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FluWatch report: May 1 to May 21, 2016 (weeks 18-20)

Overall Summary

  • In weeks 18-20, all influenza indicators declined from the previous weeks.
  • Elevated influenza B activity persisted in many regions across Canada: influenza B accounted for the majority of influenza detections in weeks 18-20.  Additionally, the majority of outbreaks reported this week were due to Influenza B.
  • This increase in influenza B is expected as influenza B often shows up later in the flu season.
  • 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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Influenza/Influenza-like Illness Activity (geographic spread)

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Organization:
Public Health Agency of Canada
Date published: 2016-05-27

Related Topics

Influenza activity continues to be reported in Canada; however, the number of regions reporting influenza activity decreased in weeks 18-20. During week 20, localized activity was reported in a total of six regions across Ontario and Nunavut. Sporadic activity levels were reported in 26 regions across all provinces and territories. A total of 13 regions reported no influenza activity.

Figure 1. Map of overall influenza/ILI activity level by province and territory, Canada, week 20

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 Canada; however, the number of regions reporting influenza activity decreased in weeks 18-20. During week 20, localized activity was reported in a total of six regions across Ontario and Nunavut. Sporadic activity levels were reported in 26 regions across all provinces and territories. A total of 13 regions reported no influenza activity and a total of 8 regions did not report data in week 20.

Laboratory Confirmed Influenza Detections

In weeks 18-20, the percentage of tests positive for influenza continued to decrease [from 17% in week 17 to 9.4% in week 20], driven by the decline in influenza A. Compared to the previous five seasons, the percent positive (9.4%) reported in week 20 was above the five year average for that week and exceeded the expected levels (confidence interval 5.0-9.2%). 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 weeks 18-20, the percentage of tests positive for influenza continued to decrease [from 17% in week 17.0 to 9.4% in week 20], driven by the decline in influenza A.

Nationally in weeks 18-20, there were 1,555 positive influenza tests reported. Influenza B continues to account for an increasing proportion of influenza detections accounting for 80% of detections in weeks 18-20. Laboratory detections of influenza in the provinces of Ontario and Quebec accounted for 78% of all detections for week 20. To date, 73% of influenza detections have been influenza A and among those subtyped, the vast majority have been influenza A(H1N1) [91% (11,003/12,103)].

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		 Weeks 18-20 (May 1 to May 21, 2016) Cumulative (August 30, 2015 to May 21, 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
BC 19 9 Table Figure 3 - Footnote x <5 22 2058 1025 361 672 1163 3141
AB 16 7 Table Figure 3 - Footnote x <5 69 3954 3636 203 115 1657 5611
SK 6 0 0 6 29 2337 1514 47 776 829 3166
MB Table Figure 3 - Footnote x <5 0 <5 60 898 185 38 675 240 1138
ON 102 29 22 51 292 7646 3260 392 3994 2770 10416
QC 98 <5 <5 95 714 8948 1024 8 7916 3685 12633
NB 51 6 <5 Table Figure 3 - Footnote x 36 1165 100 5 1060 138 1303
NS <5 0 0 <5 <5 302 Table Figure 3 - Footnote x <5 301 11 313
PE 0 0 0 0 <5 65 53 12 Table Figure 3 - Footnote x <5 68
NL 14 0 0 14 <5 473 58 <5 Table Figure 3 - Footnote x 28 501
YT 0 0 0 0 0 58 53 <5 <5 19 77
NT 0 0 0 0 <5 121 90 Table Figure 3 - Footnote x <5 20 141
NU 6 <5 0 <5 6 25 <5 Table Figure 3 - Footnote x 20 20 45
Canada 318 58 39 221 1237 28050 11003 1026 15947 10583 38633
Percentage Table Figure 3 - Footnote 2 20% 18% 12% 69% 80% 73% 39% 4% 57% 27% 100%
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.

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Cumulative data includes updates to previous weeks.

In weeks 18-20, the number of laboratory detections decreased across all age groups, most notably among individuals under the age of 5 years.  In week 20, Influenza B detections accounted for 85% of all detections (table 1).

To date this season, detailed information on age and type/subtype has been received for 33,165 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 (26-29%).

