Table 4. Secondary attack rates and risk factors for SARS-CoV-2 infection in 206 households whose members were infected with SARS-CoV-2 B.1.1.214 or Alpha variants, Kyoto, Japan, November 2020–May 2021*.
| Variables | No. contacts | No. infected | Secondary attack rate, % |
Adjusted odds ratio (95% CI)† | p value† | |
|---|---|---|---|---|---|---|
| Observed | Predicted (IQR)† | |||||
| Overall |
564 |
328 |
58.2 |
58.1 (46.5–71.1) |
NA |
NA |
| Index case age group, y | ||||||
| <18 | 57 | 23 | 40.4 | 40.4 (28.3–49.8) | 0.67 (0.35–1.28) | 0.22 |
| 19–59 | 419 | 256 | 61.1 | 61.1 (51.7–70.9) | Referent | |
|
>60 |
88 |
49 |
55.7 |
55.5 (44.9–64.0) |
0.73 (0.44–1.22) |
0.23 |
| Contacts age group, y | ||||||
| <18 | 167 | 82 | 49.1 | 49.1 (37.3–61.9) | 0.72 (0.48–1.07) | 0.10 |
| 19–59 | 344 | 208 | 60.5 | 60.4 (52.7–70.9) | Referent | |
|
>60 |
53 |
38 |
71.7 |
71.9 (66.0–81.0) |
1.75 (0.89–3.42) |
0.10 |
| Index case symptoms | ||||||
| Asymptomatic | 58 | 19 | 32.8 | 32.7 (24.1–43.4) | Referent | |
| Symptomatic |
506 |
309 |
61.1 |
61.1 (51.7–70.9) |
2.84 (1.49–5.42) |
<0.01 |
| Household size | ||||||
| 2–3 | 171 | 123 | 71.9 | 71.9 (70.9–78.7) | Referent | |
| 4 | 189 | 112 | 59.3 | 59.3 (52.0–69.3) | 0.61 (0.38–0.97) | 0.04 |
| 5 | 100 | 43 | 43.0 | 42.9 (37.3–47.5) | 0.34 (0.20–0.58) | <0.01 |
|
>6 |
104 |
50 |
48.1 |
48.0 (44.4–55.3) |
0.46 (0.26–0.79) |
<0.01 |
| SARS-CoV-2 lineage | ||||||
| Non-VOC, B.1.1.214 | 282 | 152 | 53.9 | 54.0 (44.9–64.0) | Referent | |
| Alpha, B.1.1.7 | 282 | 176 | 62.4 | 62.3 (55.3–72.7) | 1.52 (1.06–2.18) | 0.02 |
*IQR, interquartile range; NA, not applicable; VOC, variant of concern. †Calculated by using a generalized linear mixed-effects logistic regression model.