FluWatch report: December 20, 2015 – January 2, 2016 (weeks 51 & 52)

In week 52, 29 regions across Canada reported sporadic influenza/ILI activity (up from 22 regions in week 51). In week 52, 20 regions reported no influenza/ILI activity (down from 25 regions in week 51).  Localized activity was reported in two regions in Canada (one in ON and one in NL).

Laboratory Confirmed Influenza Detections

Laboratory confirmed influenza detections continue to increase steadily. The percent positive for influenza increased from 3.3% in week 51 to 4.3% in week 52. Compared to the previous five seasons, the percent positive (4.3%) reported in week 52 was below the five year average for that week and below expected levels (range 13.8%-34.3%).

In week 52, there were 183 laboratory detections of influenza reported (up from 115 detections reported in week 51). Among subtyped influenza A detections,  influenza A(H1N1) was the most common influenza A virus detected across Canada in weeks 51 and 52; and was. the most common subtype detected in AB, SK, ON, PE and NL in week 52. To date, 84% of influenza detections have been influenza A and the majority of those subtyped have been influenza A(H3N2) [63% (307/483)].

Overall, among influenza cases with reported age, the largest proportion was in those ≥65 years of age (38%) (Table 1). The largest proportion of influenza A(H3N2) cases (49%) is among those aged 65 years and older. For influenza A(H1N1) cases, the largest proportion (30%) is among those aged 45-64 years. The largest proportion of influenza B cases (49%) is among those aged 5-19 years.

For additional 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 increased from the previous week from 30.2 per 1,000 patient visits in week 51, to 50.3 per 1,000 patient visits in week 52. In week 52, the highest ILI consultation rate (120.4 per 1,000) was found in those 0-4 years of age and the lowest was found in the 20-64 yr age group (Figure 4).

Influenza Outbreak Surveillance

In week 52, three new laboratory confirmed influenza outbreaks were reported (up from two outbreaks reported in week 51). One outbreak was reported in a hospital and due to influenza A and the other two outbreaks were reported in institutional or community settings.  To date this season, 33 outbreaks have been reported (15 of which occurred in LTCFs). In comparison, at week 52 in the 2014-15 season, 435 outbreaks were reported (349 of which occurred in LTCFs) and in the 2013-14 season, 25 outbreaks were reported (15 of which occurred in LTCFs).

Sentinel Pediatric Hospital Influenza Surveillance

Paediatric Influenza Hospitalizations and Deaths

In week 52, thirteen hospitalizations were reported by the the Immunization Monitoring Program Active (IMPACT) network, up from seven hospitalizations reported in week 51 (Figure 6).

To date this season, 38 laboratory-confirmed influenza-associated paediatric (≤16 years of age) hospitalizations have been reported by  the IMPACT network. Thirty-one hospitalized cases were due to influenza A and seven cases were due to influenza B. The majority of hospitalized cases were among children  aged 2-4 years (34%).  Additionally, not included in Table 2 and Figure 6, two cases were due to co-infections of influenza A and B. To date, six  intensive care unit (ICU) admissions and less than five influenza-associated deaths have been reported.

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

Adult Influenza Hospitalizations and Deaths

In week 52, four hospitalizations were reported by the Canadian Immunization Research Network Serious Outcome Surveillance (CIRN-SOS), up from three hospitalizations reported in week 51 (Figure 6).

To date this season, 17 laboratory-confirmed influenza-associated adult (≥16 years of age) hospitalizations have been reported by CIRN-SOS. The majority of hospitalized cases were due to influenza A (94%) and were among adults ≥65 years of age (53%). To date, no intensive care unit (ICU) admissions or deaths have been reported.

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

Provincial/Territorial Influenza Hospitalizations and Deaths

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 122 influenza viruses [71 A(H3N2), 26 A(H1N1) and 25 influenza B].

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

Sequence analysis was done on 65 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): Twenty-six H1N1 virus characterized were antigenically similar to A/California/7/2009, the A(H1N1) component of the 2015-16 influenza vaccine.

Influenza B: Sixteen influenza B viruses characterized  were antigenically similar to the vaccine strain B/Phuket/3073/2013. Nine influenza B virus was 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 /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 number of 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 129 influenza viruses for resistance to oseltamivir and zanamivir and 106 influenza viruses for resistance to amantadine. All viruses were sensitive to zanamivir and oseltamivir. A total of 105 influenza A viruses (99%) were resistant to amantadine.(Table 4).

International Influenza Reports

• World Health Organization influenza update
• World Health Organization FluNet
• WHO Influenza at the human-animal interface
• Centers for Disease Control and Prevention seasonal influenza report
• European Centre for Disease Prevention and Control - epidemiological data
• South Africa Influenza surveillance report
• New Zealand Public Health Surveillance
• Australia Influenza Report
• Pan-American Health Organization Influenza Situation Report

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 ILI^{Footnote *} 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 region^{Footnote †}

4 = Widespread:

1. evidence of increased ILI^{Footnote *} 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 region^{Footnote †}

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