TABLE 2.
Comparison of the risk of transmission between the Delta and Alpha variants of SARS-CoV-2 by characteristics of the close contacts
| All close contacts |
Unvaccinated close contacts |
|||||||
|---|---|---|---|---|---|---|---|---|
| Characteristics of the close contacts and variant | Infections/contacts | SAR % | Adjusted RR (95% CI)a |
P | Infections/contacts | SAR % | Adjusted RR (95% CI)a |
P |
| Total | ||||||||
| Alpha variant | 506/2,139 | 24 | 1 | 424/1,430 | 30 | 1 | ||
| Delta variant | 1,394/5,439 | 26 | 1.32 (1.13 to 1.53) | <0.001 | 911/2,102 | 43 | 1.30 (1.10 to 1.54) | 0.002 |
| Aged <18 yrs | ||||||||
| Alpha variant | 190/594 | 32 | 1 | 190/592 | 32 | 1 | ||
| Delta variant | 443/1,120 | 40 | 0.98 (0.75 to 1.28) | 0.879 | 430/1,075 | 40 | 0.98 (0.75 to 1.27) | 0.859 |
| Aged 18 to 39 yrs | ||||||||
| Alpha variant | 195/730 | 27 | 1 | 182/649 | 28 | 1 | ||
| Delta variant | 521/1,486 | 35 | 1.62 (1.29 to 2.03) | <0.001 | 427/869 | 49 | 1.64 (1.30 to 2.08) | <0.001 |
| Aged ≥40 yrs | ||||||||
| Alpha variant | 121/815 | 15 | 1 | 52/189 | 28 | 1 | ||
| Delta variant | 430/2,833 | 15 | 1.31 (0.97 to 1.79) | 0.083 | 54/158 | 34 | 1.26 (0.62 to 2.54) | 0.528 |
| Male | ||||||||
| Alpha variant | 268/1,058 | 25 | 1b | 229/761 | 30 | 1 | ||
| Delta variant | 661/2,563 | 26 | 1.21 (0.98 to 1.50) | 0.075 | 451/1,035 | 44 | 1.20 (0.95 to 1.52) | 0.129 |
| Female | ||||||||
| Alpha variant | 238/1,081 | 22 | 1 | 195/669 | 29 | 1b | ||
| Delta variant | 733/2,876 | 26 | 1.42 (1.15 to 1.75) | 0.001 | 460/1,067 | 43 | 1.40 (1.10 to 1.79) | 0.007 |
| Major chronic condition | ||||||||
| Alpha variant | 108/499 | 22 | 1 | 77/237 | 33 | 1 | ||
| Delta variant | 328/1,459 | 23 | 1.17 (0.85 to 1.60) | 0.349 | 155/385 | 40 | 1.06 (0.71 to 1.59) | 0.763 |
| No major chronic condition | ||||||||
| Alpha variant | 398/1,640 | 24 | 1 | 347/1,193 | 29 | 1 | ||
| Delta variant | 1,066/3,980 | 27 | 1.37 (1.15 to 1.62) | <0.001 | 756/1,717 | 44 | 1.36 (1.13 to 1.65) | 0.001 |
| Household contact | ||||||||
| Alpha variant | 346/1,082 | 32 | 1 | 281/633 | 44 | 1 | ||
| Delta variant | 839/2,970 | 28 | 1.10 (0.91 to 1.34) | 0.324 | 479/973 | 49 | 1.02 (0.80 to 1.29) | 0.878 |
| Non-household contact | ||||||||
| Alpha variant | 160/1,057 | 15 | 1 | 143/797 | 18 | 1 | ||
| Delta variant | 555/2,469 | 23 | 1.71 (1.35 to 2.16) | <0.001 | 432/1,129 | 38 | 1.69 (1.31 to 2.18) | <0.001 |
| Unvaccinated | ||||||||
| Alpha variant | 424/1,430 | 30 | 1 | |||||
| Delta variant | 911/2,102 | 43 | 1.30 (1.10 to 1.54) | 0.002 | ||||
| Vaccinated | ||||||||
| Alpha variant | 82/709 | 12 | 1 | |||||
| Delta variant | 483/3,337 | 15 | 1.34 (0.97 to 1.84) | 0.077 | ||||
a
SAR, secondary attack rate; RR, relative risk; CI confidence interval.
b
RR, relative risk adjusted by age group (≤5, 6–11, 12–17, 18–39, 40–59, and ≥60 years), sex, contact setting (household or other), major chronic conditions, and COVID-19 vaccination status of the close contact, as well as age group, COVID-19 vaccination status, and month of the index case.