Since the beginning of the pandemic, we have been hoping for a vaccine to defeat the virus and get back to “normality”, a more lighthearted daily life that we had to put aside.
On March 16th, a few days after the WHO had declared the pandemic, the biotech company Moderna announced the beginning of phase 1 clinical trial to test the safety of its vaccine candidate, foreseeing that, in case of positive results, it could be available on the market in 12 to 18 months.
On November 16th, the same company announced that the first results of phase 3 clinical trial showing 94.5% efficacy two weeks after the administration of the second dose.
Two days later, another pharmaceutical company, Pfizer, made public the preliminary results of phase 3 clinical trial for its own vaccine candidate: 94% efficacy 28 days after the administration of the first dose.
How do these two vaccines work? And how has it been possible to have results in such a brief time?
Both Moderna and Pfizer vaccines are based on a more recent technology than the other vaccines available in the market for other infectious diseases: they are mRNA vaccines.
“Conventional” vaccines are based on attenuated or killed pathogens that need to be cultured and produced in large quantities, or are based on proteins, or part of them, that need to be produced and purified.
In this new type of vaccine, however, it is a fragment of RNA that is administered, carrying the instructions to produce the S protein of SARS-CoV-2. After the injection, the RNA is absorbed by the cells that start producing the viral protein and present it to the B and t lymphocytes of the immune system.
The S protein alone does not cause any disease and it is not dangerous for the cells that are producing it; moreover, the RNA of the vaccines stays in the cells for a short time and the S protein is produced only temporarily; then both the RNA and the protein are demolished by the cell itself.
If, after the vaccination, a person is infected by SARS-CoV-2, their lymphocytes will immediately recognize the S protein on its surface and will be ready to attack the virus because the vaccine had already taught them to do so.
With this technology, the cells are somehow producing the vaccine by themselves: there is no need for the production of the viral protein in cells of bacteria and its purification, and, similarly, there is no need to culture the virus, inactivate and purify it as for the vaccines based whole microorganisms.
The steps for the production of an mRNA vaccine are the synthesis of a nucleotide sequence (A, U, C, G) with the instructions for the cell, and the production of a lipidic envelope for the RNA that makes it easier for the cells to absorb it. This why Moderna and Pfizer have been able to reduce the production times and obtain the first results on the efficacy of their vaccines less than one year from the beginning of the pandemic.
(If you want more details on how vaccines work click here, if you want to know how proteins are produced from instructions written on DNA and RNA click here)
Picture: “Doctor writing Covid-19 Vaccine Loading” by focusonmore.com is licensed under CC BY 2.0
Bibliography
Moderna press release: https://investors.modernatx.com/news-releases/news-release-details/modernas-covid-19-vaccine-candidate-meets-its-primary-efficacy
Pfeizer press release: https://www.pfizer.com/news/press-release/press-release-detail/pfizer-and-biontech-conclude-phase-3-study-covid-19-vaccine
Moderna explanatory video on YouTube: https://youtu.be/qJlP91xjvsQ
mRNA vaccines — a new era in vaccinology, Pardi N. et al., Nature Reviews Drug Discovery, 2018, https://doi.org/10.1038/nrd.2017.243
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