Science’s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.
Today’s dramatic news that Moderna’s COVID-19 vaccine might work as well as one made by Pfizer and BioNTech means the world could have two powerful weapons to fight the COVID-19 pandemic. Now, the next hot vaccine topic is, well, heat. Both vaccines use a novel technology—strands of messenger RNA (mRNA), held within lipid particles—that is vulnerable to degradation at room temperature and requires doses to be frozen for transportation, then thawed for use.
That’s where the Moderna vaccine may have an edge: Unlike Pfizer’s and BioNTech’s offering, it does not have to be stored at –70°C, but can tolerate a much warmer –20°C, which is standard for most hospital and pharmacy freezers. That difference means Moderna’s vaccine should be easier to distribute and store, particularly in the rural United States and developing countries that lack ultracold freezers. Moderna says years of development work enabled its vaccine to be stored at higher temperatures, but last week another mRNA vaccine company announced it is testing a COVID-19 vaccine that early studies suggest can survive at the even warmer temperatures of 2°C to 8°C found in refrigerators.
Many types of vaccines must be stored and transported frozen, via a cold supply chain. Public health officials have even found ways to keep a vaccine ultracold, between –60°C to –80°C, in places like sub-Saharan Africa. There, for the past 5 years, a high-tech thermos called Arktek has helped distribute Ebola vaccines that must be kept ultracold. “In all likelihood, we’ll need a wide range of supply chain tools” to distribute COVID-19 vaccines, says Daniel Lieberman, a mechanical engineer with Global Health Labs in Seattle, a nonprofit created by the Bill & Melinda Gates Foundation, which funded Arktek’s development, and by a venture capital firm started by Bill Gates. Still, relying on an ultracold chain is expensive, and in some places it may make more sense to distribute a vaccine that can tolerate warmer temperatures even if it’s less effective.
Both the Moderna and Pfizer/BioNTech vaccines give the body’s cells an mRNA template for making the spike protein of SARS-CoV-2, the virus that causes COVID-19. The protein then moves to the cell’s surface and triggers an immune response. This mRNA is relatively fragile compared with the proteins or protein fragments that often make up conventional vaccines, and it cleaves easily at room temperature, says Alana Gerhardt, who studies vaccine product development at the nonprofit Infectious Disease Research Institute (IDRI) in Seattle. Also, enzymes called ribonucleases that chew up mRNAs “are everywhere, even in the controlled environment of the lab,” from sources such as lab workers’ breath and skin, Gerhardt says.
The companies give the mRNA some protection during production and storage by inserting it into a carrier, a fatlike substance called a lipid nanoparticle. The lipid also shields the mRNA from enzymes in the blood once it has been injected. But the nanoparticle is deliberately designed to slowly degrade, so it won’t build up in the liver and cause harm, says Massachusetts Institute of Technology geneticist and chemical engineer Daniel Anderson.