Muscle pain
- Joint pain
While there were no serious allergic reactions reported during the Moderna clinical trials, it's expected that the FDA and CDC will still recommend caution is exercised when those with a history of severe allergic reactions receive the vaccine.
What are the ingredients of each vaccine?
Technically, the companies behind each vaccine don't need to release specifics until they're actually authorized by the FDA. Since the Pfizer vaccine has been granted an EUA, its ingredients are available to view online in a fact sheet. Those ingredients include:
- Potassium chloride
- Monobasic potassium phosphate
- Sodium chloride
- Dibasic sodium phosphate dehydrate
Moderna also recently released its ingredients through the FDA:
- Tromethamine hydrochloride
- Acetic acid
- Sodium acetate
Generally-speaking, the mRNA does the heavy lifting for the vaccine, while the lipids help deliver the mRNA to your body, and the other ingredients help with pH maintenance and stability of the vaccine. "At the end of the day, these two vaccines are pretty similar," Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo in New York, tells Health. "There are proprietary tweaks between the two of them but, at the end of the day, they're very similar."
Rajeev Fernando, MD, an infectious disease specialist working in COVID-19 field hospitals across the country, agrees. "They're pretty much the same thing," he tells Health.
How are the vaccines stored?
This is where things are a bit different. The Pfizer vaccine has to be shipped in specially-designed, temperature-controlled thermal shippers that keep conditions around -70 degrees Celsius (-94 degrees Fahrenheit), per a Pfizer fact sheet. The vaccine can be stored in those conditions for up to 10 days. From there, it needs to be stored in "ultra-low temperature freezers" for up to six months. The Pfizer vaccine can also be stored in refrigeration units that are "commonly available in hospitals" at temperatures between 36 to 46 degrees Fahrenheit for five days, Pfizer says.
The Moderna vaccine is a little different. It should be shipped at -20 degrees Celsius (-4 degrees Fahrenheit) and can stay stable in refrigeration units between 2 to 8 degrees Celsius (36 to 46 degrees Fahrenheit) for 30 days, the company says online. The vaccine will stay stable at -20 degrees Celsius for up to six months and at room temperature for up to 12 hours.
Why the difference? Sant-Angelo says, "There's more of a difference in the lipids and likely the reason why Pfizer needs to be stored a much lower temperatures." Overall, though, "the Pfizer vaccine is less stable," Dr. Russo says.
How many doses does each vaccine require, and how many people will be vaccinated?
Each vaccine requires two doses, given a few weeks apart, Dr. Fernando says.
According to Pfizer's fact sheet, that vaccine specifically is administered intramuscularly (injected into the muscle—commonly the deltoid in the upper arm), in a series of two shots, spaced 21 days apart. Similarly, the Moderna vaccine is also administered intramuscularly as two doses spaced 28 days apart, per the company.
The New York Times reported that Pfizer has a deal with the US government to provide 100 million doses of the vaccine—enough to vaccinate 50 million people in the US—by March 2021. The US is currently trying to make another deal with Pfizer to get more doses of the vaccine later in 2021. Meanwhile Moderna made a deal with the US to supply 300 million doses of the vaccine—enough to vaccinate 150 million people— in the first and second quarters of 2021.
Overall, whether you get the Pfizer or Moderna vaccine—or another vaccine that may be authorized by the FDA down the line—experts recommend getting vaccinated, period. "Take what you can get," Dr. Fernando says. Dr. Russo agrees. "Grab it while you can," he says.
The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.
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