FluWatch report: April 17 to April 23, 2016 (week 16)

Overall Summary

  • In week 16, all influenza indicators remained similar to, or declined from the previous week.
  • Elevated influenza activity was mostly reported in the Ontario, Quebec and Atlantic provinces.
  • Many regions across Canada are reporting a greater proportion of influenza B detections. This increase in influenza B is expected as influenza B often shows up later in the flu season.
  • Influenza B continues to account for a increasing proportion of hospitalizations among children and adults.
  • Hospitalizations, ICU admissions and deaths among the pediatric population, while declining, continue to remain above expected levels based on the past several influenza seasons.
  • For more information on the flu, see our Flu(influenza) web page.

Are you a primary health care practitioner (General Practitioner, Nurse Practitioner or Registered Nurse) interested in becoming a FluWatch sentinel for the 2015-16 influenza season? Contact us at FluWatch@phac-aspc.gc.ca

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Date published: 2016-04-29

Influenza/Influenza-like Illness Activity (geographic spread)

Influenza activity continues to be reported in the majority of regions in Canada; however, the number of regions reporting influenza activity decreased in week 16.  Localized activity was reported in a total of 13 regions across five provinces. Sporadic activity levels were reported in 30 regions across all provinces and territories.

Figure 1. Map of overall influenza/ILI activity level by province and territory, Canada, week 16

Figure 1
Figure 1 Legend

Note: Influenza/ILI activity levels, as represented on this map, are assigned and reported by Provincial and Territorial Ministries of Health, based on laboratory confirmations, sentinel ILI rates and reported outbreaks. Please refer to detailed definitions at the end of the report. Maps from previous weeks, including any retrospective updates, are available in the mapping feature found in the Weekly Influenza Reports.

Figure 1 - Text Description

Influenza activity continues to be reported in the majority of regions in Canada; however, the number of regions reporting influenza activity decreased in week 16.  Localized activity was reported in a total of 13 regions across five provinces. Sporadic activity levels were reported in 30 regions across all provinces and territories.

Laboratory Confirmed Influenza Detections

In week 16, the percentage of tests positive for influenza continued to decrease from the previous week [from 21% in week 15 to 19% in week 16], driven by the decline in influenza A. Compared to the previous five seasons, the percent positive (19%) reported in week 16 was above the five year average for that week and exceeded the expected levels (range 10.1%-16.2%). With the late start to the 2015-16 influenza season, these elevated levels are not unexpected.

Figure 2. Number of positive influenza tests and percentage of tests positive, by type, subtype and report week, Canada, 2015-16

Figure 2
Figure 2 - Text Description

In week 16, the percentage of tests positive for influenza continued to decrease from the previous week [from 21% in week 15 to 19% in week 16].

Nationally in week 16, there were 1,140 positive influenza tests reported. Influenza B continues to account for an increasing proportion of influenza detections accounting for 64% of detections in week 16. Detections remain predominantly influenza A in British Columbia and the Atlantic Region. To date, 76% of influenza detections have been influenza A and among those subtyped, the vast majority have been influenza A(H1N1) [92% (10866/11855)].

Figure 3. Cumulative numbers of positive influenza specimens by type/subtype and province, Canada, 2015-16

Figure 3

Note: Specimens from NT, YT, and NU are sent to reference laboratories in other provinces. Cumulative data includes updates to previous weeks.

Figure 3 - Text Description
Reporting
provincesTable Figure 3 - Footnote 1
Weekly (April 17 to April 23, 2016) Cumulative (August 30, 2015 to April 23, 2016)
Influenza A B Influenza A B A & B
Total
A
Total
A
(H1)pdm09
A
(H3)
A Table Figure 3 - Footnote UnS B
Total
A
Total
A
(H1)pdm09
A
(H3)
ATable Figure 3 - Footnote UnS B
Total
Table Figure 3 - Footnote 1

Specimens from NT, YT, and NU are sent to reference laboratories in other provinces. Cumulative data includes updates to previous weeks.

Return to Table Figure 3 - Footnote 1 referrer

Table Figure 3 - Footnote 2

Percentage of tests positive for sub-types of influenza A are a percentage of all influenza A detections.

