"Shortness of breath" is listed by the NHS as a sign of Covid and not flu. This is more commonly found in the case of illnesses such as Covid or respiratory syncytial virus (RSV) - This is the one key symptom that means you have Coronavirus, and not flu https://www.independent.co.uk/news/health/do-i-have-covid-or-flu-symptoms-vaccine-b2846347.html