Discussion

The present results show that although OLP did not significantly improve the severity of allergic symptoms in comparison to TAU, it did reduce the frequency of symptoms and the degree of allergy-related impairment. By distinguishing between symptom severity and symptom frequency, the current results can resolve the inconclusive findings from previous research on the effects of OLP in allergic rhinitis: Previous studies revealing a significant effect of OLP assessed symptom frequency only21,22, whereas a previous study that failed to find a significant effect of OLP over TAU focused only on symptom severity23. Assessing both symptom severity and frequency, the present results suggest that OLP has medium to large effects on the frequency of allergic symptoms, whereas its effects on symptom severity are only small to medium (which failed to reach significance in the present study, possibly due to insufficient power to uncover such effects).
An altnernative interpretation of the discrepant results for the two symptom measures might be the breadth of symptoms being considered. More specifically, the symptom severity measure focuses on allergic symptoms related only to the nose and eyes, whereas the frequency questionnaire also considers additional symptoms. Indeed, the analysis of more specific symptom clusters suggests that the beneficial effects of OLP over TAU were particularly pronounced in symptoms pertaining to the eyes, breathing, and more general symptoms such as tiredness or trouble concentrating, whereas the majority of the items of the CSMS refers to nose-related symptoms. Thus, it might be that the CSMS is not ideal to capture the beneficial effects of OLP on the variety of allergic symptoms.
With regard to symptom-related impairment, it is worth noting that the present study, unlike previous studies 21-23, found that OLP significantly reduced impairment, with medium to large effects. Possibly, this is related to the fact that the present study, unlike previous work, used a questionnaire that assesses impairment specifically in the context of allergic symptoms. Thus, combining the present findings with prior research 21-23, it seems that OLP has the potential to reduce allergy-specific impairment, but not impairment more generally.
Given the positive results regarding the reduction of symptom frequency and impairment through OLP, the current findings suggest that a physical patient-provider interaction might not be necessary for OLP to work. This is an important finding with respect to the still ongoing COVID-19 pandemic and its restrictions of physical contact as well as increasing demand for online therapy. Taking the less positive results of a previous study administering OLP remotely into account23, it may be important to make sure that patients can start taking the placebos immediately after the virtual encounter, as it was done in the present study. In line with that notion, participants from the TAU group who started taking placebos with a few days delay did not show that much improvement as well. In addition, the correlational results suggest that it is important to ensure that patients are properly informed about the effects of OLP, since the extent to which participants felt informed about placebos was associated with lower symptom burden after taking OLP.

Limitations

The most significant limitation of the present study is its relatively low sample size. Althought the current sample size was larger than in all previous studies examining OLP in allergic rhinitis21-23, it did not offer enough power to uncover small to medium effects, as it would have been necessary for the effects of OLP on symptom severity. Furthermore, the current study focused on self-reported allergic symptoms only, and future research may examine whether beneficial effects of OLP can also be observed on a physiological or immunological level.

Conclusions

The present results suggest that remotely provided OLP can improve the frequency of allergic symptoms as well as allergy-related impairment over TAU. OLP might also have beneficial effects on symptom severity, but this effect warrants further exploration using larger samples.