Return to Table Figure 3 - Footnote 2 referrer

Table Figure 3 - Footnote UnS

Unsubtyped: The specimen was typed as influenza A, but no result for subtyping was available.

Return to first Table Figure 3 - Footnote UnS referrer

Table Figure 3 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table Figure 3 - Footnote x referrer

BC 22 16 <5 <5 15 2013 993 353 667 1130 3063
AB 16 7 <5 Table Figure 3 - Footnote x 68 3924 3618 187 119 1543 5467
SK 14 0 0 14 31 2321 1467 46 808 749 3070
MB 11 <5 <5 Table Figure 3 - Footnote x 22 885 180 38 667 162 1047
ON 113 34 8 70 185 7479 3199 364 3915 2355 9834
QC 177 26 0 151 389 8758 1016 6 7736 2607 11365
NB 43 0 0 43 8 1068 86 <5 Table Figure 3 - Footnote x 71 1139
NS 9 0 0 9 <5 295 0 <5 Table Figure 3 - Footnote x 9 304
PE <5 <5 0 Table Figure 3 - Footnote x <5 Table Figure 3 - Footnote x 53 12 Table Figure 3 - Footnote x <5 66
NL 5 0 0 5 <5 458 58 <5 Table Figure 3 - Footnote x 23 481
YT <5 0 Table Figure 3 - Footnote x <5 0 83 49 <5 Table Figure 3 - Footnote x 19 102
NT 0 0 0 0 <5 120 90 Table Figure 3 - Footnote x <5 15 135
NU <5 0 Table Figure 3 - Footnote x <5 0 18 <5 Table Figure 3 - Footnote x 17 14 32
Canada 415 85 14 315 725 27487 10810 1044 15632 8698 36185
Percentage Table Figure 3 - Footnote 2 36% 20% 3% 76% 64% 76% 39% 4% 57% 24% 100%

In week 16, individuals under the age of 44 accounted for 69% of influenza B detections.

To date this season, detailed information on age and type/subtype has been received for 31,063 cases. Children and teenagers (0-19) accounted for 47% of influenza B cases and approximately one third of all influenza cases. Children and teenagers (0-19), young adults (20-44) and middle-aged adults (45-64) accounted for approximately an equal proportion of influenza A(H1N1) cases (28%).

Table 1. Weekly and cumulative numbers of positive influenza specimens by type, subtype and age-group reported through case-based laboratory reporting Table 1 - Footnote 1, Canada, 2015-16
Age groups (years) Weekly (April 17 to April 23, 2016) Cumulative (August 30, 2015 to April 23, 2016)
Influenza A B Influenza A B Influenza A and B
A Total A(H1) pdm09 A(H3) A UnSTable 1 - Footnote 3 Total A Total A(H1) pdm09 A(H3) A UnSTable 1 - Footnote 3 Total # %
Table 1 - Footnote 1

Table 1 includes specimens for which demographic information was reported. These represent a subset of all positive influenza cases reported.

Return to Table 1 - Footnote 1 referrer

Table 1 - Footnote 2

Percentage of tests positive for sub-types of influenza A are a percentage of all influenza A detections.

Return to Table 1 - Footnote 2 referrer

Table 1 - Footnote 3

Unsubtyped: The specimen was typed as influenza A, but no result for subtyping was available.

Return to first Table 1 - Footnote 3 referrer

Table 1 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table 1 - Footnote x referrer

<5 41 <5 <5 34 112 4450 1698 69 2683 1309 5761 19%
5-19 14 <5 Table 1 - Footnote x 12 153 2373 1017 99 1257 2231 4606 15%
20-44 51 10 0 41 114 5735 2717 154 2864 1876 7611 25%
45-64 65 13 <5 Table 1 - Footnote x 66 6255 2739 185 3331 905 7160 23%
65+ 82 18 <5 Table 1 - Footnote x 105 4712 1587 417 2708 1212 5925 19%
Total 253 47 <5 202 550 23525 9758 924 12843 7533 31063 100%
PercentageTable 1 - Footnote 2 32% 19% 2% 80% 68% 76% 41% 4% 55% 24%    

For data on other respiratory virus detections see the Respiratory Virus Detections in Canada Report on the Public Health Agency of Canada website.

