A killed whole-virus vaccine is one of the oldest vaccine technologies. To make such a vaccine for Covid, a manufacturer will grow large quantities of the SARS-CoV-2 virus, and inactivate it using heat or a chemical. The surface of this inactivated virus is dotted with dozens of antigens, or proteins. When this virus is injected into the body, these antigens provoke an immune response.
In contrast, the mRNA vaccine consists only of the genetic code for a single antigen of the SARS-CoV-2 virus, called the spike antigen, wrapped in an oily shell. When this is injected into the body, human cells use this code to churn out the antigen. The body then reacts by making antibodies.
This explains why mRNA vaccine manufacturers don’t need Bio Safety Level 3 labs, as inactivated vaccine manufacturers do. No live virus capable of infecting humans needs to be grown; the genetic code is just a harmless chemical. It also explains why the risk of enhanced respiratory disease (ERD) is different for mRNA vaccines and inactivated vaccines. The former introduce a single antigen into the body, while the latter introduce a large number.
Adenovirus vector vaccines also use the genetic code of the spike antigen, but deploy it differently. Instead of using an oily shell to carry the code into the body, they use a harmless virus as a vehicle. For instance, the AstraZeneca vaccine and Covishield use an adenovirus that causes common cold in chimpanzees. Russia’s Sputnik V uses human adenoviruses engineered to prevent replication. When these adenoviruses, containing DNA for the spike protein, enter the body, human cells start making the antigen.
But making such an adenovirus vector vaccine requires live adenovirus to be grown in large quantities first. Again, as with inactivated vaccines, precautions have to be taken to ensure the virus is contained in the lab, because adenoviruses can infect humans too. However, because these viruses do not pose as great a danger to humans as SARS-CoV-2 does, a BSL-2 facility is enough.
livemint.com, 26 May 2021