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Two breakthrough vaccines using the same breakthrough technology have been shown to be very effective in preventing Covid-19, but differences in how vaccines are designed affect how quickly production can be scaled up and how they are distributed.
The vaccines – one produced by the American company Moderna and the other as part of a partnership between Pfizer and Germany’s BioNTech – have both recorded efficacy rates of over 94% in clinical trials, this which arouses hope around the world to be able to emerge from the pandemic.
At the heart of both injections is a strand of messenger ribonucleic acid, or mRNA – a sequence of approximately 2,000 biochemical letters of genetic code that transmit instructions to the recipient’s immune system to recognize and fight the coronavirus infection. The technology has never been used in a vaccine before.
Moderna’s vaccine uses 100 micrograms of RNA per dose, while Pfizer-BioNTech’s vaccine uses only 30 micrograms, making it easier to produce and less expensive, explained Zoltán Kis, researcher at Imperial. College London’s Future Vaccine Manufacturing Hub.
This should allow Pfizer-BioNTech to increase production of its vaccine faster than its American competitor. Immunologists added that it was not yet clear why Moderna’s shot needed the largest dose of RNA.
“It is notoriously difficult for outsiders to find out exactly what is in a vaccine,” said Alexander Edwards, associate professor of biomedical technology at the University of Reading. “But the way it’s put together can have a big effect on how it works.” Although the RNA in each is essentially the same, there can be tiny differences in the genetic sequence that make Pfizer-BioNTech more effective at lower doses.
“RNA in mRNA vaccines is produced by a chemical process rather than by the biological processes used to produce other vaccines, which involve the growth of cell cultures,” said Professor Edwards. It is slightly modified from the “wild-type” RNA naturally present in the virus, to make it more stable and more easily read by human cells, he said.
In Moderna and Pfizer-BioNTech vaccines, RNA is encapsulated in “lipid nanoparticles”. These microscopic droplets of oily liquid – about 0.1 microns in diameter – enclose and protect fragile genetic instructions as they are made, transported, and ultimately injected into humans. The composition of the lipid nanoparticles is slightly different in the two vaccines, the scientists said, with a number of implications.
“These nanoparticles can give the formulation a magical boost,” said Professor Edwards. “You might have a list of ingredients but you don’t know how they are combined to produce the best size and shape particles. There are strong parallels with food production – you might know the ingredients in Heinz Ketchup, but you can’t make it.
Pfizer and BioNTech obtain their nanoparticles from Acuitas, a specialized Canadian company, while Moderna has developed its own lipid technology.
“The art and challenge of nanoparticle development is to combine lipids with different physical characteristics in a way that stabilizes RNA as efficiently as possible,” said Mike Watson, former head of vaccines at Moderna.
In both cases, cold storage is necessary to keep the nanoparticles in good condition and to prevent degradation of the mRNA. But while Moderna’s vaccine is stable enough to survive storage for six months at -20 ° C, the temperature of a standard household or medical freezer, Pfizer / BioNTech vaccine must be stored and transported at -70 ° C.
As a result, once approved by regulatory authorities, Moderna’s vaccine can be distributed “more easily and cheaply,” said Dr Kis of Imperial.
Pfizer and BioNTech had to design special “thermal senders” capable of keeping the product for up to 15 days at this temperature when regularly filled with dry ice. Each package has a thermometer linked to GPS, which tracks its temperature and location on the Pfizer distribution network. Even so, the temperature requirement will make it more difficult to distribute the vaccine in countries without sufficient cold chain storage capacity, such as many in Africa and Asia.
In contrast, adenoviral vaccines in development, such as the one produced by the University of Oxford and AstraZeneca, can be stored for several months without freezing. Rather than using mRNA, the Oxford vaccine attaches the coronavirus spike protein genes used to elicit the immune response to a harmless adenovirus, which carries them into human cells. Sarah Gilbert, head of the Oxford team, said her vaccine was stable at ordinary refrigerator temperatures between 2 ° C and 8 ° C.
The differences in how the lipids from Pfizer-BioNTech and Moderna are formulated are also likely to affect how each injection works. “The lipid nanoparticles have some adjuvant activity, causing mild inflammation with vaccination that helps the immune system make antibodies and T cells that target the Sars-Cov-2 virus,” said Brian Ferguson, immunology researcher at Cambridge University.
Another approach, under development by Imperial College and in early clinical trials, is a self-amplifying RNA vaccine that makes more copies of itself after injection into human cells. This approach could ultimately reduce the amount of RNA needed to as little as 1 microgram per dose, Dr. Kis said.
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