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) Weeks 18-20 (May. 1, 2016 to May. 21, 2016) Cumulative (August 30, 2015 to May 21, 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 # %
<5 24 <5 0 Table 1 - Footnote x 252 4036 1225 75 2736 1701 6229 19%
5-19 11 <5 0 Table 1 - Footnote x 261 2081 700 101 1280 2649 5058 15%
20-44 26 <5 <5 21 154 4764 1713 163 2888 2168 7981 24%
45-64 52 9 6 37 95 5382 1792 199 3391 1088 7476 23%
65+ 57 7 15 35 178 4453 1225 440 2788 1563 6421 19%
Total 170 23 24 123 940 20716 6655 978 13083 9169 33165 100%
PercentageTable 1 - Footnote 2 15% 14% 14% 72% 85% 62% 32% 5% 63% 28%    
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

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 remained constant from previous weeks from 31.1 per 1,000 patient visits in week 17, to 31.1 per 1,000 patient visits in week 20. The highest ILI consultation rate was found in the 0-4 years age group (65.0 per 1,000) and the lowest was found in the 20-64 years age group (23.4 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 20 for the 2015-16 season:
Age 0-4: 65.0; Age 5-19: 47.9; Age 20-64: 23.4; Age 65+: 29.6

Pharmacy Surveillance

In the period of weeks 18-20, the proportion of prescriptions for antivirals decreased by 73% compared to week 17. The antiviral prescriptions per 100,000 total prescriptions in week 20 was 6.0; this rate is lower than the five year historical average for week 20. The proportion of prescriptions for antivirals remains highest among children. In week 20, the proportion reported among children was 11.3 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): 36.3.; Rate wk 20: 6.1
Proportion of antiviral prescriptions by age-group in week 20 for the 2015-16 season:
Infant: 0.0; child: 11.3; adult: 7.0; senior: 5.0

Influenza Outbreak Surveillance

In weeks 18-20, fourteen new laboratory confirmed influenza outbreaks were reported: twelve in long-term care facilities (LTCF), one in a hospital and and one in an institution or community setting. Of the outbreaks with known strains or subtypes, six outbreaks were due to influenza B, one was due to A(H3N2) and three were due to influenza A(unsubtyped).

To date this season, 423 outbreaks have been reported. At week 20 in the 2014-15 season, 1,724 outbreaks were reported and in the 2013-14 season, 260 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
17 0 7 1
18 0 9 1
19 0 2 0
20 1 1 0

Sentinel Pediatric Hospital Influenza Surveillance

Paediatric Influenza Hospitalizations and Deaths

In weeks 18-20, 66 laboratory-confirmed influenza-associated pediatric (≤16 years of age) hospitalizations were reported by the Immunization Monitoring Program Active (IMPACT) network (Figure 7). An equal proportion of hospitalizations were reported in children 6-23 months, 2-4 years and 5-9 years, accounting for  26%, 23% and 26% of the hospitalizations respectively.  Similar to the trend of increased laboratory detections of influenza B, 88% of pediatric hospitalizations reported in weeks 18-20 were due to influenza B.

To date this season, 1,337 hospitalizations have been reported by  the IMPACT network: 893 hospitalized cases (67%) were due to influenza A and 444 cases (33%) were due to influenza B. This season’s count of pediatric hospitalizations is nearly double that reported up to week 20 in the 2014-15 season (n=697). The current year total number of cases also exceeds the total number of cases reported in the past five seasons.

A total of 205 intensive care unit (ICU) admissions have been reported. Children aged 2 to 4 and 5 to 9 years accounted for 29% and 26% of ICU admissions respectively . A total of 131 ICU cases (64%) 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 21 May 2016)
Influenza A Influenza B Influenza A and B (#(%))
A Total A(H1) pdm09 A(H3) A (UnS) B Total
0-5m 122 32 <5 Table 2 - Footnote x 40 162 (12%)
6-23m 275 76 7 192 92 367 (27%)
2-4y 257 82 <5 Table 2 - Footnote x 114 371 (28%)
5-9y 181 46 <5 Table 2 - Footnote x 142 323 (24%)
10-16y 58 18 <5 Table 2 - Footnote x 56 114 (9%)
Total 893 254 21 618 444 1337 (100%)
Table 2 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table 2 - Footnote x referrer

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 19 20
2 15 14
3 23 26
4 40 21
5 49 45
6 77 54
7 106 93
8 146 131
9 147 117
10 146 130
11 114 120
12 89 88
13 65 91
14 58 69
15 66 44
16 39 43
17 35 17
18 29 0
19 21 0
20 16 0

Adult Influenza Hospitalizations and Deaths

Surveillance for the 2015-2016 influenza season ended on April 30th, 2016.

