The effects can be surprisingly serious for a healthy 28 year old male. COVID-19 does an astonishing amount of damage to your respritory and cardiovascular system with even mild cases impacting your endurance for weeks. Your risk of stroke and heart attack go permanently up and studies are showing that you are more likely to develop Parkinsons or Alzheimers. https://www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/coronavirus-long-term-effects/art-20490351

As for the Pfizer and Moderna vaccines, they are the first of their kind (although the research to get them goes back years to the discovery of the first covid viruses).

They work by conditioning your body to recognize specific pieces of the virus - in this case the "foot" that helps it attach to your cells. After injection, your cells are very briefly given instructions to start producing just the "foot" protein. Your immune system sees this, says