Since you are already taking Montair LC (which contains montelukast and levocetirizine), new or worsening generalized itching suggests your allergies might be active right now. While having allergic asthma doesn't automatically prevent you from getting the Covishield vaccine, it is safest to have a doctor evaluate your itching before you get the injection tomorrow. This ensures you aren't having an active, severe allergic reaction that could complicate your vaccination. Consult the medical officer at your vaccination center before getting the shot.