What’s In a Vaccine?

Vaccines all work on the same basic principle: Scientists try to make something that closely resembles a pathogen, and then expose a person’s immune system to it through a small dose administered as an injection. Ideally, the immune system develops a strong memory of the pathogen, so that the next time the person is exposed, their body will mount an attack before the infection can take hold. The trick is to do this without making the person seriously ill from the vaccine itself. There are a few different methods for making vaccines, but they all must strike this delicate balance.

One way to make a vaccine is to weaken, or attenuate, the microorganism while still keeping it alive. The most common method for doing this is growing several generations of the pathogen in environments other than human cells, so that it evolves away from causing disease in humans. By repeatedly culturing live viruses or bacteria in animal cells, scientists can essentially create a bunch of mutants. Then it’s a matter of selecting the mutant strains that can replicate in human cells but don’t cause disease like their wild ancestor. The trick is that these imposters still have to look enough like the original virus to accurately train the immune system to fend it off. Examples of attenuated vaccines include those for measles, mumps, and tuberculosis.

Another type is called an inactivated vaccine, which is made from a dead version of the whole virus or bacteria after it’s been killed with heat or chemicals. This type of vaccine can also be made using smaller pieces of the microbe, which by themselves are not considered alive.

One common approach is to locate the protein that a virus uses like a key to get into human cells, which is usually on its surface. Once scientists know the genetic code for this protein, they can paste it into bacteria or yeast and use these microbial factories to produce huge quantities of it to be used as the basis of the vaccine. The protein alone is often enough to be easily recognized by the immune system and to trigger a defense on subsequent exposure. Alternately, sometimes scientists will genetically modify the virus instead, swapping bits of the disease-causing pathogen into a harmless virus shell. These types of inactivated vaccines almost always require multiple doses, because they’re not as good at stimulating the immune system as a live microbe. But they come with a lower risk of severe reactions. Examples of inactivated vaccines include those for polio, rabies, and hepatitis A and B.

All the approved vaccines on the market use one of these two techniques. But newer methods still in development may get their debut with the Covid-19 outbreak. One such promising technology is nucleotide-based vaccines. Nucleotides are the chemical building blocks that make up genetic material, both DNA and RNA. The virus that causes Covid-19, known as SARS-CoV-2, consists of a strand of RNA enclosed in a spike-covered capsule. It uses these spikes to invade human lung cells. Vaccine makers can copy the genetic instructions for making these spikes and package them up into a shot. Once inside the body, human cells will make the viral proteins, which the immune system will then recognize as foreign. It will produce antibodies against them and learn how to attack any future invaders carrying these protein spikes.

Source: www.wired.com/story/everything-you-need-to-know-about-coronavirus-vaccines/