Table 1.
Secondary attack rate among household close contacts (n=1,001)a of COVID-19 index cases, by variant status.
| Variant status of linked index case | ||
|---|---|---|
| Delta variant | non-Delta variant | |
| Number of contacts | 753 | 248 |
| Median age of contacts, in years (IQR) | 36 (25–51) |
35 (27–47) |
| Number of female contacts (%) | 354 (47.0) | 90 (36.3) |
| Number of unique index cases linked to all contacts | 228 | 73 |
| Median age of unique index cases linked to all contacts, in years (IQR) | 40b (28–53) |
36 (28–48) |
| Number of females among unique index cases linked to all contacts (%) | 91 (39.9) | 17 (23.3) |
| Unvaccinated indexes (%) | 156 (68.4) | 70 (95.9) |
| Vaccinated indexes (%) | 72 (31.6) | 3 (4.1) |
| Partially vaccinated (%) | 21 (9.2) | 3 (4.1) |
| Fully vaccinated (%) | 51 (22.4) | 0 |
| Number of unique contact groupsc,d,e | 228 | 73 |
| 1 contact | 45 | 22 |
| 2 contacts | 45 | 12 |
| 3 to 5 contacts | 112 | 31 |
| ≥ 6 contacts | 26 | 8 |
| Median number of contacts in each contact group (IQR)c | 3 (2–4) |
3 (1–5) |
| Mean number of contacts in each contact group (SD)c | 3 (2) | 3 (3) |
| Median days from symptom onset or notification of diagnosis (if asymptomatic) to hospital admission of index case (IQR) |
2 (1–3) |
2 (0–4) |
| Number of contact groups with no cases among contacts (%) | 135 (59.2) | 56 (76.7) |
| Number of contact groups with cases among contacts (%) | 93 (40.8) | 17 (23.3) |
| 1 case | 448 | 7 |
| 2 cases | 27 | 4 |
| 3 cases | 9 | 6 |
| ≥ 4 cases | 9 | 0 |
| Unvaccinated contacts (%) | 530 (70.4) | 241 (97.2) |
| Vaccinated contactsf (%) | 223 (29.6) | 7 (2.8) |
| Partially vaccinatedg (%) | 90 (12.0) | 4 (1.6) |
| Fully vaccinatedh (%) | 133 (17.7) | 3 (1.2) |
| Type of Vaccine (contacts) | ||
| Pfizer/BioNTech (% of vaccinated) | 185 (83.0) | 7 (100.0) |
| Moderna (% of vaccinated) | 38 (17.0) | 0 |
| Others | 0 | 0 |
| Number of cases among all contacts | 169 | 33 |
| Secondary attack rate (%) (95% BCI) | 22.4 (18.1–27.1) |
13.3 (7.3–20.6) |
| Number of cases among fully vaccinated contacts | 15 | 1 |
| Secondary attack rate (fully vaccinated) (%) (95%BCI) | 11.3 (6.1–17.3) |
33.3 (0–100)i |
| Number of cases among unvaccinated contacts | 137 | 31 |
| Secondary attack rate (unvaccinated) (%) (95% BCI) | 25.8 (20.6–31.5) |
12.9 (7.0–20.0) |
Abbreviations: BCI, bootstrap confidence interval; IQR, inter-quartile range; SD, standard deviation
Of 1024 household close contacts, 1001 (97.8%) contacts were linked to an index case with variant status data. 23 (2.2%) contacts did not have variant status data of the index case.
Based on 227 of 228 contacts, as data (age) was missing for one person
Contact group refers to a group consisting of an index case and their close contacts.
Refers to the number of contacts, excluding the linked index case(s).
Data and analysis for these endpoints for non-Delta variants were not available as linked severity data was not available prior to 1 Jan 2021.
Vaccination is defined as having received at least one dose of vaccine before the start date of quarantine.
Partially vaccinated is defined to mean having received one vaccine dose or to be within 14 days of the second vaccine dose on the start date of quarantine.
Fully vaccinated is defined to mean having received both doses of vaccine more than 14 days prior to the start date of quarantine.
Due to the small number of fully vaccinated non-Delta contacts, a usable estimate of the CI could not be obtained