Table 3.

The relationship between history of smoking and the severity of COVID-19.

Studies	Severe COVID-19 with history of smoking			Nonsevere COVID-19 with history of smoking
Authors	Event	Total	%	Event	Total	%
CDC report [4]	38	457	8.3	208	6180	3.3
Chen et al. [10]	3	43	7.0	12	102	11.0
Feng et al. [11]	17	121	14.0	27	333	8.1
Goyal et al. [12]	6	130	4.6	14	263	5.3
Guan et al. [13]	38	172	22.0	120	913	13.0
Liu et al. [14]	3	11	27.3	2	67	2.0
Mo et al. [15]	4	85	4.7	2	70	2.9
Qi et al. [16]	31	50	62.0	22	217	10.1
Shi et al. [17]	6	49	12.2	34	438	7.8
Wang et al. [18]	7	25	28.0	9	100	9.0
Wan et al. [19]	1	40	2.5	8	95	8.4
Yang et al. [20]	0	32	0	2	20	10.0
Yu et al. [21]	2	26	8.0	24	307	8.9
Zhang et al. [22]	6	58	10.3	3	82	3.6
Zhang et al. [23]	6	30	20.0	0	90	0
Zhou et al. [24]	5	54	9.0	6	137	4.0
Total	173	1383	12.5	493	9414	5.2