It is possible to experience swelling, redness, pain, and itching at the injection site after receiving a vaccine, whether it is Moderna or Pfizer. This can last for a few days. However, if you notice signs of a skin infection such as increased redness, warmth, pain, and inflammation, it is important to monitor the progression by tracing the red area with a marker on your skin. If the redness expands beyond the line you traced, it is important to consult a doctor promptly. If the redness decreases, the situation should improve within 1-2 days.