Infection Control & Hospital Epidemiology
957
P<.0001), the number of days from declaring an outbreak to starting oseltamivir (t=3.48; df=51; P=.001), the prevalence of ILI among residents at the start of the outbreak (t=2.04; df=51; P=.047), the number of residents at risk at the start of the outbreak (t=4.02; df=51; P=.0002), and the staff vacci- nation rate at the start of the outbreak (t=2.09; df=25; P=.047). Using a stepwise forward modeling strategy, where initial
criteria for being included in the model was having a P value<.15 and removal from the model with a P value>.20, only 3 variables were found to be statistically significant (Table 2): the number of days from the second case to starting oseltamivir (t=6.04; df= 49; P<.0001), the number of days from the first case to the second case (t=3.35; df=49; P=.002), and the number of resi- dents at risk at the start of the outbreak (t=4.22; df=49; P=.0001). The inclusion of the prevalence of ILI in the model did not
Fig. 1. Outbreaks included and excluded from analysis.
Table 1. Summary of Influenza A H3N2 Outbreak Characteristics Characteristic
No. of cases (n=53)
Total no. of residents (n=53) Secondary cases (n=53)a
Residents excluding primary cases (n=53)a Secondary case attack rate (%) (n=53)a
No. days from 2nd case to chemoprophylaxis (n=53) No. days between 1st and 2nd cases (n=53)
No. days between 2nd case and declaring outbreak (n=53) Prevalence of ILI among residents (n=53), %b,c Prevalence of ILI among staff (n=26), %b,c Staff vaccinated (n=27), %c Residents vaccinated (n=40), %c Hygiene score (n=27)d Rural (Y/N) (n=53)e Private6 (Y/N) (n=53)f
change the odds ratios (ORs) of the 3 statistically significant independent variables. In addition, no other control variables significantly affected the model because the timing of oseltamivir chemoprophylaxis remained statistically significant (P values ranging from .002 to <.0001) in all other 4 variable models with an OR ranging from 1.27 to 1.37 in these models. Therefore, the final main effects model includes the 3 statistically significant variables. The main effects model was assessed for collinearity. The variance inflation factor was well below 10, so there was no
Average 14.57 99.21 10.38 95.02 14.24 3.85 1.17 2.08 6.29 0.55
34.07 82.73 74.29 0.51 0.30
Minimum Value 3
16 0
13 0 0 0 0
0.9 0 8 3
49 0 0
Maximum Value 81
431 77
427 67 11 6
10 19
6.6 96
100 100 1 1
Standard Deviation 13.41 76.72 12.23 76.46 13.73 2.52 1.65 2.20 5.03 1.42
22.16 16.71 14.23 0.50 0.46
NOTE. n, number of facilities with available information; ILI, influenza-like illness. aPrimary cases are defined as cases of ILI occurring on or before the day that the second case occurred; and Secondary cases are defined as all cases of ILI occurring after the primary cases. bILI is characterized as acute onset of respiratory illness with fever and cough and with 1 or more of the following: sore throat, arthralgia, myalgia, or prostration that could be due to
influenza.9 cAt the start of the outbreak. dHand hygiene score in the facility during the 2014–2015 influenza season. If >1 audit occurred during this time, scores were averaged. eRural=a population less than 10,000 in the 2011 Health Canada Census (1=yes, 0=no). fFacilities not directly operated by the Regional Health Authority (1=yes, 0=no).
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