Influenza-like Illness Consultation Rate

The national ILI consultation rate decreased from the previous week from 56.4 per 1,000 patient visits in week 15, to 36.1 per 1,000 patient visits in week 16. The highest ILI consultation rate was found in the 0-4 years age group (104.4 per 1,000) and the lowest was found in the ≥65 years age group (18.8 per 1,000) (Figure 4).

Figure 4. Influenza-like-illness (ILI) consultation rates by age group and week, Canada, 2015-16

Figure 4

Delays in the reporting of data may cause data to change retrospectively. In BC, AB, and SK, data are compiled by a provincial sentinel surveillance program for reporting to FluWatch. Not all sentinel physicians report every week.

Figure 4 - Text Description

Influenza-like illness consultation rate by age-group in week 16 for the 2015-16 season:
Age 0-4: 104.4; Age 5-19: 63.6; Age 20-64: 25.2; Age 65+: 18.8

Pharmacy Surveillance

During week 16, the proportion of prescriptions for antivirals decreased from the previous week to 27.7 antiviral prescriptions per 100,000 total prescriptions, which is lower than the five year historical average for week 16. The proportion of prescriptions for antivirals remains highest among children. In week 16, the proportion reported among children was 71.5 per 100,000 total prescriptions.

Figure 5. Proportion of prescription sales for influenza antivirals by age group and week, Canada, 2015-16

Figure 5

Note: Pharmacy sales data are provided to the Public Health Agency of Canada by Rx Canada Inc. and sourced from major retail drug chains representing over 3,000 stores nationwide (excluding Nunavut) in 85% of Health Regions. Data provided include the number of new antiviral prescriptions (for Tamiflu[oseltamivir] and Relenza [zanamivir]) and the total number of new prescriptions dispensed by Province/Territory and age group.

* The average weekly proportion includes data from April 2011 to March 2015.

Figure 5 - Text Description

Proportion of antiviral prescriptions per 100,000 total prescriptions
Average National Rate (Yrs 10-11 to 14-15): 47.5.; Rate wk 16: 27.7
Proportion of antiviral prescriptions by age-group in week 16 for the 2015-16 season:
Infant: 25.8; child: 71.5; adult: 25.1; senior: 27.1

Influenza Outbreak Surveillance

In week 16, 13 new laboratory confirmed influenza outbreaks were reported: nine in long-term care facilities (LTCF). three in hospitals and one in an institution or community setting. Of the outbreaks with known strains or subtypes, all three outbreaks were due to influenza B (two in LTCFs and one in an institution or community setting). Additionally, one ILI outbreak was reported in a school.

To date this season, 401 outbreaks have been reported. At week 16 in the 2014-15 season, 1,607 outbreaks were reported and in the 2013-14 season, 218 outbreaks were reported.

Figure 6. Overall number of new laboratory-confirmed influenza outbreaksFigure 5 - Footnote 1 by report week, Canada, 2015-2016

Figure 6
Figure 6 - Text Description
Report week Hospitals Long Term Care Facilities Other
35 0 0 0
36 0 0 0
37 1 1 0
38 0 0 0
39 0 2 0
40 0 2 1
41 0 0 0
42 0 0 0
43 0 1 0
44 1 3 1
45 1 1 0
46 0 0 0
47 0 0 0
48 0 1 0
49 0 1 0
50 0 2 0
51 1 1 0
52 1 0 2
1 0 2 1
2 0 2 0
3 1 4 1
4 4 6 3
5 8 6 3
6 8 10 3
7 2 17 8
8 6 27 7
9 13 24 8
10 9 23 14
11 2 21 4
12 9 15 5
13 5 12 4
14 2 14 1
15 1 11 0
16 3 9 1

Sentinel Pediatric Hospital Influenza Surveillance

Paediatric Influenza Hospitalizations and Deaths

In week 16, 33 laboratory-confirmed influenza-associated pediatric (≤16 years of age) hospitalizations were reported by the Immunization Monitoring Program Active (IMPACT) network (Figure 7). The largest proportion of hospitalizations were in children aged 5-9 years, accounting for 30% of the hospitalizations. Similar to the trend of increased laboratory detections of influenza B, 82% of pediatric hospitalizations reported in week 16 were due to influenza B.

