Table 1.
Characteristics of infants younger than six months of age tested for SARS-CoV-2 infection, May 2021 to September 2022, and their mothers, Ontario, Canada. Data are number (percentage), unless otherwise specified
| Characteristic | Infants (<6 months) tested for SARS-CoV-2 | Maternal covid-19 vaccination status | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Case (n=1600) | Control (n=7209) | SD* | Unvaccinated (n=3293) |
Primary and primary + booster† (n=5516) | SD* | Primary† (n=4825) | Primary plus booster† (n=691) | SD* | ||
| Infant sex, female | 742 (46.4) | 3161 (43.8) | 0.05 | 1506 (45.7) | 2397 (43.5) | 0.05 | 2111 (43.8) | 286 (41.4) | 0.05 | |
| Infant age when tested: | ||||||||||
| 0-8 weeks | 525 (32.8) | 4111 (57.0) | 0.50 | 1849 (56.1) | 2787 (50.5) | 0.11 | 2352 (48.7) | 435 (63.0) | 0.29 | |
| 9-16 weeks | 585 (36.6) | 1816 (25.2) | 0.25 | 802 (24.4) | 1599 (29.0) | 0.10 | 1435 (29.7) | 164 (23.7) | 0.14 | |
| >16 weeks | 490 (30.6) | 1282 (17.8) | 0.30 | 642 (19.5) | 1130 (20.5) | 0.02 | 1038 (21.5) | 92 (13.3) | 0.22 | |
| Gestational age at birth (weeks), median (interquartile range) | 39 (38-40) | 39 (37-39) | 0.23 | 39 (37-40) | 39 (38-39) | 0.01 | 39 (38-39) | 39 (38-39) | 0.02 | |
| Preterm birth‡ | 109 (6.8) | 985 (13.7) | 0.23 | 426 (12.9) | 668 (12.1) | 0.02 | 576 (11.9) | 92 (13.3) | 0.04 | |
| Low birthweight§ | 86 (5.4) | 813 (11.3) | 0.21 | 365 (11.1) | 534 (9.7) | 0.05 | 472 (9.8) | 62 (9.0) | 0.03 | |
| Mother's age at birth: | ||||||||||
| <25 years | 192 (12.0) | 703 (9.8) | 0.07 | 571 (17.3) | 324 (5.9) | 0.36 | 297 (6.2) | 27 (3.9) | 0.10 | |
| 25-29 years | 399 (24.9) | 1745 (24.2) | 0.02 | 956 (29.0) | 1188 (21.5) | 0.17 | 1076 (22.3) | 112 (16.2) | 0.15 | |
| 30-34 years | 591 (36.9) | 2768 (38.4) | 0.03 | 1038 (31.5) | 2321 (42.1) | 0.22 | 2009 (41.6) | 312 (45.2) | 0.07 | |
| 35-39 years | 346 (21.6) | 1641 (22.8) | 0.03 | 577 (17.5) | 1410 (25.6) | 0.20 | 1205 (25.0) | 205 (29.7) | 0.11 | |
| ≥40 years | 72 (4.5) | 352 (4.9) | 0.02 | 151 (4.6) | 273 (4.9) | 0.02 | 238 (4.9) | 35 (5.1) | 0.01 | |
| Nulliparous | 853 (53.3) | 4393 (60.9) | 0.15 | 1935 (58.8) | 3311 (60.0) | 0.03 | 2939 (60.9) | 372 (53.8) | 0.14 | |
| Pre-pregnancy maternal comorbidities: | ||||||||||
| Diabetes mellitus | 93 (5.8) | 447 (6.2) | 0.02 | 198 (6.0) | 342 (6.2) | 0.01 | 317 (6.6) | 25 (3.6) | 0.13 | |
| Hypertension | 42 (2.6) | 179 (2.5) | 0.01 | 81 (2.5) | 140 (2.5) | 0.01 | 118 (2.4) | 22 (3.2) | 0.04 | |
