Table 2.
Characteristics of the household members n = 644
| Characteristics | No. (%)$ | P-value Univariable RR | Multivariable RR (95% CI) | ||||
|---|---|---|---|---|---|---|---|
| Total | Secondary case | No secondary case | Per person SAR (95% CI)¥ | Univariable RR (95% CI) | |||
| Household members at risk | 644 | 200 | 444 | ||||
| Age group in years | 0.60 | ||||||
| > 18 | 394 (61.3%) | 129 (64.5%) | 265 (59.8%) | 33.8 (28.8–39.2) | Ref | ||
| 12–18 | 107 (16.6%) | 28 (14.0%) | 79 (17.8%) | 29.4 (21.1–39.3) | 0.87 (0.63–1.19) | ||
| < 12 | 142 (22.1%) | 43 (21.5%) | 99 (22.3%) | 30.9 (23.3–39.7) | 0.91 (0.70–1.19) | ||
| Sex | 0.30 | ||||||
| Female | 315 (48.9%) | 93 (46.5%) | 222 (50.0%) | 31.0 (25.7–36.8) | Ref | ||
| Male | 329 (51.1%) | 107 (53.5%) | 222 (50.0%) | 34.0 (28.8–39.7) | 1.10 (0.92–1.32) | ||
| Underlying medical condition | 0.37 | ||||||
| No underlying medical condition | 566 (89.1%) | 174 (87.0%) | 392 (90.1%) | 32.4 (27.8–37.4) | Ref | ||
| Any underlying medical condition | 69 (10.9%) | 26 (13.0%) | 43 (9.9%) | 37.1 (27.0–48.4) | 1.14 (0.85–1.54) | ||
| Relationship to index case | 0.01 | ||||||
| Spouse | 171 (26.9%) | 71 (36.6%) | 100 (22.6%) | 42.3 (35.2–49.7) | Ref | Ref | |
| Child | 211 (33.2%) | 66 (34.0%) | 145 (32.8%) | 33.4 (26.3–41.3) | 0.79 (0.62–1.00) | 0.78 (0.62–0.98) | |
| Parent | 122 (19.2%) | 28 (14.4%) | 94 (21.3%) | 22.5 (14.8–32.7) | 0.53 (0.35–0.82) | 0.66 (0.37–1.19) | |
| Siblings | 79 (12.4%) | 16 (8.2%) | 63 (14.3%) | 19.5 (11.3–31.5) | 0.46 (0.27–0.79) | 0.58 (0.31–1.10) | |
| Friend/other housemate | 53 (8.3%) | 13 (6.7%) | 40 (9.0%) | 22.6 (12.2–38.2) | 0.54 (0.29–0.97) | 0.50 (0.28–0.89) | |
| Index case age group, years | 0.03 | ||||||
| > 18 | 489 (76.6%) | 172 (86.4%) | 317 (72.2%) | 36.2 (31.1–41.7) | Ref | Ref | |
| 12–18 | 85 (13.3%) | 19 (9.5%) | 66 (15.0%) | 22.6 (12.0–38.4) | 0.62 (0.34–1.14) | 0.85 (0.39–1.85) | |
| < 12 | 64 (10.0%) | 8 (4.0%) | 56 (12.8%) | 13.4 (5.3–29.9) | 0.37 (0.15–0.90) | 0.60 (0.20–1.82) | |
| Index case sex | 0.95 | ||||||
| Female | 400 (62.1%) | 119 (59.5%) | 281 (63.3%) | 32.4 (27.0–38.4) | Ref | ||
| Male | 244 (37.9%) | 81 (40.5%) | 163 (36.7%) | 33.8 (25.3–41.3) | 1.01 (0.75–1.37) | ||
| Index case BMI* | 0.23 | ||||||
| Age < 2 years | 4 (0.6%) | 1 (0.5%) | 3 (0.7%) | - | - | ||
| Normal weight | 367 (58.6%) | 123 (62.1%) | 244 (57.0%) | 35.8 (29.5–42.6) | Ref | ||
| Underweight | 9 (1.4%) | 1 (0.5%) | 8 (1.9%) | 11.1 (0.2–43.1) | 0.31 (0.06–1.55) | ||
