Influenza in Canada: 2005-2006 Season Update

15 February 2006

Volume 32

Number 04

Introduction

Canada's national influenza surveillance system, the FluWatch program, is now in its 10^th season. FluWatch is coordinated through the Immunization and Respiratory Infections Division (IRID), Public Health Agency of Canada. The program collects data and information from various sources in order to provide a national picture of influenza activity. This report provides a summary of influenza activity in Canada during the current 2005-2006 season from 28 August 2005 up to and including 14 January 2006.

Methods

FluWatch reports include data and information from five main sources:

1. laboratory reports of influenza tests in Canada;

2. sentinel physician reporting of influenza-like illness (ILI) consultations;

3. regional influenza activity levels as assigned by provincial and territorial FluWatch representatives;

4. hospitalbased surveillance information on influenza in children through the Immunization Monitoring Program ACTive (IMPACT);

5. international surveillance system reports of influenza activity (from the CDC, EISS, and WHO).

Results

Respiratory Virus Detections
Across Canada, 33 laboratories submit respiratory virus detection data on a weekly basis to the IRID. Since 28 August 2005, IRID has received reports of 25,685 tests for influenza, of which 454 (1.8%) have tested positive for influenza viruses: 232 (51.1%) were influenza A and 222 (48.9%) were influenza B. The provincial distribution of positive tests is shown in Table 1. Most of the influenza A isolates (65.1%) were from British Columbia, whereas most of the influenza B isolates (93.2%) were from Alberta. The percent of influenza detections to date is low compared to the previous two seasons (1.8% in 2005-2006 compared to 9.7% and 17.7% in 2004-2005 and 2003-2004 respectively) suggesting that the current influenza season is a mild one thus far.

Table 1. Total number of influenza tests performed and number of positive tests by province/territory of testing laboratory, Canada, 2005-2006

Detailed case-by-case epidemiological reports were sent to the IRID for 367/454 (80.8%) laboratory-confirmed influenza cases, from six provinces (British Columbia, Alberta, Saskatchewan, Manitoba, Ontario and Quebec) and one territory (Yukon). Of these, 155 (42.2%) were influenza A infections and 212 (57.8%) were influenza B infections. The majority of the influenza A cases were reported in the older age groups; including 22% of cases in 25 to 44 year olds, 19% in 45 to 64 year olds and 26% in those ≥ 65. This is reflected by a majority of outbreaks being reported in British Columbia and Ontario long-term care facilities (LTCFs) this season. In contrast, the majority of the influenza B infections were reported among children; including 25.0% of cases in those < 5 years of age, 27% in 5 to 9 year olds and 24% in 10 to 14 year olds, with the majority of influenza B-associated outbreaks occurring in Alberta schools.

Influenza Virus Strain Identification & Vaccine Match

To date, 96 influenza viruses have been antigenically characterized by the National Microbiology Laboratory (NML): 56 were A/California/07/2004 (H3N2)-like; one was A/New Caledonia/ 20/1999 (H1N1)-like; 36 were B/Hong Kong/330/2001-like; and three were B/Shanghai/361/2002-like. All 57 (100%) of the influenza A strains characterized by the NML have matched those included in the 2005-2006 Canadian vaccine.

The influenza B viruses have evolved into two antigenically distinct lineages since the mid-1980s, represented by B/Yamagata/ 16/1988-like and B/Victoria/2/1987-like viruses. Of the 39 influenza B viruses characterized by the NML to date, three (8%) were B/Shanghai/361/2002-like, belonging to the B/Yamagata/166/1988 lineage, and match the influenza B component of the current 2005-2006 vaccine. However, the majority of influenza B viruses characterized to date have been B/Hong Kong/330/2001-like (36/39 or 92%), belonging to the B/Victoria/2/1987 lineage, which are not covered by the B component in the current vaccine.

Antiviral Resistance

The Public Health Agency of Canada recommends that health care providers in Canada not prescribe amantadine to treat and prevent influenza during the current flu season. The interim recommendation, released on 16 January 2006, followed NML testing which demonstrated that most influenza A (H3N2) isolates tested in Canada and the U.S. are resistant to the drug. At this time, PHAC is recommending that oseltamivir (Tamiflu) or zanamivir (Relenza) should be selected if an antiviral medication is recommended for treatment or prevention of influenza for the remainder of the 2005/2006 season.

The results are an important reminder that antiviral medications - which do not provide immunity - should not replace annual influenza immunizations for the prevention of influenza. Influenza immunization - or the annual “flu shot” - remains the most effective method of avoiding contracting influenza when combined with proper hygiene, including frequent hand-washing and staying home when you are ill¹. ILI Consultations Reported by Sentinel Clinical Practices For one-clinic day each week, sentinel clinic sites are asked to report the total number of patients seen for any reason (denominator) and the total number of patients meeting a nationally standard case definition for ILI (numerator). Data from sentinel physicians are weighted by the estimated population in the census division being represented each week. Weighted rates are summed to create a national ILI rate each week. Weekly ILI rates have remained at or below baseline levels since the beginning of the season. Figure 1 shows the Canadian age-standardized, census- division weighted ILI consultation rates for the current season, compared to the mean rate and 95% confidence intervals for the previous 9 years of the FluWatch program. To date this season, the median sentinel participation rate has been fairly good at 71.8% (range from 47.2 to 79.8%) per week. Participation rates tend to be lowest during the summer months and during the Christmas holidays.

