While vaccines like Covishield can cause mild, temporary physical stress that might slightly elevate blood sugar, a level of 556 mg/dL is critically high and unlikely to be caused by the vaccine alone. Since he is not feeling better despite starting Zavamet (vildagliptin/metformin), this situation requires immediate medical evaluation. Please contact his doctor or take him to the nearest emergency care facility right away. Extremely high blood sugar can lead to serious complications if left untreated. In the meantime, keep him hydrated with water and avoid any sugary drinks while you seek urgent professional medical care.