Table 1 Characteristics of participants in pre- and post-COVID-19 groups before and after PSM

Age (y), median (IQR)

5.4 (3.6, 6.5)

4.6 (3.3, 5.9)

0.001

5.1 (3.5, 6.3)

4.7 (3.3, 6.1)

0.236

Male, n (%)

250 (58.0)

215 (66.0)

0.026

190 (65.3%)

183 (62.9%)

0.545

BMI (kg/m²), median (IQR)

18.0 (16.5, 19.5)

17.1 (15.5, 18.9)

< 0.001

17.6 (16.3, 19.1)

17.3 (15.9, 19.1)

0.470

Vaccination status, n (%)

 H1N1 vaccination

16 (3.7)

9 (2.8)

0.468

10 (3.4)

6 (2.1)

0.311

 COVID-19 vaccination

400 (92.8)

N/A

N/A

N/A

N/A

N/A

  One-dose vaccination

89 (20.6)

N/A

N/A

N/A

N/A

N/A

  Two-dose vaccination

201 (46.6)

N/A

N/A

N/A

N/A

N/A

  Three-dose vaccination

110 (25.5)

N/A

N/A

N/A

N/A

N/A

Symptoms, n (%)

 Fever

420 (97.4)

319 (97.9)

0.717

282 (96.9)

285 (97.9)

0.433

  Duration of fever (d), median (IQR)

4.0 (3.0, 6.0)

2.5 (2.0, 4.0)

< 0.001

4.0 (3.0, 6.0)

3.0 (2.0, 4.0)

< 0.001

  Fever peak (°C), median (IQR)

39.0 (38.7, 39.4)

38.8 (38.6, 39.3)

< 0.001

39.0 (38.7, 39.4)

38.8 (38.6, 39.3)

< 0.001

 Cough

395 (91.6)

273 (83.7)

0.001

264 (90.7)

240 (82.5)

0.003

 Rhinorrhea

157 (36.4)

114 (35.0)

0.679

106 (36.4)

99 (34.0)

0.544

 Wheezing

131 (30.4)

75 (23.0)

0.024

80 (27.5)

67 (23.0)

0.215

 Sore throat

83 (19.3)

61 (18.7)

0.850

55 (18.9)

59 (20.3)

0.676

 Headache

49 (11.4)

33 (10.1)

0.585

28 (9.6)

28 (9.6)

1.000

 Myalgia

105 (24.4)

86 (26.4)

0.527

75 (25.8)

74 (25.4)

0.924

 Seizures

78 (18.1)

23 (7.1)

< 0.001

53 (18.2)

22 (7.6)

< 0.001

 Drowsiness

150 (34.8)

104 (31.9)

0.403

96 (33.0)

98 (33.7)

0.860

 Diarrhea

49 (11.4)

34 (10.4)

0.682

29 (10.0)

32 (11.0)

0.685

 Vomiting

62 (14.4)

50 (15.3)

0.715

37 (12.7)

47 (16.2)

0.238

 Abdominal pain

32 (7.4)

21 (6.4)

0.600

22 (7.6)

18 (6.2)

0.512

Laboratory findings, median (IQR)

 Leukocyte count (×10⁹/L)

6.3 (4.3, 9.0)

6.0 (4.3, 8.5)

0.341

6.1 (4.2, 8.7)

6.0 (4.3, 8.7)

0.707

 Neutrophil proportion (%)

47.0 (31.0, 60.7)

47.4 (31.8, 61.0)

0.953

46.5 (30.7, 60.2)

47.4 (32.4, 61.2)

0.623

 Lymphocyte proportion (%)

41.5 (28.0, 56.4)

40.6 (28.2, 55.1)

0.921

41.8 (29.6, 58.3)

40.5 (27.6, 55.1)

0.474

  Proportion of CD3⁺T cells (%)

69.7 (64.2, 73.2)

68.3 (64.8, 72.2)

0.198

69.8 (65.6, 73.2)

68.1 (64.1, 71.7)

0.126

  Proportion of CD3⁺8⁺T cells (%)

25.4 (21.3, 31.1)

26.2 (21.5, 31.0)

0.454

25.3 (21.2, 30.5)

26.4 (22.3, 31.0)

0.108

  Proportion of CD3⁺4⁺T cells (%)

35.8 (28.3, 42.3)

36.7 (30.9, 42.4)

0.189

36.3 (30.0, 42.4)

36.7 (31.1, 42.4)

0.544

  Proportion of NK cells (%)

12.7 (9.1, 16.6)

12.1 (8.5, 15.6)

0.071

12.7 (9.0, 16.4)

12.2 (8.5, 15.6)

0.208

  Proportion of B cells (%)

18.9 (15.2, 22.5)

18.4 (13.9, 22.8)

0.417

18.7 (15.6, 22.5)

18.5 (14.0, 23.0)

0.665

 CRP (mg/L)

15.4 (12.3, 23.1)

11.7 (9.1, 14.6)

< 0.001

14.9 (12.5, 18.2)

11.7 (8.8, 14.5)

< 0.001

 IL-6 (pg/mL)

20.8 (16.9, 31.6)

11.1 (5.6, 20.4)

< 0.001

20.5 (16.9, 30.1)

11.0 (5.6, 21.0)

< 0.001

 IL-10 (pg/mL)

16.6 (12.7, 22.3)

15.0 (12.0, 20.5)

0.020

16.5 (12.8, 22.2)

14.9 (11.9, 20.6)

0.030

 TNF-α (pg/mL)

17.9 (14.0, 23.0)

16.8 (12.8, 22.5)

0.027

18.0 (13.8, 22.8)

17.1 (13.3, 22.7)

0.161

 CK-MB (U/L)

44.9 (40.1, 53.1)

36.0 (30.4, 42.8)

< 0.001

44.9 (40.7, 52.0)

36.0 (30.2, 42.8)

< 0.001

 hs-TnT (pg/mL)

4.6 (3.0, 6.9)

4.3 (2.9, 7.0)

0.401

4.7 (3.0, 6.9)

4.3 (2.9, 7.2)

0.433

 ALT (U/L)

16.0 (12.0, 23.0)

17.0 (12.0, 26.0)

0.098

16.0 (12.0, 23.0)

17.0 (12.0, 26.0)

0.125

 AST (U/L)

32.0 (24.0, 41.0)

33.0 (24.0, 43.0)

0.290

32.0 (23.0, 42.0)

33.0 (24.0, 43.0)

0.345

 Fibrinogen (g/L)

3.9 (3.5, 4.6)

3.7 (3.2, 4.6)

0.005

3.9 (3.5, 4.7)

3.7 (3.2, 4.7)

0.046

Co-infection with SARS-CoV-2, n (%)

55 (12.8)

N/A

N/A

N/A

N/A

N/A

Mechanical ventilation, n (%)

20 (4.6)

4 (1.2)

0.008

12 (4.1)

4 (1.4)

0.043

LOS (d), median (IQR)

8.0 (7.0, 9.0)

6.0 (5.0, 7.0)

< 0.001

8.0 (7.0, 9.0)

6.0 (5.0, 7.0)

< 0.001

Severe H1N1 infection, n (%)

93 (21.6)

42 (12.9)

0.002

60 (20.6)

41 (14.1)

0.038