Figure 1. Census-division weighted age-standardized ILI consultation rates, by influenza season and report week, Canada, 2005-2006, compared with seasons 1996-1997 to 2004-2005 (average with 95% confidence intervals)

Regional Influenza Activity Levels Assessed by Provincial and Territorial Epidemiologists

To date, the 2005-2006 season has been relatively mild with mostly sporadic activity reported across Canada except in the Atlantic provinces where no activity has been reported. Sporadic activity was first reported in British Columbia in late August 2005, whereas localized influenza activity was first reported in Saskatchewan in late September to mid-October 2005 due to an influenza outbreak in a nursing home. By late November, more outbreaks were being reported in LTCFs in British Columbia and in schools in Alberta. The former were associated with influenza A, while the latter were associated with influenza B. To date, only one region (southern Alberta) has reported widespread influenza activity, which occurred in week 51 associated with the influenza B outbreaks in schools.

Influenza Hospitalizations in Children

Laboratory-confirmed cases of influenza among children that required admission to one of the sentinel paediatric hospitals located across Canada aremonitored through the Immunization Monitoring Program ACTive (IMPACT) network. To date this season, a total of 23 cases have been reported through the IMPACT network. The majority of the cases were from paediatric hospitals in Vancouver (15, 65%) and Edmonton (5, 22%). Elsewhere, Ottawa, Quebec City and Montreal sites each reported a single case. Influenza A was identified in 70% (16/23) of the cases and influenza B was identified in the remaining 30% (7/23). Eighty-six percent (6/7) of the influenza B cases were from Edmonton. Sixty-five percent (15/23) of the paediatric hospitalizations reported to date were hospitalized during the last 3 weeks of December in Vancouver and Edmonton. Thirty percent of these were between 10 to 16 years of age and 26% were < 1 year of age. To date, no influenza-associated paediatric deaths have been reported in Canada.

International

United States

In the United States, influenza activity remained low from October 2005 to early December 2005 but steadily increased through to January 2006, mostly in the Mountain and Pacific regions. Since early December 2005, the proportion of patient visits to sentinel providers for ILI has been above baseline levels except in mid-January 2006 when the proportion dropped below baseline levels. The proportion of deaths due to pneumonia and influenza has remained below baseline levels since the start of the season. To date, the CDC has received reports of 10 influenza associated paediatric deaths. Since October 2005, the CDC has tested a total of 43,434 specimens for influenza viruses of which 2,092 (4.8%) were positive. Of those, 2,026 (96.8%) were influenza A viruses and 66 (3.2%) were influenza B viruses. In addition, the CDC has antigenically characterized 77 influenza viruses: 65 influenza A(H3N2), one influenza A(H1) and 11 influenza B. Of the 65 influenza A viruses, 54 were A/California/ 07/2004 (H3N2)-like viruses and 11 were H3N2-like viruses showing reduced titers with antisera produced against A/California/07/2004 (H3N2)-like viruses. The hemagglutinin protein of the influenza A(H1) virus was similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/ 1999 (H1N1). Of the 11 influenza B viruses, eight belonged to the B/Yamagata/16/1988 lineage (one B/Shanghai/361/2002-like virus and seven B/Florida/07/2004-like viruses) and three belonged to the B/Victoria/2/1987 lineage².

Worldwide

From early December 2005 to early January 2006, influenza activity increased in several countries of North America and South-East Asia while sporadic detections of influenza viruses were reported in some countries of Central Asia and Europe. However, overall influenza activity remained low. Among the countries experiencing increased influenza activity, the viruses identified so far were: influenza A(H1) and B in China; influenza A(H3N2) in Japan and Mongolia; influenza A(H1) in Madagascar and Tunisia; influenza A(H3N2) and B in Canada; and influenza A(H3 and H1) and B in the U.S3.

In Europe, clinical influenza activity to date has remained at or below baseline levels except in the Netherlands where activity has been slightly above baseline. Since the beginning of the season, influenza A and B viruses have been detected in Europe; however in early January 2006, more influenza B virus (64%) than influenza A virus (36%) detections were reported in Europe as a whole for the first time since 19964.

FluWatch reports are published weekly during the influenza season (October to May) and biweekly during the off season (June to September). They can be accessed through the Public Health Agency of Canada's FluWatch website.

Please note that the above graphs may change as late returns come in.

References

* The 2005-2006 season Canadian vaccine contains an A/New Caledonia/ 20/1999 (H1N1)-like, an A/California/7/2004 (H3N2)-like, and a B/Shanghai/ 361/2002-like virus strain.

1. Public Health Agency of Canada. Interim recommendation for use of Amantadine for influenza. January 2006.

2. Centers for Disease Control and Prevention. Weekly report: Influenza summary update, week ending January 7, 2006 - week 1. URL: http://www.cdc.gov/flu/weekly/.

3. World Health Organization. Seasonal influenza activity in the world, 2005/2006 - January 18,2006.URL: http://www.who.int/csr/disease/influenza/update/en/index.html

4. European Influenza Surveillance Scheme. EISS weekly electronic bulletin, Influenza season 2005-2006. 20 January 2006, Issue No 171. URL: http://www.eiss.org/cgi-files/bulletin_v2.cgi.

Source: F Reyes, MHSc, SG Squires, MSc, JF Macey, MA, MSc, Immunization and Respiratory Infections Division, CIDPC, Public Health Agency of Canada; Y Li, PhD, Influenza and Respiratory Virus Section, National Microbiology Laboratory, Public Health Agency of Canada, Winnipeg; B Winchester, BSc, MSc, P Zabchuk, H Zheng, E Arseneault, Immunization and Respiratory Infections Division, CIDPC, Public Health Agency of Canada, Ottawa, Ontario.