The coronavirus’s RNA codes for at least 24 proteins, including the spike protein, which covers its exterior. The virus uses its spike protein to bind to and enter our cells, where it then injects its RNA, hijacking our human cells to make many more copies of the virus.

“It’s almost like a skeleton key for a lock, opening a portal within human cells that the virus can slip in,” Das told NOVA.

Both Moderna’s and Pfizer’s mRNA vaccines prompt a person’s cells to produce the SARS-CoV-2’s spike protein, so their immune system can learn to recognize it and develop Covid-19-fighting antibodies without having any contact with the actual virus.

“Our bodies won’t make a full-fledged infectious virus,” Das said. “They’ll just make a little piece and then learn to recognize it and then get ready to destroy the virus if it then later comes and invades us.”

Moderna and Pfizer are still gathering safety data that the Food and Drug Administration requires for consideration of an emergency use authorization. This authorization would allow vaccine manufacturers to distribute their vaccines during the pandemic at an earlier date than otherwise possible. “To apply for an emergency use authorization, the FDA requires that half of participants have at least two months of data regarding a Covid-19 vaccine’s safety after they receive the final dose. For Pfizer and BioNTech’s vaccine, that point is expected to come in the third week of November,” Erin Garcia de Jesús writes for Science News.

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Both companies plan to apply for FDA emergency use authorization this month. Moderna stated Monday that it expects to be able to ship about 20 million vaccine doses in the U.S. by the end of this year and another 500 million to 1 billion in 2021, if authorized to do so. Pfizer expects to produce up to 50 million vaccine doses in 2020, and 1.3 billion in 2021, if authorized.

To prepare to store a hopefully forthcoming coronavirus vaccine, large urban hospitals across the U.S. are rushing to buy expensive ultra-cold freezers. (Last week, Pfizer and BioNTech said that their vaccine needs to be stored at minus 94 F. Conversely, Moderna states that its vaccine can be safely stored in freezers at about 25 F for 30-day storage, “a temperature easily reached by a home refrigerator freezer,” Palca writes. However, safe 6-month storage would require a temperature of minus 4 F, Moderna stated in a press release.) Most rural hospitals can’t afford ultra-cold freezers, “meaning health workers and residents in those communities may have difficulty getting the shots,” Olivia Goldhill reports for STAT News.

Medical experts call the temperature-controlled supply chain of vaccines and other protein-based pharmaceuticals the “cold chain,” because the protein molecules in these products can break down at higher temperatures, rendering them ineffective.

“Maintaining vaccines continuously at their required temperature is already one of the biggest challenges developing countries face with routine immunization, and this will only be exacerbated with the introduction of a new [coronavirus] vaccine,” Toby Peters writes for WIRED.

In a paper published in May in the journal Science, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, suggested that the U.S. population alone may require several different vaccines made and distributed by different labs to bring an end to the pandemic, which, according to Johns Hopkins University’s Covid-19 dashboard, has so far sickened more than 11.3 million Americans and more than 55.7 million people worldwide.

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