Conclusion
Our review has shown that there is already significant evidence which
demonstrates an association between OD and the novel coronavirus –
COVID-19. It is unclear if this finding is unique to this coronavirus as
individual viral phenotypes rarely present in such concentrated large
numbers. Classically patients present with persistent symptoms following
a viral illness many weeks or months after. The symptomology during the
infective phase of the virus has not previously been studied and
therefore it is not possible to draw direct comparison between other
similar viruses. Walker et al have however demonstrated trends between
increasing cases of COVID-19 and the increase in positive novel
coronavirus cases that was not mirrored during the previous H1N1
pandemic in 2009 .
Due to the rapid spread of COVID-19 in this pandemic it is
understandable that there are a lack of studies using objective measures
and rigorous controls. The most common methodologies used were
cross-sectional questionnaires and case series. These approaches are at
risk of bias and we can only discuss associations as a result. Further
research will be required to demonstrate clearer links between OD and
COVID-19 going forwards.
In the meta-analysis of studies with cohorts of patients who had a
positive PCR test the prevalence of OD was 62%. Both Menni et al and
Yan et al compared OD to other associated symptoms seen in COVID-19 such
as fever, cough, fatigue, dyspnoea and diarrhoea. They both found that
OD was the strongest predictor of a positive PCR result for COVID-19
when compared to the other symptoms when applying their logistic
regression analysis . It is worth noting that the current self-isolation
advice, released by Public Health England, only applies to people
experiencing a new continuous cough and/or a high temperature .
When we looked at patients who had experienced OD during the outbreak
there were several studies that demonstrated an increase in the
prevalence of loss of smell in their populations when compared to
previous estimates . The largest data sets, conducted predominantly in
the outpatient setting, by Hopkins and Bagheri et al indicated a female
preponderance in their cohorts (73% and 71% respectively). These two
studies also demonstrated an average affected age between 30-40. It has
been demonstrated that both advanced age and the male sex are risk
factors for the severe form of the disease and an increased rate of
mortality . It could be that this cohort of patients were not targeted
by this study due to the more elderly populations not interacting with
web-based surveys or being within the inpatient population due to their
disease severity. Moein et al demonstrated in their study of inpatients
that OD was a common finding in this population too when they applied
objective UPSIT testing to confirmed cases . Further research is needed
to identify if the incidence of OD varies between different ages and
genders and as such if particular disease phenotypes for COVID-19 can
give clinicians prognostic information.
In areas where testing has not been adopted widely tracking of this OD
could be vital in identifying hot-spots where population-based
management strategies can then be targeted. Tracking OD using
mobile-based applications, such as the one developed by Menni et al,
will allow real time data tracking for aid models in the prediction of
national or regional COVID-19 cases . This approach could lead to
specific social distancing measures being implemented in areas where OD
is wide-spread and will also help in modelling when these measures could
be relaxed as most patients seem to recover their sense of smell
following the illness.
Given the growing evidence to support an association between OD and
COVID-19 it is paramount that national and international health bodies
recognise this association. The World Health Organisation and national
public health bodies have not yet accepted this link and as such
patients with OD are not self-isolating. We have demonstrated that OD
can be the first symptom to arise, and in some cases, may be the only
symptom(s) that patients experience. We recommend that people who
develop OD during the pandemic should be self-isolating and this
guidance should be adopted internationally to prevent transmission.