Table 4. Univariate analysis of the factors associated with the frequency of the pH1N1 HA E374K mutants isolated in Taiwan, 2009–2010.

Factors	Numbers of isolates	E374K	Fisher's exact p-values
A. Epidemic periods
1. pre-peak period of the epidemic * (<Wk 37, before September 13, 2009)	97	9 (9.3%)
2. Post-peak period of the epidemic (≧Wk 37, September 13, 2009∼)	99	64(64.7%)	<0.0001
B. Class suspension in Taipei City
1. Before suspension intervention (<Wk35, before September 1, 2009)	63	3(4.8%)
2. Peak of suspension wave (Weeks 41–45; October 11–November 14,,2009)	6	6(100%)	<0.0001
C. Vaccination program intervention
1. Peak-epidemic to pre-vaccination [Peak-Epi-Pre-Vac](Wk 37–43, September 13–October 31, 2009)	40	20(50%)
2. During vaccination campaign [D-Vac] (Wk 44–50, November 1–December 19, 2009)	17	15(88.2%)
3. Post–national vaccination campaign [Post-Nat- Vac](>Wk 50, After December 20,2009)	42	29(69.%)	0.017
D. Gender Males	118	41(34.8%)
Females	78	32(41%)	0.451
E. Age (Years)
1. Pre-peak period of the epidemic
<12	45	3(6.7%)
12–18	27	4(14.8%)
>18	25	2(8%)	0.552
2. Post-peak period of the epidemic
<12	41	29(70.7%)
12–18	18	12(66.7%)
>18	40	23(57.5%)	0.472
F. Gender and Age
a) Males <12	52	21(40.4%)
12–18	33	10(30.3%)
>18	33	10(30.3%)	0.534
b) Females <12	34	11(32.4%)
12–18	12	6(50%)
>18	32	15(46.9%)	0.394
G. Population density
a) Pre-peak period of the epidemic
1) >20000/km2	42	9.5%(4/42)
2) <20000/km2	40	7.5%(3/40)	1
b) Post-peak period of the epidemic
1) >20000/km2	38	25(69.8%)
2) <20000/km2	28	19(67.9%)	1

*: Based on the positive rate detected by RT-PCR or viral culture.