A total of 2,899 hospital staff members responded to the online questionnaire (response rate, 93.5%). Among the participants, 1,628 (56.2%) tested positive for SARS-CoV-2, of whom 1,331 (81.8%) used an antibody test, 398 (24.4%) used a PCR test, and 101 (6.2%) used both. Moreover, 877 (30.3%) staff members reported highly suspected COVID-19 symptoms, such as fever, dry cough and fatigue, but did not test for COVID-19. The cumulative incidence of SARS-CoV-2 infection between 7 December 2022 and 17 January 2023 was 86.4% (95% CI 85.2–87.7%), according to the case definition presented above.
Table 1 presents the demographic characteristics of the participants in the three groups (confirmed COVID-19 diagnosis, probable COVID-19, and no COVID-19-like symptoms). The majority of patients received at least three doses of a COVID-19 vaccine (90.5%) and were without any chronic medical conditions (86.7%). Participants reporting no distinct symptoms tended to be male (p < 0.001), be 40–59 years old (p < 0.001), have a BMI of 24 kg/m2 or greater (p < 0.001), work in nonclinical areas (p < 0.001), have no need to work night shifts (p < 0.001) and have a duration of less than six months between the latest dose of vaccine and 7 December 2022 (p = 0.022), compared with individuals diagnosed with COVID-19 and those who were highly suspected of having COVID-19.
Table 1
Demographic and health characteristics of the participants.
Variable | Total (n = 2899) | Confirmed COVID-19 diagnosis (n = 1628) | Probable COVID-19 (n = 877) | No COVID-19-like symptoms (n = 394) | p- valuea |
| n | % | n | % | n | % | n | % | |
Gender | | | | | | | | | |
Male | 567 | 19.6 | 280 | 17.2 | 154 | 17.6 | 133 | 33.8 | < 0.001 |
Female | 2332 | 80.4 | 1348 | 82.8 | 723 | 82.4 | 261 | 66.2 | |
Age, years | | | | | | | | | |
Mean, SD | 39.3 | 11.2 | 37.1 | 10.1 | 40.2 | 11.0 | 46.3 | 12.7 | < 0.001 |
18–39 | 1646 | 57.0 | 1064 | 65.6 | 452 | 51.5 | 130 | 33.2 | < 0.001 |
40–59 | 1126 | 38.9 | 523 | 32.2 | 397 | 45.3 | 206 | 52.6 | |
≥ 60 | 120 | 4.1 | 36 | 2.2 | 28 | 3.2 | 56 | 14.3 | |
BMI, kg/m2 | | | | | | | | | |
Mean, SD | 22.2 | 2.9 | 22.0 | 2.8 | 22.2 | 2.8 | 23.0 | 3.0 | < 0.001 |
< 18.5 | 218 | 7.6 | 140 | 8.6 | 60 | 6.9 | 18 | 4.6 | < 0.001 |
18.5–23.9 | 1944 | 67.3 | 1118 | 68.8 | 592 | 68.2 | 234 | 59.5 | |
≥ 24 | 725 | 25.1 | 368 | 22.6 | 216 | 24.9 | 141 | 35.9 | |
Chronic medical conditions | | | | | | | | | |
No condition | 2514 | 86.7 | 1418 | 87.1 | 744 | 84.8 | 352 | 89.3 | 0.168 |
One condition | 342 | 11.8 | 188 | 11.5 | 119 | 13.6 | 35 | 8.9 | |
More than one condition | 43 | 1.5 | 22 | 1.4 | 14 | 1.6 | 7 | 1.8 | |
Occupation | | | | | | | | | |
Clinical practitioner | 581 | 20.0 | 344 | 21.1 | 211 | 24.1 | 26 | 6.6 | < 0.001 |
Nurse | 1075 | 37.1 | 684 | 42.0 | 315 | 35.9 | 76 | 19.3 | |
Medical technician | 272 | 9.4 | 180 | 11.1 | 72 | 8.2 | 20 | 5.1 | |
Administrative or logistics personnel | 315 | 10.9 | 186 | 11.4 | 102 | 11.6 | 27 | 6.9 | |
Others (e.g. cleaning/food service, security) | 656 | 22.6 | 234 | 14.4 | 177 | 20.2 | 245 | 62.2 | |
Working night shifts | | | | | | | | | |
No | 1130 | 39.0 | 936 | 57.5 | 550 | 62.7 | 283 | 71.8 | < 0.001 |
Yes | 1769 | 61.0 | 692 | 42.5 | 327 | 37.3 | 111 | 28.2 | |
COVID-19 vaccine doses | | | | | | | | | |
0 | 42 | 1.4 | 22 | 1.4 | 14 | 1.6 | 6 | 1.5 | 0.654 |
1 or 2 | 234 | 8.1 | 132 | 8.1 | 77 | 8.8 | 25 | 6.3 | |
2623 | 90.5 | 1474 | 90.5 | 786 | 89.6 | 363 | 92.1 | |
Duration between the latest vaccine dose and 7 Dec. 2022 | | | | | | | | | |
No vaccination or < 1 month | 165 | 5.7 | 91 | 5.6 | 48 | 5.5 | 26 | 6.6 | 0.022 |
1–3 months | 74 | 2.6 | 38 | 2.3 | 16 | 1.8 | 38 | 2.3 | |
3–6 months | 147 | 5.1 | 82 | 5.0 | 41 | 4.7 | 24 | 6.1 | |
> 6 months | 2513 | 86.7 | 1417 | 87.0 | 772 | 88.0 | 324 | 82.2 | |
BMI: body mass index, SD: standard deviation.