To date this season, 1,233 hospitalizations have been reported by  the IMPACT network: 878 hospitalized cases (71%) were due to influenza A and 355 cases (29%) were due to influenza B. This season's count of pediatric hospitalizations is nearly double that reported up to week 16 (667) in the 2015-16 season. The current year total number of cases also exceeds the total number of cases reported in the past five seasons.

A total of 193 intensive care unit (ICU) admissions have been reported. Children aged 2 to 4 and 5 to 9 years accounted for 29% and 26% respectively of ICU admissions. A total of 128 ICU cases (66%) reported at least one underlying condition or comorbidity. Eight influenza-associated deaths have been reported.

Table 2 - Cumulative numbers of peadiatric hospitalizations (≤16 years of age) with influenza reported by the IMPACT network, Canada, 2015-16
Age Groups Cumulative (30 Aug. 2015 to 23 April 2016)
Influenza A Influenza B Influenza A and B (#(%))
A Total A(H1) pdm09 A(H3) A (UnS) B Total
Table 2 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table 2 - Footnote x referrer

0-5m 118 32 5 81 29 147 (12%)
6-23m 271 73 7 191 71 342 (28%)
2-4y 250 79 <5 Table 2 - Footnote x 96 346 (28%)
5-9y 181 47 <5 Table 2 - Footnote x 115 296 (24%)
10-16y 58 18 <5 Table 2 - Footnote x 44 102 (8%)
Total 878 249 21 608 355 1233 (100%)

Figure 7. Number of cases of influenza reported by sentinel hospital networks, by week, Canada, 2015-16, paediatric and adult hospitalizations (≤16 years of age, IMPACT; ≥16 years of age, CIRN-SOS)

Figure 7

Not included in Table 2 and Figure 7 are two IMPACT cases that were due to co-infections of influenza A and B.

Figure 7 - Text Description
Report week IMPACT CIRN-SOS
35 0 0
36 0 0
37 1 0
38 2 0
39 0 0
40 0 0
41 1 0
42 0 0
43 1 0
44 0 2
45 2 0
46 1 3
47 2 1
48 2 1
49 3 7
50 3 3
51 6 7
52 13 11
1 20 19
2 13 13
3 23 26
4 39 21
5 48 45
6 77 55
7 108 92
8 148 129
9 147 116
10 147 130
11 114 121
12 90 88
13 65 89
14 58 63
15 66 40
16 33 28

Adult Influenza Hospitalizations and Deaths

In week 16, 28 laboratory-confirmed influenza-associated adult (≥16 years of age) hospitalizations were reported by the Canadian Immunization Research Network Serious Outcome Surveillance (CIRN-SOS) (Figure 7). The largest proportion of hospitalizations was in adults 65+ years of age (53%) and due to influenza A (53%). In recent weeks, the weekly number of hospitalizations reported by CIRN has been decreasing and an increasing proportion of hospitalizations have been due to influenza B.

To date this season, 1,108 hospitalizations have been reported by CIRN-SOS (Table 3). The majority of hospitalized cases were due to influenza A (82%) and the largest reported proportion was among adults ≥65 years of age (50%). One hundred and seventy-four intensive care unit (ICU) admissions have been reported of which 118 cases reported at least one underlying condition or comorbidity. A total of 48 deaths have been reported this season with the majority of deaths reported in adults ≥65 years of age (67%).

Table 3 - Cumulative numbers of adult hospitalizations (≥16 years of age) with influenza reported by the CIRN-SOS network, Canada, 2015-16
Age groups (years) Cumulative (1 Nov. 2015 to 23 April 2016)
Influenza A B Influenza A and B
A Total A(H1) pdm09 A(H3) A(UnS) Total # (%)
Table 3 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table 3 - Footnote x referrer

16-20 Table 3 - Footnote x <5 0 <5 <5 7 (1%)
20-44 140 48 <5 Table 3 - Footnote x 39 179 (16%)
45-64 321 97 <5 Table 3 - Footnote x 40 361 (33%)
65+ 443 123 23 297 113 556 (50%)
Unknown <5 0 0 <5 <5 5 (%)
Total 912 273 27 612 196 1108
% 82% 30% 3% 67% 18% 100%

Figure 8. Percentage of hospitalizations, ICU admissions and deaths with influenza reported by age-group (≥16 year of age), Canada 2015-16

Figure 8

Note: The number of hospitalizations reported through CIRN-SOS and IMPACT represents a subset of all influenza-associated adult and paediatric hospitalizations in Canada. Delays in the reporting of data may cause data to change retrospectively.

x - Suppressed to prevent residual disclosure.

