system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we use live SARS-CoV-2 as a Covid-19 vaccine?

The SARS-CoV-2 virus infects the upper and lower respiratory tract in humans. As far as I understand, it does not infect other parts of the body, e.g. muscle tissue. So I would expect that if the virus is injected into muscle tissue, it should help the recipient develop immunity, which could then protect them from a conventional Covid-19 infection. This is not being done, so either:

* the virus does infect muscle tissue
* the virus is not infectious enough in muscle tissue to trigger an immune response
* the virus would reach the respiratory tract too quickly from the injection site

Is any of the above the right answer? Or is it something else entirely?
gpt
Because once it gets into the bloodstream, or into the lymphatic system, it can go anywhere in the body, so you would still have a very high risk of getting a full blown infection. You also would have a potential risk of infecting others.

But... there are vaccines that use attenuated (as in less virulent) live viruses (such as the MMR and chickenpox vaccines) and this could be a way to make an effective COVID vaccine.

To attenuate a virus, you infect a host that isn’t the viruses’ normal host, and if you can get the virus to do this successfully, it means it’s mutated sufficiently to get better at infecting this new host - which also usually means it’s no longer so good at infecting its original host.

Once you’ve done testing to verify that this attenuated virus will no longer successfully infect the original host, you can use it as a vaccine in the original host - it will still elicit an immune response, but it won’t replicate much/at all and therefore won’t cause an infection.

These types of vaccines can’t be used in people with compromised immune systems.