High prevalence of olfactory disorders 18 months after contracting COVID-19: a case-control study

Background More than a year after recovering from COVID-19, a large proportion of individuals still report olfactory dysfunctions. However, olfactory dysfunction was common also before the COVID-19 pandemic. To establish the prevalence of olfactory dysfunction while controlling for existing cases, we tested individuals with established COVID-19 in the first wave of the pandemic using psychophysical testing of smell functions. Methods Participants were regularly tested for SARS-CoV-2 IgG antibodies since the start of the pandemic. To assess normal rate of dysfunction in the population and skewed recruitment of individuals with prior olfactory dysfunction, SARS-CoV-2 IgG naïve individuals were tested as a control group. Results One and a half years after COVID-19, 37% of individuals who recovered from COVID-19 demonstrated a clinical reduction in their sense of smell, compared to only 20% of the control group. Critically, 51% of COVID-19-recovered individuals reported parosmia symptoms, compared to only 5% in the control group. Conclusions In summary, a full 65% of individuals who recovered from COVID-19 experienced some form of olfactory dysfunction 18 months later. This means that the first wave of COVID-19 increased the prevalence of a clinical reduction in the sense of smell in the population with 17 percentage points and the prevalence of any form of olfactory dysfunction with a staggering 41 percentage points when compared to individuals not infected by the virus. Given the amount of time since the initial insult to the olfactory system, it is likely that these olfactory problems are permanent in a plurality of patients. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement Funding was provided by grants awarded to JNL and CT from the Knut and Alice Wallenberg Foundation (KAW 2018.0152 and KAW 2020.0182, respectively), the Swedish Research Council (2021-06527) and a donation from Stiftelsen Bygg-Gota for Vetenskaplig forskning to JNL, as well as the Swedish Heart Lung Foundation to CT. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Informed consent was obtained and all aspects of the study were approved by the Swedish Ethical Review Authority. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes The datasets generated and associated data scripts to generate results and figures are available on the Open Science Framework data depository.