a Chi-square test or ANOVA for differences among the three groups: confirmed COVID-19 diagnosis, probable COVID-19, and no COVID-19-like symptoms.
Overall, cough (83.3%), fatigue (79.8%) and fever (74.3%) were the most prevalent symptoms. Among the 1,628 participants who tested positive, the most frequently reported symptoms were cough (1292 [92.9%]), fatigue (1255 [90.2%]), fever (1205 [86.6%]), productive cough (1088 [78.2%]), muscle or joint pain (1048 [75.3%]), sore throat (1018 [73.2%]) and stuffy or runny nose (988 [71.0%], Fig. 1a). Similarly, among the 877 participants without a diagnosis but who reported one or more potential symptoms of COVID-19, the most frequently reported symptoms were cough (796 [91.1%]), fatigue (743 [85.0%]), fever (656 [75.1%], productive cough (649 [74.3%]), muscle or joint pain (615 [70.4%]), sore throat (612 [70.0%]), and stuffy or runny nose (531 [60.8%], Fig. 1b).
Table 2 compares the COVID-19-related characteristics between individuals with a COVID-19 diagnosis and highly suspected COVID-19 patients. The former group reported a greater median number of COVID-19 symptoms (14, IQR [11–18]) and a higher percentage of individuals taking leave from work due to illness (44.5%) than did participants in the latter group (13, IQR [9–17]; 30%). However, no statistically significant differences in disease severity (using either admission to hospital or diagnosis of pneumonia as a proxy) were detected between the two groups (p > 0.05).
Table 2
COVID-19-related characteristics of the participants.
Variable | Confirmed COVID-19 diagnosis (n = 1628) | Probable COVID-19 (n = 877) | p valuea |
| n | % | n | % | |
Number of symptoms, median (IQR) | 14 | 11–18 | 13 | 9–17 | < 0.001 |
Illness severity: admitted to hospital | | | | | 0.982 |
Not admitted to hospital | 1613 | 99.1 | 869 | 99.1 | |
Admitted to hospital | 15 | 0.9 | 8 | 0.9 | |
Illness severity: diagnosis of pneumonia | | | | | 0.199 |
No | 1469 | 90.2 | 805 | 91.8 | |
Yes | 159 | 9.8 | 72 | 8.2 | |
Took a leave from work due to the illness | | | | | < 0.001 |
No | 903 | 55.5 | 614 | 70.0 | |
Yes | 725 | 44.5 | 263 | 30.0 | |
IQR, interquartile range.
aChi-square test or Mann‒Whitney U test for differences between two groups.
Table 3 compares the participants with and without SARS-CoV-2 infection. Based on the univariable analysis, the following six factors appeared to be potentially associated with SARS-CoV-2 infection: gender, age, BMI, occupation, working night shifts, and at least 3 vaccine doses, with the latest dose administered 1–3 months before 7 December 2022.
Table 3
Univariable logistic regression analysis of SARS-CoV-2 infection among the participants.
