Olfactory dysfunction is a defining symptom of COVID-19 infection. Studies have demonstrated improved olfaction in patients with post infectious olfactory dysfunction after an olfactory training (OT). The aim of this study is to assess the clinical outcomes of olfactory training (12 weeks) therapy in the treatment of persistent olfactory dysfunctions after COVID-19. Specially, we aim to compare the effectiveness of two different olfactory training (different odors).

A group will train themselves with 4 scents (rose, orange, clove and eucalyptus) and another group with 4 different scents (cheese, coffee, strawberries and lemon). Olfaction sensory evaluation will be performed by using different olfaction tests (Sniffin' Sticks and UPSIT) and complete questionnaires to assess olfactory perception and particularly parosmia and phantosmia.

Patients who meet the criteria, who have agreed to participate in this research project and sign the consent form will be placed in one of the following two groups:

● Experimental group (olfactory training)

1. Initial meeting: Each participant will be asked for a session of approximately 1:30 h which will take place in a ventilated room in the laboratory of Dr. Johannes Frasnelli at UQTR. During the hour preceding this evaluation, subjects will be asked not to smoke, eat or drink anything other than water and not to wear perfume. A research assistant will be responsible for completing the various tests. During this meeting, the following tests will be done:

   • Data collection (case report form): medical and socio-demographic questionnaire that will be used for data analysis.
   • A nasal obstruction criterion scale - NOSE
   • A subjective questionnaire on the impact of OD on quality of life - sQOD-NS
   • Chemosensory Dysfunction Home Test - TMSC
   • The UPSIT 40 test
   • The Sniffin' Sticks test
   • Test to assess the manifestation of parosmia and phantosmia
   • Teaching on the olfactory rehabilitation kit and explanation of the training protocol.

2. Treatment (12 weeks): Following the initial meeting, patients can begin training:

   ■ Participants will bring their training kit with them following the olfactory evaluation.

3. Final meeting: After 12 weeks of treatment, an appointment will be scheduled with the participant. The olfactory evaluation will be done as described in the initial meeting. During the hour preceding this evaluation, subjects will be asked not to smoke, eat or drink anything other than water and not to wear perfume. A research assistant will be responsible for administering the various tests. During this meeting, the following tests will be done:

   • A subjective questionnaire on the impact of OD on quality of life - sQOD-NS
   • Chemosensory Dysfunction Home Test - TMSC
   • The UPSIT 40 test
   • The Sniffin' Sticks test
   • Test to assess the manifestation of parosmia and phantosmia

Inclusion Criteria:

• Patient diagnosed positive for COVID-19 with persistent olfactory dysfunction
• Willing and able to provide written informed consent
• Understand and read the French language
• Have an internet connection and a working email address

Exclusion Criteria:

• Anosmia and hyposmia pre-covid-19
• Be known chronic rhinosinusitis with or without nasal polyposis
• Have received radiotherapy or chemotherapy for Head and Neck Tumors
• Have a diagnosis of Alzheimer's, Parkinson's, multiple sclerosis or any other neurodegenerative disease 5. Have a brain tumor or ENT diagnosis 6. History of naso-sinus surgery; 7. History of traumatic brain injury