Population Characteristics and Participants’ Behaviour
In total, 261 of 600 (43.5%) participants had at least one episode of ILI influenza-like illness during January 2019. Of these, 194 (75.5%) visited healthcare facilities, 167 (86.1%) of those within 2 days of symptom onset. A total of 88.5% of these patients were tested using RIDT and 101 were diagnosed as having influenza, of whom 95.0% were given antivirals. The details of the descriptive analysis are shown in Table 1.
Table 1. Demographic and behavioural characteristics of the participants
Variable
|
Number (Percentage) or median (IQR)
|
Number of household members
|
4 (3-4)
|
Male
|
295/600 (49.2%)
|
Age (year)
|
42 (21-57)
|
High-risk group*
|
46/201 (22.9%)
|
Smoker
|
81/600 (13.5%)
|
Healthcare facility visit
|
194/261 (75.5%)
|
Duration of symptoms (days)
|
2 (1-3)
|
Day of healthcare facility visit
(days from symptom onset)
|
1 (0-2)
|
Patients examined by RIDT
|
169/191 (88.5%)
|
Patients diagnosed as influenza at healthcare facility
|
101/194 (52.1%)
|
Influenza diagnosed by RIDT
|
97/101 (96.0%)
|
Treated by antivirals among influenza cases diagnosed by RIDT
|
96/101 (95.0%)
|
Class of antivirals prescribed
|
Oseltamivir
|
22 (37.3%)
|
Baloxavir
|
20 (33.9%)
|
Laninamivir
|
9 (15.3%)
|
Zanamivir
|
4 (6.8%)
|
Unknown
|
4 (6.8%)
|
Vaccinated for seasonal influenza
|
87 (34.5%)
|
Income level of household
|
< 50,000 USD**/year
|
64 (32.0%)
|
50,000 USD/year <
< 100,000 USD/year
|
86 (43.0%)
|
> 100,000 USD/year
|
28 (14.0%)
|
Education level of householder
|
Primary
|
1 (0.5%)
|
Secondary
|
83 (46.5%)
|
Tertiary
|
95 (47.5%)
|
Advanced
|
11 (5.5%)
|
IQR: Interquartile range, RIDT: Rapid influenza diagnostic test, USD: US dollars
*Participants who have past medical history associated with high-risk of severe influenza
**1 USD = 100 Japanese yen
Disease Burden
The median value of QOL and QALYs lost during the period of ILI were 0.67 (interquartile range [IQR] 0.60-0.79) and 0.0055 (IQR 0.0040–0.0072). The median duration of symptoms and absence were 2 days (IQR 1-3 days) and 2 days (IQR 1-5 days).
Difference between Influenza and Other ILIs
The median QOL score during symptomatic period of influenza group and ILI group was 0.66 (IQR 0.58-0.79) and 0.66 (IQR 0.59-0.79). The median QALYs lost per episode was 0.0044 (IQR 0.0034-0.0066) in the influenza group and ILI group were 0.0044 (IQR, 0.0034-0.0066) and 0.0036 (IQR 0.0018-0.0054), respectively. The basic characteristics of influenza group and ILI group are shown in Table 2. Figure 1, Figure 2, and Figure 3 show the difference between two groups in duration of symptoms, QALYs lost per episode, and duration of absenteeism, respectively.
Table 2. Characteristics and disease burden of the influenza group and the ILI group
Variable
|
Influenza group (N=72)
|
ILI group (N=73)
|
Number of household members
|
4 (2-4)
|
3 (2-4)
|
Male
|
34 (47.9%)
|
46 (63.8%)
|
Age
|
42 (17-53)
|
42 (22-55)
|
High-risk group
|
15 (20.8%)
|
22 (30.1%)
|
Smoker
|
10 (13.9%)
|
19 (26.0%)
|
Day of healthcare facility visit
(days from symptom onset)
|
1 (1-2)
|
1 (1-2)
|
Patients examined by RIDT
|
71 (97.2%)
|
57 (80.3%)
|
Treated by antivirals
|
38 (90.5%)
|
1 (4.3%)
|
Vaccinated for seasonal influenza
|
28 (38.9%)
|
22 (31.9%)
|
Income level
|
|
|
< 50,000 USD*/year
|
20 (30.3%)
|
22 (33.8%)
|
50,000 USD/year <
< 100,000 USD/year
|
34 (51.5%)
|
34 (52.3%)
|
> 100,000 USD/year
|
12 (18.2%)
|
9 (13.8%)
|
Education level of householder
|
|
|
Primary
|
0
|
0
|
Secondary
|
33 (45.8%)
|
32 (43.8%)
|
Tertiary
|
37 (51.4%)
|
34 (46.6%)
|
Advanced
|
2 (2.8%)
|
7 (9.6%)
|
Duration of symptoms (days)
|
2 (2-3)
|
2 (1-3)
|
QOL during symptomatic period
|
0.66 (0.58-0.79)
|
0.66 (0.59-0.79)
|
QALYs lost per episode
|
0.0044
(0.0034-0.0066)
|
0.0036
(0.0018-0.0054)
|
Duration of absenteeism (days)
|
5 (4-6)
|
2 (1-3)
|
Values are shown as absolute number (percentage) or median (interquartile range).
RIDT: Rapid influenza diagnostic test, QOL: quality of life, QALYs: quality-adjusted life-years, USD: US dollars
*1 USD = 100 Japanese Yen
(Figure 1)
(Figure 2)
(Figure 3)
We imputed the missing values in the dataset before comparing the two groups. The number of missing items is shown in Table S2 in the Appendix. After one-to-one propensity score matching for the influenza group versus the ILI group with imputed data, differences in QOL score, QALYs lost, and duration of symptoms were not statistically significant. However, those with influenza were off work about two days longer than the other group. The comparison is shown in Table 3.
Table 3. Differences in outcomes between the two group after propensity score matching
Outcome
|
Estimate
|
SE
|
p-value
|
QOL score
Intercept
Influenza group
|
0.698
-0.0231
|
0.0174
0.0228
|
< 0.001
0.314
|
QALYs lost
Intercept
Influenza group
|
0.00413
0.000650
|
0.000503
0.000620
|
< 0.001
0.297
|
Duration of symptoms
Intercept
Influenza group
|
2.189
0.456
|
0.250
0.346
|
< 0.001
0.192
|
Duration of absenteeism
Intercept
Influenza group
|
2.099
2.010
|
0.277
0.392
|
< 0.001
< 0.001
|
ILI: influenza like illness, QOL: quality of life, QALYs: quality-adjusted life-years,
SE: standard error
Sensitivity Analysis
For sensitivity analyses, we used one-to-one propensity score matching for the influenza group versus the other group for responses with complete data only. We also used IPW-PS analysis with the imputed dataset.
Only 58 of the 200 cases contained complete data. After one-to-one propensity score matching, we had 13 pairs of cases, providing 26 complete cases for analysis. Linear regression analysis showed that duration of absence was significantly different between the two groups, but there were no significant differences between QOL score, QALYs lost, and duration of symptoms. The details are shown in Table S3 in the Appendix.
IPW-PS analysis with the imputed dataset showed that QOL scores were similar between the two groups, but duration of symptoms and absence were longer for those with influenza. As a result, QALYs lost was also greater for those with influenza. These results are shown in Table S4 in the Appendix.