This study was designed to investigate the recovery process of OD and GD over time in patients with COVID-19. This study showed that approximately 81% of OD and 84% of GD recovered in the first 30 days of their onsets and with a mean recovery time of 35.93 ± 27.71 days and 37.75 ± 29.52 days for OD and GD respectively. In addition, age less than 20 years, incidence at the time of the second peak of pandemic and partial grade of OD are among the factors affecting the recovery from OD. Among these factors, age and incidence in the second pandemic peak were also factors affecting the recovery from GD.
In the present study 52.5% of cases were female. This higher incidence of OD in COVID-19 women patients was similar with other reports. In the study of Romero-Gameros, C.A. et al. the incidence of OD in women was 63.5% (21). In addition, the study by Chary et al. and Gorzkowski et al. reported higher incidence of OD in women than in men (22, 23). The study by Lechien et al. showed that 59.1% of 252 patients with COVID-19-induced OD were female (24). The results of these studies are consistent with the results of our study. However, a study by Meini et al. On 100 hospitalized patients with COVID-19 showed that men had a 2: 1 ratio of olfactory and taste disorder compared to women (25). This difference may be due to the inpatient setting of this study.
Our study showed that the mean age of patients was 39.12 ± 11.89 years and most patients were in the age group of 20 to 40 years. Other studies have shown almost the same findings as our study, including the study of Gorzkowski et al. that reported a mean age of 39.7 ± 13.7 years (23) and the study of Romero-Gameros et al. that reported a mean age of patients of 41 ± 13 years (21). In the study of Lechien et al., the mean age of patients was 36.9 ± 11.4 years (24) and in the study of Amer et al. the mean age of patients was 34.26 ± 11.91 years (26); but in the study of Meini et al., that conducted on hospitalized patients, the mean age was 65 ± 15 years, this difference may to be due to the difference in the studied population (25).
Based on the results of our study, in 82.6% of cases the onset of OD and general symptoms in COVID-19 patients was at the same time or simultaneous, in 11.2% of cases OD start before general symptoms, and in 6.2% of cases the onset of OD was after the general symptoms. This observation was similar to other reports. Lv et al. showed that in more than 87% of cases, the onset of OD and GD coincided with the onset of general symptoms of COVID-19 (27). Ramasamy et al. found that 15.9% of patients reported onset of OD or GD before the onset of general symptoms, 34.1% after, and 36.4% with the onset of general symptoms of COVID-19 (28). Therefore, the most common time of onset of OD coincides with the onset of general symptoms, which is consistent with the findings of our study. In addition, our study showed that in COVID-19 patients who did not develop OD at the onset of the general symptoms, the mean of time window was 5.03 ± 2.32 days before and 10 ± 10.51 days after the general symptoms. Ramasamy et al. reported that in most COVID-19 patients in their study the median duration of OD and also the complete recovery time from it was 7 days (28). In addition, the study by Chary et al., showed that OD occurred from 3 days before to 7 days after general symptoms of COVID-19 (22).
The recovery rate of OD in this study was 80.99% at the end of the first 30 days, this rate reaches 81.82% by 90 days and 98% of patients recover by 6 months. This trend was also observed for GD and in our study all cases of GD had complete recovery within 6 months of follow-up. Gorzkowski et al., showed that 95.71% of OD recovered within 26 days after the onset of OD, and 51.43% of patients achieved complete recovery (23). Meini et al. reported 82% improvement for OD during the 4-week follow-up (25). Lv et al., showed that during 4 weeks 89.7% of COVID-19 patients had GD and OD recovery (27). Paderno et al., reported 87% and 82% recovery rates for OD and GD during one month respectively (29). Hopkins et al., reported a 79% recovery rate over the course of a month (30). Amer et al., reported complete and partial OD recovery rate of 96 patients with COVID-19; this rate was 33% and 41% for complete and partial recovery respectively within 11 days, while 25% did not achieve any recovery during one month (26). In a study by Vaira et al., 7.3% of patients still had severe disorders during the two-month follow-up (31). Cho et al. also reported an OD recovery rate of 71.8% (32). In general, the results of these studies are consistent with our findings and the comparison of the results of the above studies shows that a significant percentage of patients achieve OD and GD recovery in a short time and the difference in the percentages of recovery rates may be attributed to the assessment method of OD or GD, the duration of follow-up or genetic susceptibility to the disease.
In the present study, the mean and median of recovery time were 28.05 ± 2.33 and 14 ± 0.56 days for OD, and 29.52 ± 2.55 and 14 ± 0.60 days for GD. In study of Chary et al., the recovery time for OD and GD was 15 days (22). The OD recovery time in the study of Meini et al., was 18 days (25). In the study by Cho et al., the mean time for OD recovery was 10.3 ± 8.3 days (30). Gorzkowski et al., and Romero-Gameros, C.A. et al., reported 11.6 and 10 days, respectively as the mean recovery time from onset of OD (21, 23). In the study by Cho et al., mean time for GD recovery was 9.5 ± 6.8 days and the complete recovery rate of GD was 83.3% (32).
The present study did not show a significant difference for the median recovery time in patients with OD in terms of gender, age groups and ethnicity. So that the findings of the study of Chary et al. and the study of Lv et al. are in line with the results of our study (22, 27). However, our study showed that the median recovery time was higher in people infected with the first peak of the epidemic than in those infected in the second peak of the epidemic, which may be due to the fact that OD and GD was known in the first peak, therefore there was more knowledge about the management of OD or GD cases in the second peak and this may be the reason of the reduction of the recovery time. It may also be related to changes in the pathogenicity of SARS-Cov-2 virus. Gorzkowski et al., studied 229 patients with COVID-19 and reported higher incidence of OD in cases that were infected before March 20, 2020 and lower incidence in cases that were infected later (70.3% vs. 53.9%) (23).
Our study showed that the probability of recovery for OD and GD over time in the under 20 years’ age group is about 4 times higher than in the over 60 years’ age group. The study by Paderno et al. also showed that the patients with the partial OD were more likely to recover than patients with complete OD. However, this study did not show a significant difference for recovery of OD or GD among age groups (29). Lv et al., also showed that age and gender had no effect on the improvement of OD or GD (27).
We studied the recovery of OD and GD in COVID-19 patients over a long period of time, e.g., 6 months and our result showed that 98% of patients had been recovered during this time. However, it is necessary to point out the limitations of this study. The evaluation of OD and GD in COVID-19 patients was done by means of a self-reported method and there was no objective assessment. In addition, no quantitative scale was used to classify the severity of OD or GD. The status of smoking and other co-morbidities’ or diseases with potential effect on the OD or GD recovery process were not investigated. These limitations can be applied in future studies.