Table 2.

Two-level logistic regression model comparing serious outbreaks and mild outbreaks in Beijing, from August 2018 to July 2019.

Variables	β	t	p	OR (95%CI)
Intercept	−2.19	−3.61	0.003	0.11 (0.03–0.42)
Primary school	−0.27	−0.81	0.43	0.76 (0.37–1.54)
Urban	0.57	1.09	0.29	1.78 (0.57–5.44)
More male illnesses	−0.02	−0.08	0.94	0.98 (0.56–1.66)
More male students in class	0.26	0.93	0.37	1.29 (0.72–2.36)
Large class	0.87	3.08	0.008	2.38 (1.28–4.26)
High level influenza activity when outbreak reported	0.17	0.65	0.53	1.19 (0.68–2.14)
A(H1N1)pdm09 vs. A(H3N2)	−0.63	−1.44	0.17	0.53 (0.21–1.36)
B(Victoria) vs. A(H3N2)	−0.24	−0.72	0.48	0.79 (0.4–1.63)
Other vs. A(H3N2)	0.02	0.04	0.97	1.02 (0.33–3.21)
Days from first case to intervention	0.16	2.37	0.03	1.17 (1.02–1.34)
High vaccination coverage of class	−0.27	−0.85	0.41	0.77 (0.35–1.35)
High vaccination coverage of school	−0.10	−0.28	0.78	0.9 (0.4–1.89)

Notes: Large class: number of students in class was more than the median of the number of students in all classes with febrile outbreaks in Beijing, which was 39 students. High level influenza activity when outbreak reported: weekly positive proportion of ILI for influenza in the district when outbreak was reported was higher than 40%. High vaccination coverage: a vaccination coverage higher than 50%.