| Overweight | 192 (30.7%) | 54 (27.3%) | 138 (32.2%) | 28.0 (20.8–36.6) | 0.78 (0.56–1.10) | ||
| Obesity | 54 (8.6%) | 19 (9.6%) | 35 (8.2%) | 38.3 (24.2–54.7) | 1.07 (0.68–1.68) | ||
| Index case having any underlying medical condition | 0.57 | ||||||
| No | 572 (90.1%) | 184 (92.0%) | 388 (89.2%) | 33.6 (28.7–38.8) | Ref | ||
| Yes | 63 (9.9%) | 16 (8.0%) | 47 (10.8%) | 29.0 (17.3–44.5) | 0.87 (0.53–1.43) | ||
| Index case symptom status | 0.01 | ||||||
| ARI | 482 (75.3%) | 171 (85.5%) | 311 (70.7%) | 37.3 (31.9–43.0) | Ref | Ref | |
| Mild symptoms | 132 (20.6%) | 27 (13.5%) | 105 (23.9%) | 21.3 (13.8–31.5) | 0.57 (0.37–0.89) | 0.57 (0.36–0.90) | |
| Asymptomatic | 26 (4.1%) | 2 (1.0%) | 24 (5.5%) | 8.0 (1.2–38.9) | 0.21 (0.03–1.35) | 0.29 (0.05–1.89) | |
| Index case hospitalized at time of enrollment | 0.87 | ||||||
| Yes | 21 (3.3%) | 6 (3.0%) | 15 (3.4%) | 30.4 (11.1–60.5) | Ref | ||
| No | 620 (96.7%) | 193 (97.0%) | 427 (96.6%) | 32.6 (28.0–37.6) | 1.07 (0.44–2.61) | ||
| Inclusion season | 0.20 | ||||||
| Winter (Dec–Feb) | 154 (23.9%) | 63 (31.5%) | 91 (20.5%) | 41.1 (31.0–51.9) | Ref | ||
| Spring (March–May) | 78 (12.1%) | 23 (11.5%) | 55 (12.4%) | 33.8 (21.5–48.7) | 0.82 (0.51–1.34) | ||
| Summer (June–August) | 34 (5.3%) | 7 (3.5%) | 27 (6.1%) | 23.9 (10.7–45.1) | 0.58 (0.27–1.27) | ||
| Autumn (Sept–Nov) | 378 (58.7%) | 107 (53.5%) | 271 (61.0%) | 29.8 (24.3–35.9) | 0.73 (0.53–1.00) | ||
| Inclusion site | 0.32 | ||||||
| UA (Belgium) | 74 (11.5%) | 26 (13.0%) | 48 (10.8%) | 40.5 (27.8–54.5) | Ref | ||
| UCHB (Switzerland) | 103 (16.0%) | 30 (15.0%) | 73 (16.4%) | 26.9 (16.7–40.2) | 0.66 (0.38–1.16) | ||
| UMCU (Netherlands) | 467 (72.5%) | 144 (72.0%) | 323 (72.7%) | 32.5 (27.3–38.3) | 0.80 (0.55–1.17) | ||
$Some numbers might not add up to 644 due to missing values
¥Per-person SARs and related relative risks are estimated by using GEE models taking into account clustering within households
*BMI only available for index cases ≥ 2 years. BMI categories for index cases 2–20 year defined as BMI z-score < -2 = underweight, BMI z-score -2–1 = normal weight, BMI z-score 1–2 = overweight, BMI z-score > 2 = obesity. BMI categories for index cases ≥ 21 years defined as BMI < 18.5 = Underweight, BMI 18.5–25 = Normal weight, BMI 25–30 = Overweight" & BMI > 30 = Obesity [47]
Abbreviations: 95% CI: 95% confidence interval, SAR: Secondary attack rate, RR: Relative Risk, BMI: Body Mass Index, UA: University Hospital of Antwerp, UCHB: University Children’s Hospital Basel University, UMCU: University Medical Centre Utrecht