| Heart disease | ≤5 (≤0.3)¶ | ≤5 (≤0.1)¶ | 0-0.06¶ | ≤5 (≤0.2)¶ | ≤5 (≤0.1)¶ | 0-0.02¶ | ≤5 (≤0.1)¶ | 0 (0.0) | 0-0.01¶ | |
| Asthma | 307 (19.2) | 1498 (20.8) | 0.04 | 686 (20.8) | 1119 (20.3) | 0.01 | 981 (20.3) | 138 (20.0) | 0.01 | |
| Autoimmune disease | 46 (2.9) | 203 (2.8) | 0 | 72 (2.2) | 177 (3.2) | 0.06 | 148 (3.1) | 29 (4.2) | 0.06 | |
| Immunosuppression** | 31 (1.9) | 189 (2.6) | 0.05 | 72 (2.2) | 148 (2.7) | 0.03 | 127 (2.6) | 21 (3.0) | 0.02 | |
| Prenatal care index††: | ||||||||||
| Intensive/intermediate | 845 (52.8) | 3598 (49.9) | 0.06 | 1606 (48.8) | 2837 (51.4) | 0.05 | 2480 (51.4) | 357 (51.7) | 0.01 | |
| Adequate | 253 (15.8) | 1109 (15.4) | 0.01 | 483 (14.7) | 879 (15.9) | 0.04 | 767 (15.9) | 112 (16.2) | 0.01 | |
| Inadequate | 370 (23.1) | 1796 (24.9) | 0.04 | 872 (26.5) | 1294 (23.5) | 0.07 | 1120 (23.2) | 174 (25.2) | 0.05 | |
| No care | 132 (8.3) | 706 (9.8) | 0.05 | 332 (10.1) | 506 (9.2) | 0.03 | 458 (9.5) | 48 (6.9) | 0.09 | |
| Maternal influenza vaccine‡‡ | 381 (23.8) | 2095 (29.1) | 0.12 | 381 (11.6) | 2095 (38.0) | 0.64 | 1804 (37.4) | 291 (42.1) | 0.10 | |
| Maternal SARS-CoV-2 infection: | ||||||||||
| Pre-pregnancy | 28 (1.8) | 179 (2.5) | 0.05 | 80 (2.4) | 127 (2.3) | 0.01 | 100 (2.1) | 27 (3.9) | 0.11 | |
| During pregnancy | 69 (4.3) | 834 (11.6) | 0.27 | 425 (12.9) | 478 (8.7) | 0.14 | 381 (7.9) | 97 (14.0) | 0.20 | |
| Post partum | 324 (20.3) | 365 (5.1) | 0.47 | 309 (9.4) | 380 (6.9) | 0.09 | 348 (7.2) | 32 (4.6) | 0.11 | |
| Median household income quintile§§¶¶: | ||||||||||
| 1 (lowest) | 385 (24.1) | 1548 (21.5) | 0.06 | 1004 (30.5) | 929 (16.8) | 0.33 | 824 (17.1) | 105 (15.2) | 0.05 | |
| 2 | 348 (21.8) | 1406 (19.5) | 0.06 | 744 (22.6) | 1010 (18.3) | 0.11 | 894 (18.5) | 116 (16.8) | 0.05 | |
| 3 | 312 (19.5) | 1535 (21.3) | 0.04 | 645 (19.6) | 1202 (21.8) | 0.05 | 1057 (21.9) | 145 (21.0) | 0.02 | |
| 4 | 310 (19.4) | 1472 (20.4) | 0.03 | 539 (16.4) | 1243 (22.5) | 0.16 | 1066 (22.1) | 177 (25.6) | 0.08 | |
| 5 (highest) | 245 (15.3) | 1248 (17.3) | 0.05 | 361 (11.0) | 1132 (20.5) | 0.26 | 984 (20.4) | 148 (21.4) | 0.03 | |
| Self-identified visible minority quintile§§***: | ||||||||||
| 1 (lowest) | 245 (15.3) | 1225 (17.0) | 0.05 | 640 (19.4) | 830 (15.0) | 0.12 | 737 (15.3) | 93 (13.5) | 0.05 | |
| 2 | 246 (15.4) | 1260 (17.5) | 0.06 | 566 (17.2) | 940 (17.0) | 0 | 821 (17.0) | 119 (17.2) | 0.01 | |
| 3 | 273 (17.1) | 1343 (18.6) | 0.04 | 555 (16.9) | 1061 (19.2) | 0.06 | 890 (18.4) | 171 (24.7) | 0.15 | |
