What are the differences between Moderna and Pfizer vaccines?

(CNN) – Moderna’s coronavirus candidate vaccine is similar to the Pfizer-BioNTech vaccine, which was licensed and shipped to early Americans earlier this week.

But there are some key differences. Most importantly, Moderna’s vaccine can be stored in normal freezers and does not require a very cold transportation network, making it more accessible to smaller facilities and local communities.

The US Food and Drug Administration (FDA) Vaccine Advisory Committee is meeting Thursday to review whether it recommends emergency use authorization for Moderna’s vaccine. The FDA’s decision is expected on Friday.

Here’s a look at what we know about the Moderna vaccine and how it compares to Pfizer’s.

What is Moderna?

While Pfizer is a pharmaceutical giant, Moderna (short for modified RNA, which is RNA in English) is a biotech company based in Cambridge, Massachusetts.

Moderna, which was established in 2010, has never brought a product to market, nor has it had any of its nine vaccine candidates approved for use by the FDA. Also, it had never taken a product to phase three of a clinical trial before.

Company scientists had already been collaborating with researchers at the National Institutes of Health (NIH) on a vaccine for another coronavirus, Middle East respiratory syndrome (MERS). When Chinese researchers published the genomic sequence of the new coronavirus in mid-January, they got a big boost.

The vaccine, called mRNA-1273, was supported by development aid from the National Institute of Allergy and Infectious Diseases. The US Advanced Biomedical Research and Development Authority, known as BARDA, supported late-stage clinical trials and has helped scale up manufacturing.

The FDA gave the green light to clinical trials of Moderna’s vaccine on March 3, the first to go public. Its late-stage clinical trial began on July 27 and was the first government-funded phase 3 clinical trial for a COVID-19 vaccine in the United States. The company applied to the FDA for an emergency use authorization for the vaccine on November 30.

Does Moderna’s vaccine work?

The Moderna vaccine was effective in preventing COVID-19 cases in total and limiting severe COVID-19 during the testing phase.

The efficacy of the COVID-19 vaccine was 94.1%, and it occurred at least 14 days after the second dose, according to an information paper released by an FDA advisory committee.

In Moderna’s trial, 15,000 study participants were given a placebo, which is a saline injection that has no effect. Over several months, 185 of them developed covid-19, and 30 developed severe forms of the disease. One of them died.

Another 15,000 participants were given the vaccine and only 11 of them developed COVID-19. None of the 11 became seriously ill.

The vaccine was effective in all age groups, genders, races and ethnic groups, and participants with medical comorbidities associated with a high risk of severe COVID-19, according to the information document. About 10% of the study participants were black, and 20% were Hispanic or Latino.

How does Moderna’s vaccine work?

Like Pfizer’s vaccine, Moderna offers messenger RNA, or mRNA, which is a genetic recipe for making some of the spikes found on the coronavirus. Once injected, the body’s immune system produces antibodies against the spikes. If a vaccinated person is later exposed to the coronavirus, those antibodies must be ready to attack the virus.

The Moderna vaccine in particular contains a synthetic mRNA that encodes a structure called the “pre-fusion stabilized peak glycoprotein” of the virus.

The vaccine also contains fatty materials called lipids, tromethamine, tromethamine hydrochloride, acetic acid, sodium acetate, and sucrose.

How is Moderna’s vaccine different from Pfizer’s?

The two vaccines are very similar, but they have some key differences that make Moderna’s vaccine “more flexible,” as US Secretary of Health and Human Services Alex Azar said last month.

Effectiveness: Both Moderna’s and Pfizer-BioNTech’s vaccines have shown similar efficacy levels close to 95%.

“They appear to be roughly equivalent,” said Dr. Paul Offit, a member of the FDA’s vaccine advisory committee, during an appearance on CNN’s “New Day” Tuesday.

Structure: Both vaccines depend on mRNA – or messenger RNA – to function, albeit with slightly different structures and composition.

“Although they are both messenger RNA vaccines, they are really different messenger RNA molecules, they have different so-called lipid release systems, that is, the kind of fat drop that messenger RNA is in,” Offit said Monday . “That is why they have different storage and handling characteristics.”

Cold storage: Most importantly, Moderna’s vaccine does not need to be kept at extremely cold temperatures, like Pfizer’s.

The Pfizer vaccine should be stored at approximately -75 degrees Celsius. This is approximately 50 degrees cooler than any vaccine currently used in the United States. The vaccine can be stored in the refrigerator for only up to five days before it expires.

To accommodate that, CDC created a complex set of handling and storage requirements known as the “cold chain” that included expensive ultra-cold freezers and lots of dry ice.

In contrast, Moderna’s vaccine can be kept at about minus -20 degrees Celsius, or roughly the temperature of a home freezer, according to Moncef Slaoui, director of the US initiative to develop a vaccine for COVID-19. Moderna’s vaccine can also be stored in the refrigerator for 30 days before it expires.

“So that’s more manageable and much more commonly available,” Slaoui said Tuesday.

These differences suggest that the Pfizer vaccine may be used more for large institutions with established infrastructure such as hospitals. In contrast, Moderna’s vaccine may be more useful for smaller facilities like a local chain or a pharmacist.

Dosage and time: Moderna’s vaccine is given in two doses of 100 micrograms 28 days apart. The Pfizer vaccine is given in two doses of 30 micrograms 21 days apart.

Age: If licensed, the Moderna vaccine would be used in people over the age of 18, while the Pfizer vaccine was licensed for people over the age of 16.

Is the vaccine safe?

The vaccine has a “favorable” safety profile, says the FDA briefing document, “with no specific safety concerns identified” preventing it from obtaining an emergency clearance.

The most common adverse reactions to the vaccine have been injection site pain, fatigue, headache, muscle pain, joint pain and chills, according to the document. Swollen lymph nodes have also been reported.

Overall, “the frequency of serious non-fatal adverse events was low and without significant imbalances” between the group that received the vaccine and the group that received the placebo.

One study participant, Yasir Batalvi, 24, told CNN he had a low fever, fatigue and chills after receiving the second injection of what he believed to be the actual vaccine. It was fine the next morning.

Slaoui, the chief scientific advisor to Operation Warp Speed, has said that around 10-15% of immunized study subjects will develop “quite noticeable side effects.”

When can I get vaccinated?

The United States initially bought 100 million doses of Moderna’s vaccine and last week agreed to buy 100 million more doses. Moderna began manufacturing the first 100 million doses while clinical trials were underway, the Department of Human Services said in a statement.

The vaccine has yet to be licensed first. The FDA advisory committee is meeting Thursday to discuss the emergency use authorization, and Slaoui said he hopes the committee will approve it on Friday.

If that happens, he said shipments are expected to begin 24 hours later for the vaccines to arrive at the facility Monday morning.

More than 6 million doses of Moderna vaccines will be shipped to more than 3,200 sites, far more than the 636 sites to which Pfizer vaccines were shipped.

From there, the vaccines will be administered in the order established by state and local authorities. CDC vaccine advisers recommended that healthcare workers and residents of long-term care facilities be first in line for any coronavirus vaccine that gets emergency approval.

CNN’s chief medical correspondent, Dr. Sanjay Gupta, estimated that the vaccines will generally be available in late spring or early summer.

– CNN’s Jen Christensen, Elizabeth Cohen and Naomi Thomas contributed to this report.