To date this season, 1,153 hospitalizations have been reported by CIRN-SOS (Table 3). The majority of hospitalized cases were due to influenza A (81%) and the largest reported proportion was among adults ≥65 years of age (50%). One hundred and ninety-one intensive care unit (ICU) admissions have been reported of which 132 cases reported at least one underlying condition or comorbidity. A total of 55 deaths have been reported this season with the majority of deaths reported in adults ≥65 years of age (62%).

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 April 30, 2016)
Influenza A B Influenza A and B
A Total A(H1) pdm09 A(H3) A(UnS) Total # (%)
16-20 Table 3 - Footnote x <5 0 <5 <5 Table 3 - Footnote x
20-44 144 50 <5 Table 3 - Footnote x 46 190(16%)
45-64 331 105 <5 Table 3 - Footnote x 46 377(33%)
65+ 452 125 24 303 123 575 (50%)
Unknown <5 Table 3 - Footnote x 0 <5 <5 <5 (x%)
Total 934 285 28 621 219 1153
% 81% 31% 3% 66% 19% 100%
Table 3 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table 3 - Footnote x referrer

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=1149) ICU admissions (n=191) Deaths (n=50)
16-20 0.6% Table Figure 8 - Footnote x% Table Figure 8 - Footnote x%
20-44 16.5% Table Figure 8 - Footnote x% Table Figure 8 - Footnote x%
45-64 32.8% 46.1% 34.5%
65+ 50.0% 34.6% 61.8%
Table Figure 8 - Footnote x

Suppressed to prevent residual disclosure.

Return to first Table Figure 8 - Fo2tn273x referrer

Provincial/Territorial Influenza Hospitalizations and Deaths

In week 20, 37 hospitalizations were reported by participating provinces and territoriesootnote *. Influenza B accounted for the greatest proportion of hospitalizations, accounting for 65% of hospitalizations reported in week 20. The largest proportion of cases reported was in adults 65+ years of age (49%). Among hospitalizations for influenza B, children (0-19 years) represented 41% of cases.

Since the start of the 2015-16 season, 5,169 laboratory-confirmed influenza-associated hospitalizations have been reported. A total of 4,054 hospitalizations (78%) were due to influenza A and 1115 (22%) were due to influenza B. Of the 529 ICU admissions reported, 271 (51%) were due to influenza A(H1N1). A total of 255 deaths have been reported; all but 36 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 45-64years of age and the highest proportion of fatal cases was reported in adults ≥65 years of age (figure 9). Pediatric (0-19 years) and young to middle-aged adults (20-44 years) accounted for 29% of all hospitalizations and 5% 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=5169) ICU admissions (n=529) Deaths (n=255)
0-4 18.9% 8.9% 2.0%
5-19 9.6% 5.9% 2.7%
20-44 13.0% 18.3% 7.5%
45-64 26.4% 44.6% 35.7%
65+ 32.1% 22.3% 52.2%

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 2,678 influenza viruses [217 A(H3N2), 1,367 A(H1N1) and 1094 influenza B].

Influenza A (H3N2):When tested by hemagglutination inhibition (HI) assays, 64 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 153 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 1,367 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 232 influenza B viruses characterized  were antigenically similar to the vaccine strain B/Phuket/3073/2013. A total of 862 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 included: 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 1,675 influenza viruses for resistance to oseltamivir, 1,596 for resistance to zanamivir and 1,585 influenza viruses for resistance to amantadine. All but nine tested viruses were sensitive to oseltamivir. The nine 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 (%)
A (H3N2) 178 0 (0%) 171 0 (0%) 219 218 (99.5%)
A (H1N1) 968 9 (0.9%) 948 0 (0%) 1366 1365 (99.9%)
B 529 0 (0%) 477 0 (%) NATable 4 - Footnote * NATable 4 - Footnote *
Total 1675 9 (0.5%) 1596 0 (0%) 1585 1583 (99.9%)
Table 4 - Footnote *

NA - not applicable

Return to first Table 4 - Footnote * referrer

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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