Characteristics of participants, IgM, IgG, and IgM or IgG apparent seroprevalence according to those characteristics among workers from the public higher education institutions of Porto, Portugal, assessed from May to July 2020
| Total number of participants | IgM seroprevalence | IgG seroprevalence | IgM or IgG | ||||
| N | % | aOR† (95% CI) | % | aOR† (95% CI) | % | aOR† (95% CI) | |
| Overall | 4592 | 142 (3.1) | 45 (1.0) | 179 (3.9) | |||
| Institution | |||||||
| Nursing School of Porto | 148 | 0.7 | 0.19 (0.03 to 1.39) | 2.0 | 2.50 (0.74 to 8.43) | 2.7 | 0.65 (0.24 to 1.80) |
| Polytechnic of Porto | 816 | 2.5 | 0.74 (0.45 to 1.20) | 0.6 | 0.65 (0.25 to 1.67) | 2.9 | 0.71 (0.46 to 1.12) |
| University of Porto | 3628 | 3.3 | Reference | 1.0 | Reference | 4.2 | Reference |
| P value | 0.094 | 0.239 | 0.198 | ||||
| Sociodemographic characteristics | |||||||
| Age strata (years) | |||||||
| <30 | 435 | 1.1 | Reference | 0.9 | Reference | 2.1 | Reference |
| 30–39 | 1097 | 1.9 | 1.56 (0.57 to 4.23) | 0.7 | 0.58 (0.17 to 2.04) | 2.6 | 1.16 (0.54 to 2.52) |
| 40–49 | 1479 | 3.0 | 2.74 (1.06 to 7.09) | 0.9 | 0.74 (0.22 to 2.47) | 3.7 | 1.80 (0.86 to 3.74) |
| 50–59 | 936 | 4.3 | 3.66 (1.38 to 9.71) | 1.3 | 1.03 (0.29 to 3.61) | 5.2 | 2.41 (1.12 to 5.15) |
| 60–69 | 505 | 5.1 | 4.18 (1.51 to 11.57) | 1.4 | 1.06 (0.27 to 4.17) | 6.3 | 2.77 (1.24 to 6.19) |
| ≥70 | 38 | 7.9 | 8.31 (1.83 to 37.73) | 2.6 | 1.67 (0.17 to 16.76) | 7.9 | 4.06 (1.01 to 16.25) |
| Missing | 102 | ||||||
| P value | <0.001 | 0.610 | <0.001 | ||||
| Gender | |||||||
| Men | 1643 | 2.9 | Reference | 1.3 | Reference | 4.0 | Reference |
| Women | 2917 | 3.2 | 1.21 (0.84 to 1.74) | 0.8 | 0.62 (0.34 to 1.13) | 3.8 | 1.03 (0.74 to 1.41) |
| Other | 7 | ||||||
| Missing | 25 | ||||||
| P value | 0.681 | 0.099 | 0.828 | ||||
| Educational level | |||||||
| Basic education | 203 | 6.9 | 2.06 (1.10 to 3.83) | 1.5 | 1.21 (0.35 to 4.19) | 8.4 | 2.00 (1.14 to 3.52) |
| Secondary or post-secondary education | 440 | 4.1 | 1.32 (0.76 to 2.28) | 0.7 | 0.56 (0.17 to 1.89) | 4.8 | 1.20 (0.73 to 1.98) |
| Bachelor’s or Master’s | 2056 | 2.4 | 0.99 (0.66 to 1.48) | 0.7 | 0.55 (0.27 to 1.15) | 3.1 | 0.91 (0.63 to 1.31) |
| Doctorate | 1852 | 3.2 | Reference | 1.3 | Reference | 4.2 | Reference |
| Missing | 41 | ||||||
| P value | 0.003 | 0.146 | 0.001 | ||||
| Nationality | |||||||
| Portuguese | 4327 | 3.1 | Reference | 0.9 | Reference | 3.8 | Reference |
| Non-Portuguese | 198 | 3.5 | 1.68 (0.76 to 3.71) | 2.5 | 2.98 (1.13 to 7.88) | 5.6 | 1.99 (1.04 to 3.79) |
| Missing | 67 | ||||||
| P value | 0.860 | 0.045* | 0.293 | ||||
| Occupational group‡ | |||||||
| High-skilled white collar | 3954 | 3.0 | Reference | 1.0 | Reference | 3.8 | Reference |
| Low-skilled white collar | 538 | 3.5 | 1.05 (0.63 to 1.74) | 0.7 | 0.78 (0.27 to 2.23) | 4.3 | 1.04 (0.66 to 1.65) |
| High/low-skilled blue collar | 36 | 8.3 | 2.82 (0.84 to 9.49) | 0.0 | – | 8.3 | 2.14 (0.64 to 7.16) |
| Missing | 64 | ||||||
| P value | 0.142 | 0.678 | 0.331 | ||||
| Currently working from home | |||||||
| No | 1260 | 3.5 | Reference | 1.1 | Reference | 4.6 | Reference |
