Table IV.
SIR of SARS-CoV-2 among household contacts, by age and SARS-CoV-2 lineage of primary cases and household contacts, California and Colorado, January to April 2021
| Characteristics | Primary cases, n | Secondary cases,∗ n | Total household contacts,† n | SIR (95% CI)‡ | OR (95% CI)§ |
|---|---|---|---|---|---|
| SIR by primary case characteristic¶ | |||||
| Primary case age | |||||
| Child | 36 | 37 | 83 | 0.45 (0.34-0.56) | 0.79 (0.41-1.54) |
| Adult | 91 | 100 | 184 | 0.54 (0.47-0.62) | Reference |
| Pediatric primary case age, y | |||||
| 0-11 | 14 | 19 | 41 | 0.46 (0.31-0.63) | 1.02 (0.35-3.00) |
| 12-17 | 22 | 18 | 42 | 0.43 (0.28-0.59) | Reference |
| Genomic lineage of pediatric primary case | |||||
| Alpha | 21 | 24 | 44 | 0.55 (0.39-0.70) | 1.52 (0.51-4.53) |
| Non-Alpha | 10 | 12 | 26 | 0.46 (0.27-0.67) | Reference |
| Alpha lineage and pediatric primary case age, y | |||||
| 0-11 | 6 | 11 | 17 | 0.65 (0.38-0.86) | 1.63 (0.40-6.70) |
| 12-17 | 15 | 13 | 27 | 0.48 (0.29-0.68) | Reference |
| Non-Alpha lineage and pediatric primary case age, y | |||||
| 0-11 | 4 | 7 | 14 | 0.50 (0.23-0.77) | 1.39 (0.28-6.99) |
| 12-17 | 6 | 5 | 12 | 0.42 (0.15-0.72) | Reference |
| SIR by household contact characteristic∗∗ | |||||
| Household contact age | |||||
| Child | – | 61 | 111 | 0.55 (0.45-0.64) | 1.01 (0.68-1.50) |
| Adult | – | 76 | 156 | 0.49 (0.41-0.57) | Reference |
| Pediatric household contact age, y | |||||
| 0-11 | – | 34 | 57 | 0.60 (0.46-0.72) | 1.09 (0.60-1.97) |
| 12-17 | – | 27 | 54 | 0.50 (0.36-0.64) | Reference |
| Genomic lineage of pediatric household contact | |||||
| Alpha | – | 39 | 61 | 0.64 (0.51-0.76) | 1.08 (0.40-2.98) |
| Non-Alpha | – | 22 | 39 | 0.56 (0.40-0.72) | Reference |
| Alpha lineage and pediatric household contact age, y | |||||
| 0-11 | – | 21 | 27 | 0.78 (0.58-0.91) | 1.37 (0.61-3.04) |
| 12-17 | – | 18 | 34 | 0.53 (0.35-0.70) | Reference |
| Non-Alpha lineage and pediatric household contact age, y | |||||
| 0-11 | – | 13 | 24 | 0.54 (0.32-0.74) | 0.45 (0.10-1.94) |
| 12-17 | – | 9 | 15 | 0.60 (0.32-0.84) | Reference |
–, not applicable.
Secondary cases were defined as household contacts who had a positive RT-PCR for SARS-CoV-2 during the investigation period, converted from SARS-CoV-2 IgG negative on enrollment to SARS-CoV-2 IgG positive on closeout without a history of vaccination, or were seropositive on enrollment with new-onset symptoms without a history of vaccination or a previous SARS-CoV-2 infection.
Four pediatric and 45 adult household contacts reported prior SARS-CoV-2 infection or seropositivity and were excluded from the SIR calculations.
SIRs were calculated as the proportion of secondary cases among all non-excluded household contacts.
Unadjusted ORs and 95% CIs were calculated using GEEs with an exchangeable correlation matrix and logit link to account for clustering within households. ORs were used to compare SIRs.
The probability of transmission among household contacts was compared by age of the primary case (pediatric vs adult). For pediatric primary cases, the probability of transmission among household contacts was compared by age of the pediatric primary case (young child [≤11 years] vs adolescent [12-17 years]) and by lineage group (B.1.1.7 [Alpha] vs non-Alpha). Specimens from 5 primary pediatric cases did not undergo sequencing and therefore a lineage was not assigned to these individuals.
The probability of infection among household contacts was compared by age of the household contact (pediatric vs adult). For pediatric household contacts, the probability of infection was compared by age of the pediatric household contact (young child [≤11 years] vs adolescent [12-17 years]) and by lineage group (B.1.1.7 [Alpha] vs non-Alpha). Lineages were assigned to uninfected pediatric household contacts based on the lineage of the household primary case.