system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do vaccines contain multiple components, not just the dead virus?

i understand preservatives and all that, but are there other things in the vaccine that will make it more effective? do these components vary from vaccine to vaccine, and are these the reason why vaccines take so long to develop?

and i guess while i’m here, would simply injecting someone with the dead virus (in solution) be enough to trigger immunity?
gpt
You need the immune system to recognize the vaccine component as a threat, not just trash. If you just inject the dead vaccine, it's possible that your macrophages (essentially trash collectors) will just eat the dead virus and move on.

Instead, if you add components called "adjuvants," the body will actually respond to this new molecule. The T and B cells will respond by making antibodies against that molecule.

So when you're choosing what molecule, you need to find something that is:

* unique to the virus (If a very similar component exists in your heart, after the vaccine, your body will destroy that part of your heart. This is called molecular mimicry and is what can happen after you get strep throat. That's why we treat strep throat a little aggressively)

* found in most versions of Covid-19 (Obviously, or this won't be helpful
 This is why the flu virus is hard to vaccinate against. We predict which strains are most likely, but we can guess wrong)

* Safe for humans to be injected with

* Doesn't create TOO strong of an immune response (If people get a fever and feel like shit for a week after the vaccine, they would rather risk getting the actual virus. This is an issue with the flu vaccine (have you heard people say they got flu from the vaccine? They didn't, but they may have gotten too strong of an immune response)).

* Creates a strong enough response in the immunocompromised. (This is a really hard line to walk. People with bad immune systems are high risk to viruses, but they're also harder to vaccinate since their body may not respond to vaccines.)