Table 1. Sociodemographic information of initial and re-infected COVID-19 cases in China from April 1 to May 30, 2023.
| Variables | Total, n (%) | Initial infection, n (%) | Reinfection, n (%) |
| Note: Initial infections were defined as cases with no history of a positive COVID-19 test before April 1, 2023. Meanwhile, reinfections were defined as cases with one or more positive COVID-19 tests before April 1, 2023. * North China: Beijing Municipality, Tianjin Municipality, Hebei Province, Shanxi Province; Northeast China: Inner Mongolia Autonomous Region, Liaoning Province, Jilin Province, Heilongjiang Province; East China: Shanghai Municipality, Jiangsu Province, Zhejiang Province, Anhui Province, Fujian Province, Jiangxi Province, Shandong Province; South Central China: Henan Province, Hubei Province, Hunan Province, Guangdong Province, Guangxi Zhuang Autonomous Region, Hainan Province; Southwest China: Chongqing Municipality, Sichuan Province, Guizhou Province, Yunnan Province, Xizang Autonomous Region; Northwest China: Shaanxi Province, Gansu Province, Qinghai Province, Ningxia Hui Autonomous Region, Xinjiang Uygur Autonomous Region; any group with less than three participants was excluded to ensure result robustness. For vaccination status, we included the number of vaccine doses and the duration from the last vaccination, categorized into three stages: <3 months (from January 1 to April 1, 2023), 3–6 months (from October 1, 2022 to January 1, 2023), and >6 months (before October 1, 2022). Abbreviation: COVID-19=coronavirus disease 2019. | |||
| Total | 42,987 | 20,798 (48.4) | 22,189 (51.6) |
| Regions* | |||
| North | 6,456 (15.0) | 3,099 (48.0) | 3,357 (52.0) |
| Northeast | 1,844 (4.3) | 1,020 (55.3) | 824 (44.7) |
| East | 14,934 (34.7) | 7,317 (49.0) | 7,617 (51.0) |
| South Central | 12,640 (29.4) | 6,069 (48.0) | 6,571 (52.0) |
| Southwest | 5,232 (12.2) | 2,410 (46.1) | 2,822 (53.9) |
| Northwest | 1,881 (4.4) | 883 (46.9) | 998 (53.1) |
| Age group (years) | |||
| 0–17 | 1,488 (3.5) | 786 (52.8) | 702 (47.2) |
| 18–29 | 26,221 (61.0) | 12,499 (47.7) | 13,722 (52.3) |
| 30–39 | 11,266 (26.2) | 5,350 (47.5) | 5,916 (52.5) |
| 40–49 | 2,621 (6.1) | 1,278 (48.8) | 1,343 (51.2) |
| 50–59 | 949 (2.2) | 582 (61.3) | 367 (38.7) |
| 60–69 | 310 (0.7) | 205 (66.1) | 105 (33.9) |
| ≥70 | 132 (0.3) | 98 (74.2) | 34 (25.8) |
| Sex | |||
| Female | 33,053 (76.9) | 15,698 (47.5) | 17,355 (52.5) |
| Male | 9,934 (23.1) | 5,100 (51.3) | 4,834 (48.7) |
| Underlying diseases | 3,084 (7.2) | 1,467 (47.6) | 1,617 (52.4) |
| Hypertension | 967 (2.3) | 496 (51.3) | 471 (48.7) |
| Diabetes | 354 (0.8) | 192 (54.2) | 162 (45.8) |
| Cardiovascular diseases | 381 (0.9) | 202 (53.0) | 179 (47.0) |
| Cerebrovascular diseases | 82 (0.2) | 58 (70.7) | 24 (29.3) |
| Respiratory diseases | 734 (1.7) | 332 (45.2) | 402 (54.8) |
| Chronic kidney disease | 157 (0.4) | 80 (51.0) | 77 (49.0) |
| Chronic liver disease | 290 (0.7) | 150 (51.7) | 140 (48.3) |
| Diseases of immune deficiency | 624 (1.5) | 262 (42.0) | 362 (58.0) |
| Tumour | 53 (0.1) | 28 (52.8) | 25 (47.2) |
| Vaccination status | |||
| None | 2,194 (5.1) | 876 (39.9) | 1,318 (60.1) |
| One dose (>6 months) | 580 (1.4) | 216 (37.2) | 364 (62.8) |
| One dose (3–6 months) | 125 (0.3) | 45 (36.0) | 80 (64.0) |
| One dose from (<3 months) | 27 (0.1) | 13 (48.1) | 14 (51.9) |
| Two doses (>6 months) | 7,721 (18) | 3,498 (45.3) | 4,223 (54.7) |
| Two doses (3–6 months) | 387 (0.9) | 209 (54) | 178 (46) |
| Two doses (<3 months) | 229 (0.5) | 143 (62.4) | 86 (37.6) |
| Three doses (>6 months) | 24,560 (57.1) | 10,780 (43.9) | 13,780 (56.1) |
| Three doses (3–6 months) | 2,252 (5.2) | 1,241 (55.1) | 1,011 (44.9) |
| Three doses (<3 months) | 1,156 (2.7) | 638 (55.2) | 518 (44.8) |
| Four doses and above (>6 months) | 333 (0.8) | 155 (46.5) | 178 (53.5) |
| Four doses and above (3–6 months) | 1,882 (4.4) | 1,512 (80.3) | 370 (19.7) |
| Four doses and above (<3 months) | 1,541 (3.6) | 1,472 (95.5) | 69 (4.5) |