A tray of purple-topped medical vials.

December 19, 2020

In one week we’ve gone from the U.S. Food and Drug Administration recommending one coronavirus vaccine to two.

So how does newcomer Moderna compare to Pfizer's vaccine? And what does that mean for you?

“Having both is not the end of the COVID-19 pandemic, but it’s a huge turning point that will help get our lives back to normal,” says Annamaria Macaluso Davidson, MD, Associate VP of medical operations for MHMG, Medical Director for Employee Health.

Dr. Macaluso Davidson delivers the scoop.

Q. Why does Pfizer’s vaccine require subarctic transportation and not Moderna’s?

A. Pfizer’s vaccine has no preservatives, so it’s short-lived. It must be shipped and stored at -80 degrees, which requires special freezers. Then refrigerated, it must be used within five days. It also must be diluted with sodium chloride beforehand.

In contrast, Moderna’s vaccine has different requirements enabling it to be shipped at -20 degrees in traditional freezers. It can be frozen up to six months, and then kept refrigerated up to 30 days.

Both come in multi-dose vials and are injected into deep muscles on the arm.

Q. What does the difference mean for patients and others waiting to be vaccinated?

A. While the Pfizer vaccine requires the equipment and skills more likely found in hospitals, the Moderna vaccine can be shipped and stored in standard freezers.

This makes it more accessible to medical clinics, CVS, Walgreens and other pharmacies as well as small and rural communities. “That is helpful in getting the vaccine to as many patients in as many places as possible.”

Now that there are two vaccines, Dr. Macaluso Davidson says, “health care systems are discussing how they can be rolled out to frontline workers, including firefighters, police and nursing home staff, as well as to the elderly and other high-risk populations.”

Current projections are that by spring or summer the general public also will be able to be vaccinated, she says.

Q. Is one vaccine better than the other?

A. Both are 94-95% effective at preventing cases of COVID-19—as long as you get two doses of the same vaccine, at least three to four weeks apart. Your body starts working on the immune response immediately; but the full response takes a few weeks to develop.

“You cannot mix the two vaccines,” Dr. Macaluso Davidson says.

Pfizer vaccine can be used in those 16 and older, while the Moderna can be used in those 18 and above, since those age groups were included in clinical trials.

Q. I heard vaccines can take decades to be developed. How can I trust these, which came to market in less than a year?

A. “No short cuts and no safety corners were cut,” she says. “The beauty of Operation Warp Speed is that it united a private company’s ingenuity (Moderna) with the operational skills and financial and operational resources of the government (the National Institutes of Health, Department of Health and Human Services and The Department of Defense).”

Moderna had been studying the groundbreaking technology behind the vaccine for years, anticipating its use against viruses.

Like Pfizer, its vaccine creates molecular couriers—mRNA, which stands for messenger ribonucleic acid—to deliver COVID-19’s genetic codes for a protein directly to cells. The body then starts making antibodies against this protein. This enables the body’s immune system to recognize and destroy the virus.

Q. How can the Moderna vaccine be rolled out in just a couple of days?

A. Through the coordination through Operation Warp Speed, Moderna started to produce and ready the vaccine for shipment in advance of FDA approval. “The vaccine can get to patients that much faster,” Dr. Macaluso Davidson says.

Q. How do side effects compare?

A. “Some—but not the majority—may experience headache, fatigue, low-grade fever, body aches and chills,” she says. “The symptoms show your body is at work building immune response memory, so if you get the virus, it knows what to do to combat it.”

Learn more about COVID-19 and the COVID-19 vaccine »

The information in this article was accurate as of December 18, 2020.

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