Table 1. Relationship between Positive PCR Tests in Contacts and the Vaccination Status of Index Patients and Contacts.*.
| Characteristic | Transmission of Alpha Variant | Transmission of Delta Variant | Delta Variant vs. Alpha Variant | ||
|---|---|---|---|---|---|
| Index Patient– Contact Pairs |
Adjusted Rate Ratio (95% CI) |
Index Patient– Contact Pairs |
Adjusted Rate Ratio (95% CI) |
Rate Ratio for Interaction (95% CI) |
|
| number | number | ||||
| Vaccination status of index patient | |||||
| Unvaccinated | 52,566 | — | 23,835 | — | — |
| Partially vaccinated† | |||||
| ChAdOx1 nCoV-19 | 3,619 | 0.90 (0.86–0.94) | 7,617 | 0.95 (0.91–0.99) | 1.06 (1.00–1.12) |
| BNT162b2 | 3,917 | 0.88 (0.85–0.91) | 27,122 | 0.83 (0.81–0.86) | 0.94 (0.90–0.99) |
| Vaccinated twice‡ | |||||
| ChAdOx1 nCoV-19 | 99 | 0.48 (0.30–0.78) | 21,322 | 0.76 (0.70–0.82) | 1.58 (0.97–2.56) |
| BNT162b2 | 176 | 0.32 (0.21–0.48) | 5,970 | 0.50 (0.39–0.65) | 1.59 (1.07–2.35) |
| Vaccination status of contact | |||||
| Unvaccinated | 52,321 | — | 12,796 | — | — |
| Partially vaccinated† | |||||
| ChAdOx1 nCoV-19 | 3,739 | 0.94 (0.91–0.98) | 8,568 | 0.69 (0.66–0.72) | 0.73 (0.69–0.77) |
| BNT162b2 | 3,829 | 0.85 (0.82–0.88) | 17,170 | 0.67 (0.65–0.69) | 0.79 (0.76–0.83) |
| Vaccinated twice‡ | |||||
| ChAdOx1 nCoV-19 | 151 | 0.40 (0.27–0.59) | 32,212 | 0.42 (0.38–0.45) | 1.04 (0.70–1.53) |
| BNT162b2 | 337 | 0.15 (0.11–0.21) | 15,120 | 0.19 (0.16–0.23) | 1.28 (0.92–1.78) |
Results for index patients and contacts who received two vaccinations were estimated 14 days after the second vaccination. Adjustment was made for the type of exposure between patients and contacts, index-patient characteristics (age, sex, and symptom status), contact characteristics (age and sex), local deprivation, local incidence of severe acute respiratory syndrome coronavirus 2 infection, and calendar time. There was no evidence that adding an interaction between the index patient and contact vaccination status improved the model fit. There was evidence of greater associated reductions in transmission of the delta variant after the second vaccination in the index patient with BNT162b2 than with ChAdOx1 nCoV-19 (heterogeneity rate ratio, 1.51; 95% confidence interval [CI], 1.15 to 1.97) but no evidence of a difference between the vaccines with respect to transmission of the alpha variant (heterogeneity rate ratio, 1.51; 95% CI, 0.81 to 2.85). Two BNT162b2 vaccinations in contacts were associated with greater reductions in the incidence of positive PCR tests than two ChAdOx1 nCoV-19 vaccinations for both the alpha variant (heterogeneity rate ratio, 2.68; 95% CI, 1.61 to 4.47) and the delta variant (heterogeneity rate ratio, 2.17; 95% CI, 1.78 to 2.65).
Partial vaccination encompasses the period from the date of the first vaccination to 13 days after the second vaccination.
Persons were considered to be vaccinated twice 14 or more days after the second vaccination.