| 4 | 390 (24.4) | 1640 (22.7) | 0.04 | 660 (20.0) | 1370 (24.8) | 0.12 | 1198 (24.8) | 172 (24.9) | 0 | |
| 5 (highest) | 446 (27.9) | 1741 (24.2) | 0.08 | 872 (26.5) | 1315 (23.8) | 0.06 | 1179 (24.4) | 136 (19.7) | 0.11 | |
| Average number of people per dwelling quintile§§: | ||||||||||
| 1 (0-2.1) | 290 (18.1) | 1230 (17.1) | 0.03 | 659 (20.0) | 861 (15.6) | 0.12 | 732 (15.2) | 129 (18.7) | 0.09 | |
| 2 (2.2-2.4) | 236 (14.8) | 1345 (18.7) | 0.10 | 656 (19.9) | 925 (16.8) | 0.08 | 806 (16.7) | 119 (17.2) | 0.01 | |
| 3 (2.5-2.6) | 230 (14.4) | 1101 (15.3) | 0.03 | 473 (14.4) | 858 (15.6) | 0.03 | 756 (15.7) | 102 (14.8) | 0.03 | |
| 4 (2.7-3.0) | 397 (24.8) | 1817 (25.2) | 0.01 | 754 (22.9) | 1460 (26.5) | 0.08 | 1279 (26.5) | 181 (26.2) | 0.01 | |
| 5 (3.1-5.7) | 447 (27.9) | 1716 (23.8) | 0.09 | 751 (22.8) | 1412 (25.6) | 0.07 | 1252 (25.9) | 160 (23.2) | 0.06 | |
| High risk non-health occupation quintile§§†††: | ||||||||||
| 1 (0%-32.5%) | 235 (14.7) | 1238 (17.2) | 0.07 | 300 (9.1) | 1173 (21.3) | 0.34 | 997 (20.7) | 176 (25.5) | 0.11 | |
| 2 (32.5%–42.3%) | 344 (21.5) | 1538 (21.3) | 0 | 558 (16.9) | 1324 (24.0) | 0.18 | 1156 (24.0) | 168 (24.3) | 0.01 | |
| 3 (42.3%–49.8%) | 333 (20.8) | 1479 (20.5) | 0.01 | 701 (21.3) | 1111 (20.1) | 0.03 | 988 (20.5) | 123 (17.8) | 0.07 | |
| 4 (50.0%–57.5%) | 350 (21.9) | 1493 (20.7) | 0.03 | 796 (24.2) | 1047 (19.0) | 0.13 | 919 (19.0) | 128 (18.5) | 0.01 | |
| 5 (57.5%–100%) | 338 (21.1) | 1461 (20.3) | 0.02 | 938 (28.5) | 861 (15.6) | 0.31 | 765 (15.9) | 96 (13.9) | 0.06 | |
| Public health unit region: | ||||||||||
| Central East | 84 (5.3) | 432 (6.0) | 0.03 | 197 (6.0) | 319 (5.8) | 0.01 | 270 (5.6) | 49 (7.1) | 0.06 | |
| Central West | 338 (21.1) | 1532 (21.3) | 0 | 739 (22.4) | 1131 (20.5) | 0.05 | 978 (20.3) | 153 (22.1) | 0.05 | |
| Durham | 89 (5.6) | 353 (4.9) | 0.03 | 148 (4.5) | 294 (5.3) | 0.04 | 254 (5.3) | 40 (5.8) | 0.02 | |
| Eastern | 56 (3.5) | 502 (7.0) | 0.16 | 220 (6.7) | 338 (6.1) | 0.02 | 300 (6.2) | 38 (5.5) | 0.03 | |
| Northern | 121 (7.6) | 337 (4.7) | 0.12 | 182 (5.5) | 276 (5.0) | 0.02 | 242 (5.0) | 34 (4.9) | 0 | |
| Ottawa | 61 (3.8) | 514 (7.1) | 0.15 | 125 (3.8) | 450 (8.2) | 0.18 | 399 (8.3) | 51 (7.4) | 0.03 | |
| Peel | 197 (12.3) | 746 (10.3) | 0.06 | 378 (11.5) | 565 (10.2) | 0.04 | 510 (10.6) | 55 (8.0) | 0.09 | |
| South West | 237 (14.8) | 1200 (16.6) | 0.05 | 634 (19.3) | 803 (14.6) | 0.13 | 716 (14.8) | 87 (12.6) | 0.07 | |
| Toronto | 299 (18.7) | 1175 (16.3) | 0.06 | 484 (14.7) | 990 (17.9) | 0.09 | 844 (17.5) | 146 (21.1) | 0.09 | |