Variable | SARS-CoV-2 infection (n = 2505) | No SARS-CoV-2 infection (n = 394) | Crude OR (95% CI) | p value |
Gender | n | % | n | % | | < 0.001 |
Male | 434 | 17.3 | 133 | 33.8 | 1.00 | |
Female | 2071 | 82.7 | 261 | 66.2 | 2.43 (1.93, 3.07) | |
Age, years | | | | | | < 0.001 |
18–39 | 1516 | 60.6 | 130 | 33.2 | 1.00 | |
40–59 | 920 | 36.8 | 206 | 52.6 | 0.38 (0.30, 0.48) | |
≥ 60 | 64 | 2.6 | 56 | 14.3 | 0.10 (0.07, 0.15) | |
BMI, kg/m2 | | | | | | < 0.001 |
< 18.5 | 200 | 8.0 | 18 | 4.6 | 1.00 | |
18.5–23.9 | 1710 | 68.6 | 234 | 59.5 | 0.66 (0.40, 1.09) | |
≥ 24 | 584 | 23.4 | 141 | 35.9 | 0.37 (0.22, 0.63) | |
Chronic medical conditions | | | | | | 0.144 |
No condition | 2162 | 86.3 | 352 | 89.3 | 1.00 | |
One condition | 307 | 12.3 | 35 | 8.9 | 1.43 (0.99, 2.06) | |
More than one condition | 36 | 1.4 | 7 | 1.8 | 0.84 (0.37, 1.90) | |
Occupation | | | | | | < 0.001 |
Clinical practitioner | 555 | 22.2 | 26 | 6.6 | 12.73 (8.33, 19.44) | |
Nurse | 999 | 39.9 | 76 | 19.3 | 7.84 (5.91, 10.39) | |
Medical technician | 252 | 10.1 | 20 | 5.1 | 6.36 (4.16, 9.73) | |
Administrative or logistics personnel | 288 | 11.5 | 27 | 6.9 | 7.51 (4.64, 12.16) | |
Others (e.g. cleaning/food service, security) | 411 | 16.4 | 245 | 62.2 | 1.00 | |
Working night shifts | | | | | | < 0.001 |
No | 1486 | 59.3 | 283 | 71.8 | 1.00 | |
Yes | 1019 | 40.7 | 111 | 28.2 | 1.75 (1.39, 2.21) | |
At least 3 vaccine doses with the latest dose administered 1–3 months before 7 Dec. 2022 | | | | | | 0.001 |
No | 2457 | 98.1 | 376 | 95.4 | 1.00 | |
Yes | 48 | 1.9 | 18 | 4.6 | 0.41 (0.24, 0.71) | |
BMI: body mass index, OR: odds ratio, 95% CI: 95% confidence interval.
Table 4 presents the results from the forward stepwise logistic regression. The association between BMI and working night shifts was no longer significant after adjusting for the effects of other confounding variables. Compared with males, females were more likely (adjusted OR [aOR]: 1.48, 95% CI: 1.13–1.96) to be infected with SARS-CoV-2. Compared with those aged 18–39 years, the likelihood of SARS-CoV-2 infection decreased with age. Specifically, those aged ≥ 60 years had the lowest likelihood of infection (aOR: 0.33, 95% CI: 0.21–0.53). In terms of occupation, our study showed that clinical practitioners had the highest odds of SARS-CoV-2 infection (aOR: 9.66, 95% CI: 6.24–14.96) compared with those working in areas such as cleaning, security, and laundry. In addition, receiving at least three doses of the vaccine with the latest dose administered 1 to 3 months prior to 7 December 2022 decreased the likelihood of SARS-CoV-2 infection by 59% (aOR: 0.41, 95% CI: 0.22–0.77).
Table 4
Factors associated with SARS-CoV-2 infection according to stepwise logistic regression analysis.
Variable | n (%) | Adjusted OR (95% CI) | p value |
Gender | | | 0.005 |
Male | 434 (17.3%) | 1.00 | |
Female | 2071 (82.7%) | 1.48 (1.13, 1.96) | |
Age, years | | | < 0.001 |
18–39 | 1516 (60.6%) | 1.00 | |
40–59 | 920 (36.8%) | 0.71 (0.54, 0.93) | |
≥ 60 | 64 (2.6%) | 0.33 (0.21, 0.53) | |
Occupation | | | < 0.001 |
Clinical practitioner | 555 (22.2%) | 9.66 (6.24, 14.96) | |
Nurse | 999 (39.9%) | 5.17 (3.70, 7.21) | |
Medical technician | 252 (10.1%) | 5.62 (3.64, 8.66) | |
Administrative or logistic staff | 288 (11.5%) | 5.52 (3.34, 9.11) | |
Others (e.g. cleaning/food service, security) | 411 (16.4%) | 1.00 | |
At least 3 vaccine doses with the latest dose administered 1–3 months before 7 Dec. 2022 | | | 0.005 |
No | 2457 (98.1%) | 1.00 | |
Yes | 48 (1.9%) | 0.41 (0.22, 0.77) | |
OR: odds ratio, 95% CI: 95% confidence interval.
Nonsignificant variables were body mass index, chronic medical conditions, and working night shifts.
One hundred and seven out of 2,505 (4.27%) SARS-CoV-2-infected cases suffered from either fatigue, brain fog or both for at least two months after the initial illness. Among the 80 participants with long COVID symptoms of fatigue, a small proportion reported severe symptoms. More specifically, 13.8%, 11.3% and 16.3% of respondents rated the symptoms as severe for the three fatigue items “feeling of physical or mental exhaustion that does not improve with rest”, “being interested and wanting to do things but not having the energy” and “worsening of fatigue following simple physical or mental activities”, respectively. Additionally, among the 100 participants reporting at least one symptom of brain fog, only a few considered their condition moderate/severe (or sometimes/always) in terms of forgetfulness (31.0%), poor concentration (24.0%), fuzzy thoughts (4.0%), lost words (2.0%), feeling sluggish (1.0%), and feeling confused (none).