Patients that present loss of smell 4 weeks after their coronavirus-19 infection and that have had a real-time polymerase chain reaction (reverse transcriptase polymerase chain reaction ) positive for coronavirus-19 will receive the phototherapy intervention. It consists of the introduction of the nasal probe of the phototherapy device in the nasal cavity of the patient. A mixture of ultraviolet light A, ultraviolet light B and red light will be applied between 2 and 3 minutes. The patient will receive a maximum of 10 interventions. Furthermore both oral corticosteroids (prednisone 40mg) and olfactory training will be applied, in the intervals of daily for 10 days and daily for the duration of the study respectively.

Patients that present loss of smell 4 weeks after their coronavirus-19 infection and that have had a real-time polymerase chain reaction (reverse transcriptase polymerase chain reaction )and both oral corticosteroids (prednisone 40mg) and olfactory training will be applied, in the intervals of daily for 10 days and daily for the duration of the study respectively.

Prior to the beginning of the procedure the patient will be administered a nasal decongestant. Second to this, the patient will be advised to blow his/her nose. This step allows the physician a better view and allows the light exposure to reach the majority of the nasal mucosa(9).

To start the intervention the physician will introduce the phototherapy device inside the patient's nasal cavity as the patient sits in an upright condition. As the procedure is ongoing the physician will slightly rotate the device in order to minimize dryness and to irradiate all the nasal cavity. After the intervention the patient will be administered an emollient/moisturizing drug to further hydrate the nasal mucosa(9).

The intervention will have a crescent duration, the first one will be only two minutes and the following ones will increase 15 seconds each until a plateau is reached at 3 minutes in the 5th intervention.

The Visual Analog Scale (VAS) scale is able to determine the grade of loss of smell. From 1 to 10, being 1 the minimum and 10 the maximum grade of loss of smell.

The Threshold Discrimination Identification score (TDI) is used when the patient undergoes olfactory training, it is comprised by 3 tests: the threshold, discrimination, and identification. Each result is interpreted according to 4 age groups.

The test is accomplished by having the patient sniff at a felt pen which has been impregnated with 4ml of odorant fluid dissolved in propylene glycol. TDI scores defined functional anosmia as a TDI score <16,5, normosmia as a TDI score >30.5 and hyposmia as a score between these two values.

The Self-reported mini olfactory questionnaire (Self-MOQ) is a simple, reliable and valid questionnaire to screen olfactory dysfunction in clinical practice. For the sample in the current study, the optimal scores will be 3.5, 4.5, and 3.5 for distinguishing normosmic from hyposmic/ anosmic patients, anosmic patients, and hyposmic patients, respectively.

The Hospital Anxiety and Depression Scale (HADS) allows for the assessment of both anxiety and depression as it has been seen that both can be present at the same time. This scale focuses on non-physical symptoms so that it can be used to diagnose depression in people with significant physical illness.

The questionnaire comprises seven questions for both anxiety and depression, and should take around 2-5min to complete. A score between 8 and 10 is considered as mild, if it is between 11 and 14 it is considered moderate and if it exceeds 15 it is considered severe.