Figure 8 - Text Description
Age-group (years) Hospitalizations (n=1103) ICU admissions (n=174) Deaths (n=48)
Table Figure 8 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table Figure 8 - Fo2tn273x referrer

16-20 0.6% Table Figure 8 - Footnote x% Table Figure 8 - Footnote x%
20-44 16.2% 18.4% Table Figure 8 - Footnote x%
45-64 32.7% 44.8% 29.2%
65+ 50.4% 35.6% 66.7%

Provincial/Territorial Influenza Hospitalizations and Deaths

In week 16, 129 hospitalizations were reported by participating provinces and territoriesFootnote *. Influenza B accounted for the greatest proportion of hospitalizations, accounting for 55% of hospitalizations reported in week 16. The largest proportion of cases reported was in adults 65+ years of age (37%). Among hospitalizations for influenza B, children represented 35% of cases.

Since the start of the 2015-16 season, 4,840 laboratory-confirmed influenza-associated hospitalizations have been reported. A total of 3,929 hospitalizations (81%) were due to influenza A and 911 (19%) were due to influenza B. Of the 510 ICU admissions reported, 267 (52%) were due to influenza A(H1N1). A total of 229 deaths have been reported; all but 30 were associated with influenza A.

Overall this season, hospitalizations have been reported more frequently among adults ≥65 years of age. The largest proportion of ICU admissions was reported in adults 45-64 years of age and the highest proportion of fatal cases was reported in adults ≥65 years of age. Pediatric (0-19 years) and young to middle-aged adults (20-44 years) accounted for 42% of all hospitalizations and 14% of all deaths reported to date this season. Similar to findings from the IMPACT network, there have been more pediatric hospitalizations reported to date compared to the year-end totals in each of the previous four influenza seasons.

Figure 9. Percentage of hospitalizations, ICU admissions and deaths with influenza reported by age-group, Canada 2015-16

Figure 9
Figure 9 - Text Description
Age-group (years) Hospitalizations (n=4840) ICU admissions (n=510) Deaths (n=229)
0-4 19.0% 8.8% 2.2%
5-19 9.3% 6.1% 3.1%
20-44 13.3% 18.2% 8.3%
45-64 26.8% 44.7% 35.8%
65+ 31.6% 22.2% 50.7%

See additional data on Reported Influenza Hospitalizations and Deaths in Canada: 2011-12 to 2015-16 on the Public Health Agency of Canada website.

Influenza Strain Characterizations

During the 2015-16 influenza season, the National Microbiology Laboratory (NML) has characterized 2,117 influenza viruses [194 A(H3N2), 1,188 A(H1N1) and 735 influenza B].

Influenza A (H3N2): When tested by hemagglutination inhibition (HI) assays, 50 A(H3N2) viruses  were antigenically characterized as A/Switzerland/9715293/2013-like using antiserum raised against cell-propagated A/Switzerland/9715293/2013.  

Sequence analysis was done on 144 A(H3N2) viruses. All viruses belonged to a genetic group for which most viruses were antigenically related to A/Switzerland/9715293/2013. A/Switzerland/9715293/2013 is the A(H3N2) component of the 2015-16 Northern Hemisphere's vaccine.

Influenza A (H1N1): All of the 1,188 A(H1N1) viruses characterized were antigenically similar to A/California/7/2009, the A(H1N1) component of the 2015-16 influenza vaccine.

Influenza B: A total of 154 influenza B viruses characterized  were antigenically similar to the vaccine strain B/Phuket/3073/2013. A total of 581 influenza B viruses were characterized as B/Brisbane/60/2008-like, one of the influenza B components of the 2015-16 Northern Hemisphere quadrivalent influenza vaccine.

