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That’s why COVID-19 vaccines like Pfizer’s should be kept as cold


Pfizer is running to get approval for its COVID-19 vaccine, requesting emergency use authorization from the U.S. Food and Drug Administration on Nov. 20. But the pharmaceutical giant faces a huge challenge when it comes to distributing its vaccine, which must be kept frozen at 70 ° Celsius, requiring special freezers and transport containers.

“It has some unique storage requirements,” says Kurt Seetoo, the immunization program manager at the Maryland Department of Public Health in Baltimore. "We don't usually store vaccines at that temperature, so it's definitely a challenge."

This means that although the vaccine developed by Pfizer and its German partner BioNTech is likely to be the first vaccine to reach the United States, its adoption may be limited. The FDA committee overseeing the vaccines will meet Dec. 10 to discuss the emergency use application. That meeting will be streamed live on the agency’s website and on YouTube, Facebook and Twitter channels.

Companies are also seeking permission to distribute the vaccine in Australia, Canada, Europe, Japan, the UK and other parts of the world, making their freezing issue a global challenge.

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A similar vaccine developed by Moderna and the U.S. National Institute of Allergy and Infectious Diseases also requires freezing. But it survives at a lower temperature of –20 ° C, so it can be stored in a standard freezer and can even be stored in the refrigerator for up to a month. Most vaccines do not require freezing, but both Pfizer and Moderna vaccines. vaccines are a new type of vaccine for which low temperatures are needed to prevent vaccines from breaking down and becoming useless.

Both vaccines are based on messenger RNA or mRNA, which carries instructions to build copies of the coronavirus ear protein. Human cells read these instructions and produce copies of the protein, which in turn presses on the immune system to attack the coronavirus in the event it calls.

So why should the Pfizer vaccine be frozen at sub-Antarctic temperatures and the Moderna vaccine not?

Answering that question requires some speculation. Companies are unlikely to reveal all the tricks and trade secrets they used to make the vaccines, says Sanjay Mishra, a protein chemist and data scientist at Vanderbilt University Medical Center in Nashville.

But there are at least four things that can determine how fragile the mRNA vaccine is and the depth of its freezing to keep it fresh and effective. How companies have addressed these four challenges is probably the key to the cold that vaccines should have, says Mishra.

The cold demand puzzle begins with the difference in chemistry between RNA and its cousin, DNA.

One of the reasons why RNA is much less stable than DNA is due to a significant difference in the sugars that form the backbones of molecules. The backbone of RNA is a sugar called ribose, while DNA is deoxyribose. The difference: DNA lacks an oxygen molecule. As a result, “DNA can survive for generations,” Mishra says, but RNA is much more transient. "And for biology, that's good."

When cells have a job to do, they usually need to put the proteins into service. But like most manufacturers, cells do not have a reserve of protein. Every time they have to make new batches. The recipe for making protein is stored in DNA.

Instead of risking damaging DNA recipes by placing them on the molecular kitchen counter while cooking a batch of proteins, cells make RNA copies of the recipe. These copies are read by mobile machinery and are used to produce proteins.

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As an impossible mission message that self-destructs once reproduced, many RNAs degrade rapidly once read. Quickly removing RNA is one way to control the amount of a particular protein. There are a number of enzymes dedicated to the destruction of RNA floating inside cells and almost everywhere. The insertion of RNA-based vaccines into the freezer prevents those enzymes from tearing the RNA and making the vaccine inert.

Another way to defer the stability of molecules lies in their architecture. The double strands of DNA emerge in a graceful double helix. But RNA does so only in a single strand that pairs with itself at some points, creating fantastic shapes reminiscent of lollipops, hair bolts, and traffic circles. Those “secondary structures” can make some RNAs more fragile than others.

Another place where chemical differences in DNA and RNA make RNA difficult is the part of the molecules that explains the instructions and ingredients in the recipe. The information transport subunits of molecules are known as nucleotides. DNA nucleotides are usually represented by the letters A, T, C, and G for adenine, thymine, cytosine, and guanine. RNA uses the same A, C, and G, but instead of thymine it has a different letter: uracil or U.

“Uracil is a problem because it excels,” Mishra says. The ones that stand out to us are like a flag waving to special proteins in the immune system called Toll-like receptors. These proteins help detect virus RNAs, such as SARS-CoV-2, the coronavirus that causes COVID-19, and destroy invaders.

All of these ways in which mRNA can collapse or be attacked by the immune system creates an obstacle course for vaccine manufacturers. Companies need to make sure that RNA remains intact long enough to enter cells and bake lots of spike protein. Both Moderna and Pfizer probably played with RNA chemistry to make a vaccine that could do the job: both reported that their vaccines are approximately 95 percent effective in preventing disease in clinical trials (SN: 16/11/20; SN: 11 / 18/20).

Although the details of each company’s approach are not known, both probably played slightly with the chemical letters of the mRNAs in order to make it easier for human cellular machinery to read the instructions. Companies also need to add additional RNA – a stopper and a tail – that flank the instructions of the spike protein to make the molecule stable and readable in human cells. Such manipulation can disrupt or create secondary structures that could affect RNA stability, Mishra says.

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The uracil problem can be treated by adding a modified version of the nucleotide, which Toll-like receptors overlook, saving RNA from an initial attack by the immune system so that the vaccine is more likely to produce the protein that will build immune defenses against the virus. . Exactly the modified version of uracil that companies were able to introduce into the vaccine could also affect the stability of the RNA and therefore the temperature at which each vaccine has to be stored.

Finally, by itself, an RNA molecule is under the warning of a cell because it is too small, Mishra says. Thus, companies coat the mRNA with a lipid emulsion, creating small bubbles known as lipid nanoparticles. Those nanoparticles need enough for the cells to grab them, bring them inside, and open the particle to release the RNA.

Some types of lipids withstand heat better than others. It's "like regular oil versus fat. You know how lard is solid at room temperature" while oil is liquid, says Mishra. For nanoparticles, "what they're made of makes a giant difference in the stability they'll have. in general to (keep) things inside. ”The lipids used by companies could make a big difference in the vaccine’s ability to withstand heat.

The need for ultra-cold storage may ultimately limit the number of people who end up getting vaccinated with the Pfizer vaccine. “We anticipate that this Pfizer vaccine will only be used in this early phase,” says Seetoo.

The first wave of vaccinations is expected to go to health workers and other essential employees, such as firefighters and police, and to people at high risk of becoming seriously ill or dying of COVID-19 if they contract it as seniors living in nursing facilities.

Pfizer told health officials that the vaccine can be stored in special cargo containers that are refilled with dry ice for 15 days and remain refrigerated for another five days after thawing, Seetoo says. That gives health officials 20 days to get the vaccine into people’s arms once it is delivered. But Moderna’s vaccine and a lot of others that are still being tested seem to last longer at warmer temperatures. If those vaccines are as effective as Pfizer’s, they may be more attractive candidates in the long run because they don’t need such extreme special handling.


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