The investigators propose a real-world study to assess the mechanism of action of long-lasting response to mepolizumab in patients with chronic rhinosinusitis with nasal polyposis (CRSwNP) and identify clinically useful predictors of response. Mepolizumab is a monoclonal antibody targeting IL-5 and is approved for use in asthma and CRSwNP. In clinical studies, 12 months of treatment with mepolizumab improved signs and symptoms of CRSwNP and reduced the need for surgery. While several biologic medications targeting facets of the Type 2 mechanism are currently indicated for chronic rhinosinusitis with nasal polyps mepolizumab alone appears capable of modifying the disease's biological behaviour and producing long-standing improvements after the cessation of treatment. In the mepolizumab for CRSwNP regulatory trial (SYNAPSE), a subset of patients experienced dramatic and long-lasting, which is over 48 months after cessation of administration of the investigational medicinal product (IMP) in our experience. This has been partially captured in a follow-on study to the registration trail, which showed that a subset of patients followed for 24 weeks after cessation of biologic therapy (with continued use of mometasone furoate) demonstrated persistent improvements over baseline. However, the mechanism of the long-lasting effect in a subset of patients is not well understood, and it is impossible currently to identify patients who will derive this maximal benefit. The mechanism for the prolonged improvements in CRSwNP seen in certain patients with mepolizumab remains to be established but suggests that effects beyond eosinophil trafficking are implicated. The investigators believe that mepolizumab has IL-5-mediated pleiotropic effects which contribute to disease modification with effects extending beyond eosinophil activation and trafficking. This may include the following primary or secondary effects: i) Improving epithelial barrier function ii) Altering mast cell dynamics iii) Reversing epigenetic modifications iv) Altering the immune response to better clear pathogenic bacteria or viruses.