The recommended components for the 2015-2016 northern hemisphere trivalent influenza vaccine include: an A/California/7/2009(H1N1)pdm09-like virus, an A/Switzerland/9715293/2013(H3N2)-like virus, and a  B/Phuket/3073/2013 -like virus (Yamagata lineage). For quadrivalent vaccines, the addition of a B/Brisbane/60/2008-like virus (Victoria lineage) is recommended.

The NML receives a proportion of the  influenza positive specimens from provincial laboratories for strain characterization and antiviral resistance testing. Characterization data reflect the results of haemagglutination inhibition testing compared to the reference influenza strains recommended by WHO.

Antiviral Resistance

During the 2015-16 season, the National Microbiology Laboratory (NML) has tested 1,288 influenza viruses for resistance to oseltamivir, 1,290 for resistance to zanamivir and 1,349 influenza viruses for resistance to amantadine. All but eight tested viruses were sensitive to oseltamivir. The eight H1N1 viruses resistant to oseltamivir had a H275Y mutation. All viruses tested for resistance were sensitive to zanamivir.  All but two influenza A viruses were resistant to amantadine (Table 4).

Table 4. Antiviral resistance by influenza virus type and subtype, Canada, 2015-16
Virus type and subtype Oseltamivir Zanamivir Amantadine
# tested # resistant (%) # tested # resistant (%) # tested # resistant (%)
Table 4 - Footnote *

NA - not applicable

Return to first Table 4 - Footnote * referrer

A (H3N2) 157 0 (0%) 157 0 (0%) 201 200 (99.5%)
A (H1N1) 794 8 (1%) 796 0 (0%) 1148 1147 (99.9%)
B 337 0 (0%) 337 0 (%) NATable 4 - Footnote * NATable 4 - Footnote *
Total 1288 8 (0.6%) 1290 0 (0%) 1349 1347 (99.9%)

International Influenza Reports


FluWatch definitions for the 2015-2016 season

Abbreviations: Newfoundland/Labrador (NL), Prince Edward Island (PE), New Brunswick (NB), Nova Scotia (NS), Quebec (QC), Ontario (ON), Manitoba (MB), Saskatchewan (SK), Alberta (AB), British Columbia (BC), Yukon (YT), Northwest Territories (NT), Nunavut (NU).

Influenza-like-illness (ILI): Acute onset of respiratory illness with fever and cough and with one or more of the following - sore throat, arthralgia, myalgia, or prostration which is likely due to influenza. In children under 5, gastrointestinal symptoms may also be present. In patients under 5 or 65 and older, fever may not be prominent.

ILI/Influenza outbreaks

Schools:
Greater than 10% absenteeism (or absenteeism that is higher (e.g. >5-10%) than expected level as determined by school or public health authority) which is likely due to ILI.
Note: it is recommended that ILI school outbreaks be laboratory confirmed at the beginning of influenza season as it may be the first indication of community transmission in an area.
Hospitals and residential institutions:
two or more cases of ILI within a seven-day period, including at least one laboratory confirmed case. Institutional outbreaks should be reported within 24 hours of identification. Residential institutions include but not limited to long-term care facilities ( LTCF) and prisons.
Workplace:
Greater than 10% absenteeism on any day which is most likely due to ILI.
Other settings:
two or more cases of ILI within a seven-day period, including at least one laboratory confirmed case; i.e. closed communities.

Note that reporting of outbreaks of influenza/ILI from different types of facilities differs between jurisdictions.

Influenza/ILI activity level

1 = No activity: no laboratory-confirmed influenza detections in the reporting week, however, sporadically occurring ILI may be reported

2 = Sporadic: sporadically occurring ILI and lab confirmed influenza detection(s) with no outbreaks detected within the influenza surveillance region Footnote

3 = Localized:

  1. evidence of increased ILIFootnote * and
  2. lab confirmed influenza detection(s) together with
  3. outbreaks in schools, hospitals, residential institutions and/or other types of facilities occurring in less than 50% of the influenza surveillance regionFootnote

4 = Widespread:

  1. evidence of increased ILIFootnote * and
  2. lab confirmed influenza detection(s) together with
  3. outbreaks in schools, hospitals, residential institutions and/or other types of facilities occurring in greater than or equal to 50% of the influenza surveillance regionFootnote

Note: ILI data may be reported through sentinel physicians, emergency room visits or health line telephone calls.

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