| York | 118 (7.4) | 418 (5.8) | 0.06 | 186 (5.6) | 350 (6.3) | 0.03 | 312 (6.5) | 38 (5.5) | 0.04 | |
| Covid-19 vaccine dose 2 or 3 during pregnancy | 906 (56.6) | 4610 (63.9) | 0.15 | 0 (0.0) | 5516 (100.0) | — | 4825 (100.0) | 691 (100.0) | 0 | |
| Covid-19 vaccine dose 3 during pregnancy | 111 (6.9) | 580 (8.0) | 0.04 | 0 (0.0) | 691 (12.5) | — | 0 (0.0) | 691 (100.0) | — | |
| Maternal covid-19 vaccine dose 1 and 2: | ||||||||||
| BNT162b2-BNT162b2 | 650 (40.6) | 3155 (43.8) | 0.06 | 0 (0.0) | 3805 (69.0) | — | 3320 (68.8) | 485 (70.2) | 0.03 | |
| mRNA-1273-mRNA-1273 | 154 (9.6) | 800 (11.1) | 0.05 | 0 (0.0) | 954 (17.3) | — | 850 (17.6) | 104 (15.1) | 0.07 | |
| Heterologous‡‡‡ | 102 (6.4) | 655 (9.1) | 0.10 | 0 (0.0) | 757 (13.7) | — | 655 (13.6) | 102 (14.8) | 0.03 | |
| Timing of maternal covid-19 vaccination: | ||||||||||
| First dose | ||||||||||
| Preconception | 215 (13.4) | 954 (13.2) | 0.01 | 0 (0.0) | 1169 (21.2) | — | 593 (12.3) | 576 (83.4) | 2.02 | |
| First trimester | 239 (14.9) | 913 (12.7) | 0.07 | 0 (0.0) | 1152 (20.9) | — | 1051 (21.8) | 101 (14.6) | 0.19 | |
| Second trimester | 346 (21.6) | 1942 (26.9) | 0.12 | 0 (0.0) | 2288 (41.5) | — | 2274 (47.1) | 14 (2.0) | 1.23 | |
| Third trimester | 106 (6.6) | 801 (11.1) | 0.16 | 0 (0.0) | 907 (16.4) | — | 907 (18.8) | 0 (0.0) | 0.68 | |
| Second dose | ||||||||||
| Preconception | 40 (2.5) | 223 (3.1) | 0.04 | 0 (0.0) | 263 (4.8) | — | 0 (0.0) | 263 (38.1) | 1.11 | |
| First trimester | 185 (11.6) | 723 (10.0) | 0.05 | 0 (0.0) | 908 (16.5) | — | 577 (12.0) | 331 (47.9) | 0.85 | |
| Second trimester | 411 (25.7) | 1576 (21.9) | 0.09 | 0 (0.0) | 1987 (36.0) | — | 1891-1895 (39.2-39.3)¶ | 92-96 (13.3)-13.9)¶ | 0.60-0.62 | |
| Third trimester | 270 (16.9) | 2088 (29.0) | 0.29 | 0 (0.0) | 2358 (42.7) | — | 2353-2357 (48.8-48.8)¶ | ≤5 (≤0.7)¶ | 1.34-1.38 | |
| Third dose | ||||||||||
| Second trimester | 12 (0.8) | 49 (0.7) | 0.01 | 0 (0.0) | 61 (1.1) | — | 0 (0.0) | 61 (8.8) | — | |
| Third trimester | 99 (6.2) | 531 (7.4) | 0.05 | 0 (0.0) | 630 (11.4) | — | 0 (0.0) | 630 (91.2) | — | |
| Interval between covid-19 vaccine doses 1 and 2: | ||||||||||
| <35 days | 226 (14.1) | 904 (12.5) | 0.05 | 0 (0.0) | 1130 (20.5) | — | 1004 (20.8) | 126 (18.2) | 0.06 | |
| 35-55 days | 296 (18.5) | 1627 (22.6) | 0.10 | 0 (0.0) | 1923 (34.9) | — | 1668 (34.6) | 255 (36.9) | 0.05 | |
| ≥56 days | 384 (24.0) | 2079 (28.8) | 0.11 | 0 (0.0) | 2463 (44.7) | — | 2153 (44.6) | 310 (44.9) | 0 | |
