Table 2.
Characteristics of study population overall and by status of third covid-19 vaccine dose received during pregnancy
| Characteristics | Unweighted | Stabilised inverse probability of treatment weighted * | ||||
| Received third covid-19 vaccine dose during pregnancy (n=18 491), n (%) | Did not receive third covid-19 vaccine dose during pregnancy (n=14 198), n (%) | Standardised difference † | Received third covid-19 vaccine dose during pregnancy, % | Did not receive third covid-19 vaccine dose during pregnancy, % | Standardised difference † | |
| Maternal age at delivery (years): | ||||||
| <25 | 453 (2.4) | 988 (7.0) | 0.21 | 3.9 | 4.8 | 0.05 |
| 25-29 | 3130 (16.9) | 3802 (26.8) | 0.24 | 20.2 | 22.2 | 0.05 |
| 30-34 | 8351 (45.2) | 5756 (40.5) | 0.09 | 45.1 | 40.1 | 0.10 |
| 35-39 | 5414 (29.3) | 2919 (20.6) | 0.20 | 25.8 | 24.9 | 0.02 |
| ≥40 | 1143 (6.2) | 733 (5.2) | 0.04 | 4.9 | 8.0 | 0.13 |
| Mean (standard deviation) | 33.5 (4.2) | 32.0 (4.8) | 0.34 | 32.9 (4.3) | 32.9 (4.8) | 0.02 |
| Parity: | ||||||
| 0 (nulliparous) | 8987 (48.6) | 6447 (45.4) | 0.06 | 47.5 | 47.6 | 0.00 |
| ≥1 (multiparous) | 9403 (50.9) | 7700 (54.2) | 0.07 | 52.5 | 52.4 | 0.00 |
| Missing | 101 (0.5) | 51 (0.4) | 0.03 | |||
| Pre-existing medical condition‡: | ||||||
| No | 16 041 (86.8) | 12 574 (88.6) | 0.06 | 87.6 | 87.4 | 0.01 |
| Yes | 2450 (13.2) | 1624 (11.4) | 0.06 | 12.4 | 12.6 | 0.01 |
| Smoked during pregnancy: | ||||||
| No | 17 576 (95.1) | 13 314 (93.8) | 0.06 | 96.8 | 96.9 | 0.00 |
| Yes | 327 (1.8) | 609 (4.3) | 0.15 | 3.2 | 3.1 | 0.00 |
| Missing | 588 (3.2) | 275 (1.9) | 0.08 | |||
| Maternal BMI: | ||||||
| <30.0 | 13 001 (70.3) | 9735 (68.6) | 0.04 | 78.9 | 78.9 | 0.00 |
| ≥30.0 | 3383 (18.3) | 2718 (19.1) | 0.02 | 21.1 | 21.1 | 0.00 |
| Missing | 2107 (11.4) | 1745 (12.3) | 0.03 | |||
| First prenatal care visit in the first trimester: | ||||||
| Yes | 16 919 (91.5) | 13 040 (91.8) | 0.01 | 95.6 | 95.6 | 0.00 |
| No | 661 (3.6) | 698 (4.9) | 0.07 | 4.4 | 4.4 | 0.00 |
| Unknown | 911 (4.9) | 460 (3.2) | 0.09 | |||
| Neighbourhood median family income quintiles: | ||||||
| Quintile 1 (lowest) | 2782 (15.0) | 2998 (21.1) | 0.16 | 17.9 | 17.7 | 0.00 |
| Quintile 2 | 3390 (18.3) | 2949 (20.8) | 0.06 | 19.4 | 19.5 | 0.00 |
| Quintile 3 | 4041 (21.9) | 3157 (22.2) | 0.01 | 22.1 | 22.0 | 0.00 |
| Quintile 4 | 4314 (23.3) | 2930 (20.6) | 0.07 | 22.3 | 22.4 | 0.00 |
| Quintile 5 (highest) | 3874 (21.0) | 2079 (14.6) | 0.17 | 18.3 | 18.4 | 0.00 |
| Missing | 90 (0.5) | 85 (0.6) | 0.02 | |||
| Rural residence: | ||||||
| No | 16 228 (87.8) | 12 444 (87.6) | 0.00 | 87.4 | 87.4 | 0.00 |
| Yes | 2263 (12.2) | 1754 (12.4) | 0.00 | 12.6 | 12.6 | 0.00 |
| Missing | 238 (1.3) | 264 (1.9) | 0.05 | |||
| Laboratory confirmed covid-19 test result before pregnancy§: | ||||||
| No | 18 473 (99.9) | 14 191 (100.0) | 0.02 | 99.9 | 100.0 | 0.02 |
| Yes | 18 (0.1) | 7 (0.0) | 0.02 | 0.1 | 0.0 | 0.02 |
| Laboratory confirmed covid-19 diagnosis during pregnancy¶: | ||||||
| No | 16 185 (87.5) | 12 175 (85.8) | 0.05 | 87.5 | 85.4 | 0.06 |
| Yes | 2306 (12.5) | 2023 (14.2) | 0.05 | 12.5 | 14.6 | 0.06 |
BMI=body mass index; LMP=last menstrual period.
*No missing values are shown in the weighted distributions of baseline characteristics from inverse probability of treatment because multiple imputation was used to address missing values. Column percentages and weights for the weighted study population were based on imputation dataset one.
†Absolute standardised difference comparing people who received a third dose of covid-19 vaccine during pregnancy and those who did not; standardised difference >0.10 indicates an imbalance in the distribution of the baseline characteristic between these two exposure groups.
‡Composite of: asthma, chronic hypertension, diabetes, heart disease, and thyroid disease. Sum of individual conditions does not equal the total number of individuals with any individual condition, as categories were not mutually exclusive (individual conditions shown in online supplemental table 3).
§Laboratory confirmed covid-19 diagnosis before the last menstrual period date. Represents pre-omicron time period.
¶Laboratory confirmed covid-19 diagnosis between the last menstrual period date up to one day before the date of birth (the specimen collection date, which was a proxy for date of infection, was lagged by two days).