Table 1.
Secondary attack rate (SAR) of SARS-CoV-2 for Japanese household contacts according to their characteristics.
| Characteristics | No. Household Contacts | No. SARS-CoV-2 Positive | SAR % | p-Value | |
|---|---|---|---|---|---|
| Sex | Female | 396 | 110 | 27.8% | 0.754 |
| Male | 291 | 84 | 28.9% | ||
| Unknown | 1 | 0 | NA | ||
| Age | 0–29 years | 278 | 66 | 22.3% | 0.002 |
| 30–59 years | 276 | 78 | 29.7% | ||
| 60 years+ | 134 | 50 | 38.8% | ||
| Relationship to index case | Spouse | 176 | 74 | 42.1% | <0.001 |
| Parent | 179 | 39 | 21.8% | ||
| Children | 220 | 55 | 25.0% | ||
| Others | 112 | 25 | 22.3% | ||
| Fever of index case | <38.0 °C | 308 | 67 | 21.7% | 0.003 |
| ≥38.0 °C | 337 | 112 | 33.2% | ||
| Unknown | 43 | 15 | 34.9% | ||
| Time from symptom onset to diagnosis of the index case | 2 days or less | 331 | 95 | 28.7% | 0.494 |
| 3 to 5 days | 224 | 56 | 25.0% | ||
| 6 days and over | 103 | 34 | 22.0% | ||
| Unknown | 30 | 9 | 30.0% | ||
| Epidemic phase based on diagnosis of index case | 1 to 20 December 2020 | 290 | 56 | 19.3% | <0.001 |
| 20 April to 3 May 2021 | 398 | 138 | 34.7% | ||
| Viral type of index case | Pre-existing virus * | 290 | 56 | 19.3% | <0.001 |
| Alpha variant | 124 | 48 | 38.7% | ||
* Index cases who were diagnosed with COVID-19 from 1 to 20 December 2020 were assumed not to be infected with the variant of concern.