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What are some big differences between the Pfizer and Moderna COVID-19 vaccines?

The FDA approved Moderna's vaccine on Friday, making it the second on the market, with two more expected by the end of January 2021.

HOUSTON — With the FDA approving Moderna's COVID-19 vaccine Friday night, there are now two vaccines on the market, with two more expected by the end of January 2021.

Both use messenger RNA or mRNA to attack the coronavirus and create an immune response. Both require two doses.

Here are some of the big differences between Pfizer and Moderna's COVID-19 vaccine.

Pfizer's is approved for people ages 16 and older. Moderna's is for people 18 years or older.

Pfizer's must be stored in ultra-cold freezers, around -80 degrees Celsius. Moderna's can last in normal refrigerators for up to 30 days.

According to Dr. Catherine Troisi, an infectious disease epidemiologist at UT Health School of Public Health, the difference is because of the lipids, or fats, that each company used for surrounding the mRNA.

She said Moderna's are more stable, but that Pfizer did not know that when creating their vaccine.

“Get whichever vaccine you can," said Dr. Troisi. "They are both equally effective.”

Pfizer requires shipment batches of 975 or more. Moderna requires 100 or more.

These differences in batch size and storage ability can affect which hospitals choose to order them.

“It’s really good news for more rural parts of the country," Troisi said of Moderna's storage ability and shipment flexibility.

One big question: are COVID-19 vaccines like the flu shot? Will we need to get a different one every year?

“We don’t know how long immunity lasts because the virus hasn’t been around long enough," said Dr. Troisi. “We are not sure with the coronavirus how quickly it’s going to mutate and if that will make a difference in our immune response.”

With two more vaccines expected in late January, people are curious if those will be just as effective as Pfizer's and Moderna's.

“We think that these vaccines will also prove to be safe and effective because they are targeting the same part of the virus, that spike, that you see in the pictures," said Troisi.

Troisi warned that widespread access to the vaccines is still several months away.

“Light at the end of the tunnel, but it’s a long tunnel."

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