I have tightness and discomfort, but not pain, in the left side of my chest. it has developed over the last few days and not gone away. breathing is a little difficult. this came up a few days after a second covid vaccine (moderna?

I assume you haven't injured your ribs. If you're experiencing sharp,shooting chest pain with a deep breath or twisting motions of your torso, you are probably dealing with a pinched intercostal nerve or muscle which runs between 2 ribs. This should resolve. If you develop cough,significant breathing difficulties or worsening chest distress see a physician for evaluation. I doubt vaccine cause.

The symptoms may be related and please follow up with your PCP. Sometimes there may be a delayed allergic reaction or wheezing due to another cause. Inflammation of the chest wall and muscular strain is another common cause which is not related to the vaccination. Watch for shortness of breath, cough and fever.