| Yes, partially | 1326 | 3.1 | 0.90 (0.55 to 1.46) | 0.8 | 0.56 (0.24 to 1.34) | 3.6 | 0.75 (0.49 to 1.17) |
| Yes, in full | 1915 | 2.9 | 0.90 (0.58 to 1.40) | 1.0 | 0.72 (0.34 to 1.55) | 3.7 | 0.82 (0.55 to 1.21) |
| Other (sick leave, paternity/maternity leave/sabbatical leave) | 23 | 0.0 | – | 4.3 | 3.28 (0.39 to 27.73) | 4.3 | 0.74 (0.10 to 5.70) |
| Missing | 68 | ||||||
| P value | 0.634 | 0.367 | 0.522 | ||||
| Infection-related characteristics | |||||||
| Contact with confirmed cases since January 2020 | |||||||
| No | 4100 | 3.0 | Reference | 0.6 | Reference | 3.6 | Reference |
| Yes | 226 | 6.2 | 2.22 (1.22 to 4.04) | 7.1 | 14.26 (7.28 to 27.91) | 11.1 | 3.69 (2.32 to 5.88) |
| Missing | 118 | ||||||
| P value | 0.014 | <0.001 | <0.001 | ||||
| Quarantine since January 2020 | |||||||
| No | 4384 | 2.9 | Reference | 0.5 | Reference | 3.4 | Reference |
| Yes | 142 | 8.5 | 2.94 (1.53 to 5.63) | 15.5 | 42.79 (22.18 to 82.54) | 19.7 | 7.31 (4.60 to 11.63) |
| Missing | 66 | ||||||
| P value | 0.001* | <0.001* | <0.001 | ||||
| Travel abroad since December 2019 | |||||||
| No | 3124 | 3.2 | Reference | 0.9 | Reference | 4.0 | Reference |
| Yes | 1210 | 3.0 | 0.97 (0.63 to 1.48) | 1.2 | 1.02 (0.51 to 2.07) | 3.9 | 0.98 (0.67 to 1.43) |
| Missing | 110 | ||||||
| P value | 0.735 | 0.540 | 0.792 | ||||
| Symptoms since January 2020 | |||||||
| Asymptomatic | 1421 | 3.4 | Reference | 0.5 | Reference | 3.8 | Reference |
| Moderately symptomatic§ | 1936 | 3.0 | 0.93 (0.63 to 1.38) | 0.6 | 1.34 (0.51 to 3.50) | 3.6 | 1.00 (0.69 to 1.46) |
| Symptomatic¶ | 1006 | 3.1 | 0.99 (0.62 to 1.60) | 2.4 | 6.09 (2.55 to 14.50) | 5.0 | 1.52 (1.01 to 2.28) |
| Missing | 229 | ||||||
| P value | 0.789 | <0.001 | 0.166 | ||||
| Symptoms unusual or sudden since January 2020 | |||||||
| Asymptomatic | 2902 | 3.3 | Reference | 0.6 | Reference | 3.8 | Reference |
| Moderately symptomatic§ | 965 | 2.9 | 1.01 (0.66 to 1.52) | 0.4 | 0.79 (0.26 to 2.36) | 3.2 | 0.96 (0.63 to 1.44) |
| Symptomatic¶ | 496 | 3.2 | 1.02 (0.58 to 1.78) | 4.2 | 8.04 (4.14 to 15.61) | 6.5 | 1.87 (1.23 to 2.84) |
| Missing | 229 | ||||||
| P value | 0.849 | <0.001 | 0.008 | ||||
| Self-perception of the probability of having already been infected (excluding those with diagnosis) | |||||||
| Very low or low | 3416 | 3.0 | Reference | 0.4 | Reference | 3.4 | Reference |
| Moderate | 724 | 2.8 | 0.94 (0.57 to 1.54) | 0.7 | 1.88 (0.66 to 5.37) | 3.3 | 1.07 (0.64 to 1.59) |
| High or very high | 142 | 4.9 | 1.47 (0.63 to 3.44) | 4.2 | 11.43 (4.16 to 31.45) | 8.5 | 2.47 (1.29 to 4.73) |
| Missing | 142 | ||||||
| P value | 0.388 | <0.001* | 0.006 | ||||
*P value for the Fisher’s exact test.
†ORs adjusted for age, gender, educational level and nationality.
‡Not adjusted for the educational level.
§Moderately symptomatic: having or having had one or two of the following symptoms: cough, dyspnoea, odynophagia, headache, vomiting or nausea, diarrhoea, fever, arthralgias, myalgia, asthenia.
¶Symptomatic defined as having or having had at least three symptoms listed before, or dysgeusia or anosmia.
aOR, adjusted OR.