Table 1. Baseline Characteristics.
| Source | Design | Country | Observational period | Predominant variant | Vaccine type | Vaccination status | Cohort size, No. | Age, y | No. (%) | Follow-up duration | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Female | At least 1 comorbidity | ||||||||||
| Cohen-Stavi et al,32 2022 | Retrospective cohort | Israel | November 23, 2021, to January 7, 2022 | Omicron | BNT162b2 | Vaccinated | 94 728 | Median (IQR), 8 (7-10) | 46 082 (49) | 31 087 (33) | 7-21 d After second dose |
| 14-27 d After first dose | |||||||||||
| Unvaccinated | 94728 | Median (IQR), 8 (7-10) | 46 083 (49) | 31 087 (33) | NA | ||||||
| Fleming-Dutra et al,31 2022 | Retrospective case-control | US | December 26, 2021, to February 21, 2022 | Omicron | BNT162b2 | Vaccinated | 15 778 | Median (IQR), 9 (7-10) | 7646 (48.5) | NA | 14-30/30-90 d After second dose |
| Unvaccinated | 58 430 | Median (IQR), 8 (6-10) | 28 958 (49.6) | NA | NA | ||||||
| Price et al,14 2022 | Retrospective case-control | US | December 19, 2021, to February 17, 2022 | Omicron | BNT162b2 | Vaccinated | 70 | NA | NA | NA | ≥14 d After second dose |
| Unvaccinated | 467 | NA | NA | NA | NA | ||||||
| Fowlkes et al,13 2022 | Prospective cohort | US | December 14, 2021, to February 12, 2022 | Omicron | BNT162b2 | 2 Doses | 682 | NA | NA | NA | 14-82 d After second dose |
| 1 Dose | 69 | NA | NA | NA | NA | ||||||
| Unvaccinated | 301 | NA | NA | NA | NA | ||||||
| Tan et al,34 2022 | Prospective cohort | Singapore | January 21, 2022, to April 8, 2022 | Omicron | BNT162b2 | 2 Doses | 173 237 | NA | NA | NA | ≥7-60 d After second dose |
| 1 Dose | 30 656 | NA | NA | NA | ≥1-30 d After first dose | ||||||
| Unvaccinated | 52 043 | NA | NA | NA | NA | ||||||
| Sacco et al,35 2022 | Retrospective cohort | Italy | January 17, 2022, to April 10, 2022 | Omicron | BNT162b2 | 2 Doses | 1 063 035 | NA | 519 315 (48.9) | NA | 0-84 d After second dose |
| 1 Dose | 134 386 | NA | 64 556 (48.0) | NA | NA | ||||||
| Unvaccinated | 1 768 497 | NA | 857 295 (48.5) | NA | NA | ||||||
| Klein et al,20 2022 | Retrospective cohort | US | December 3, 2021, to January 29, 2022 | Omicron | BNT162b2 | Vaccinated | 582 | NA | NA | NA | 14-67 d After second dose |
| Unvaccinated | 8599 | NA | NA | NA | NA | ||||||
| Creech et al,19 2022a | RCT | US and Canada | August 2021 to November 2021 | Delta | BNT162b2 | Vaccinated | 3007 | Mean (SD), 8.5 (1.7) | 1453 (48.3) | NA | ≥14 d After first/second dose |
| Unvaccinated | 995 | Mean (SD), 8.5 (1.6) | 514 (51.7) | NA | NA | ||||||
| Walter et al,15 2022 | RCT | US, Finland, Spain, and Poland | June 7, 2021, to October 8, 2021 | Delta | mRNA-1273 | Vaccinated | 1518 | Mean (SD), 8.2 (1.93) | 719 (47.4) | 312 (20.6) | ≥7 d After second dose, median follow-up time: 2.3 (0-2.5) mo |
| Unvaccinated | 750 | Mean (SD), 8.1 (1.97) | 367 (48.9) | 152 (20.3) | NA | ||||||
| Zambrano et al,12 2022 | Retrospective case-control | US | December 20, 2021, to April 7, 2022 | Omicron | BNT162b2 | Vaccinated | 53 | NA | NA | NA | 28-120 d After second dose |
| Unvaccinated | 321 | NA | NA | NA | NA | ||||||
| Amir et al,36 2023b | Retrospective cohort | Israel | December 26, 2021, to January 8, 2022 | Omicron | BNT162b2 | Vaccinated | 56 819 | NA | 48.3% | NA | 14-35 d After second dose |
| Unvaccinated | 572 859 | NA | 44.3% | NA | NA | ||||||
| Shi et al,37 2022 | Retrospective case-control | US | December 19, 2021, to February 28, 2022 | Omicron | BNT162b2 | Vaccinated | 48 | Median (IQR), 9 (8-11) | 23 (46.0) | NA | NA |
| Unvaccinated | 301 | Median (IQR), 7 (8-9) | 136 (44.4) | NA | NA | ||||||
| Block et al,38 2022 | Retrospective cohort | US | January 1, 2021, to January 31, 2022 | Delta-omicron | BNT162b2 and mRNA-1273 | 2 Doses | 41 742 | NA | NA | NA | NA |
| 1 Dose | 48 986 | NA | NA | NA | NA | ||||||
| Dose unknown | 30 199 | NA | NA | NA | NA | ||||||
| Unvaccinated | 76 960 | NA | NA | NA | NA | ||||||
| Hause et al,40 2022 | Retrospective single-arm | US | May 17, 2022, to July 31, 2022 | Omicron | BNT162b2 | 3 Doses | 3249 | NA | NA | NA | NA |
| 2 Doses | 3249 | NA | NA | NA | NA | ||||||
| 1 Dose | 3249 | NA | NA | NA | NA | ||||||
| Hause et al,39 2022 | Retrospective single-arm | US | November 3, 2021, to February 27, 2022 | Delta-omicron | BNT162b2 | 2 Doses | 7 266 633 | NA | NA | NA | 0-7 d After second dose |
| 1 Dose | 9 235 783 | NA | NA | NA | 0-7 d After first dose | ||||||
| Capponi et al,42 2022 | Retrospective single-arm | Italy | February 1, 2022, to February 28, 2022 | Omicron | BNT162b2 | 2 Doses | 345 | NA | NA | NA | 1/2/3 d After second dose |
| 1 Dose | 332 | NA | NA | NA | 1/2/3 d After first dose | ||||||
| Bloise et al,41 2022 | Prospective single-arm | Italy | December 15, 2021, to January 31, 2022 | Omicron | BNT162b2 | 2 Doses | 449 | Mean (SD), 9.5 (0.35) | NA | NA | 24-48 h/7 d/20 d after second dose |
| 1 Dose | 569 | Mean (SD), 9.5 (0.35) | 318 (55.9) | 459 (80.7) | 24-48 h/7 d/20 d after first dose | ||||||
Abbreviations: NA, not available; RCT, randomized clinical trial.
a
Age group, 6-11 years.
b
Age group, 5-10 years; the baseline is only shown for percentage because the absolute numbers were unavailable.