| Interval between covid-19 vaccine doses 2 and 3: | ||||||||||
| <168 days | 25 (1.6) | 117 (1.6) | 0 | 0 (0.0) | 142 (2.6) | — | 0 (0.0) | 142 (20.5) | — | |
| ≥168 days | 86 (5.4) | 463 (6.4) | 0.04 | 0 (0.0) | 549 (10.0) | — | 0 (0.0) | 549 (79.5) | — | |
Absolute standardised difference. Values greater than 0.10 indicate a potentially clinically important difference in the distribution between groups.
Primary covid-19 vaccination series defined as two doses up to 14 days before delivery with at least one dose after conception. Primary plus booster covid-19 vaccination series defined as three doses up to 14 days before delivery with at least one dose after conception. Unvaccinated defined as no covid-19 vaccine doses preconception, during pregnancy, and postpartum up to 14 days before the infant’s SARS-CoV-2 polymerase chain reaction test.
Preterm birth: birth at <37 weeks’ gestation.
Low birthweight: birthweight <2500 g.
In accordance with Ontario’s privacy legislation, ICES data privacy policy prohibits us from reporting cells with fewer than five people. Ranges reported in adjacent cells are to prevent back calculation of small cells.
Immunosuppression defined as solid organ or stem cell transplant, active cancer, sickle cell anaemia, HIV infection, immunosuppressing therapies, and other immune system disorders.
Adequacy of prenatal care characterised with the Revised-Graduated Prenatal Care Utilisation Index (R-GINDEX). Intensive and intermediate categories were combined due to small cells.
Maternal influenza vaccination during the 2019-20 and/or 2020-21 influenza seasons.
Dissemination area level variable.
Household income quintile has variable cut-off values in different cities; census areas to account for cost of living. A dissemination area being in quintile 1 means it is among the lowest 20% of dissemination areas in its city by income.
Percentage of people in the area who self-identified as a visible minority. Census counts for people are randomly rounded up or down to the nearest number divisible by five, which causes some minor imprecision.
Percentage of people in the area working in the following occupations: sales and service; trades, transport, and equipment operators, and related occupations; natural resources, agriculture, and related production occupations; and manufacturing and utilities. Census counts for people are randomly rounded up or down to the nearest number divisible by five, which causes some minor imprecision.
Heterologous: mRNA-1273 followed by BNT162b2 or vice versa.