Volume 29-06
15 March 2003

[Table of Contents]

INFLUENZA IN CANADA: 2001-2002 SEASON

Introduction 

Influenza activity in Canada is monitored through a network of sentinel laboratories, sentinel clinical practices, and provincial and territorial ministries of health, which provide information on rates of influenza-like illness (ILI) and types of influenza virus circulating in the population. This surveillance network is coordinated through the FluWatch program and maintained by the Centre for Infectious Disease Prevention and Control (CIDPC), Public Health Agency of Canada, Health Canada. The primary objective of FluWatch is to monitor and report on nationally standardized key indicators of influenza activity to provide a national picture of influenza on an ongoing and timely basis. 

Three main indicators of influenza activity are reported as aggregate data on a weekly basis throughout the influenza season (October to May): 1) laboratory-based influenza virus detection* (viral culture and direct antigen detections) and strain identification** in Canada; 2) sentinel surveillance of ILI consultation rates in Canada; and 3) regional influenza activity levels, as assigned by provincial and territorial epidemiologists (P/T FluWatch representatives). In addition, FluWatch reports on international influenza activity weekly, as assessed from surveillance reports published by other countries (e.g. US Centers for Disease Control and Prevention in Atlanta) and international surveillance systems (e.g. European Influenza Surveillance Scheme and the World Health Organization [WHO]). The 2001-2002 influenza season was the 6th year of the FluWatch program.

During the influenza season, FluWatch disseminates weekly reports to health professionals and the public through a variety of mechanisms, including the CIDPC FAXlink, fax, e-mail, and Health Canada's Viral Respiratory Diseases' Web site <http://www.phac-aspc.gc.ca/fluwatch/index.html>. Beginning in the 2001-2002 season, FluWatch reports were disseminated year round, with twice monthly reporting of laboratory detections in Canada and summaries of international activity during the off season (June to September). For the 2002-2003 season, sentinel surveillance of ILI will continue year round as well. In addition, summaries of influenza, respiratory syncytial virus (RSV), parainfluenza virus and adenovirus laboratory detection data are made available weekly throughout the year through the Respiratory Virus Detections (RVD) Web site <http://www.hc-sc.gc.ca/ pphb-dgspsp/bid-bmi/dsd-dsm/rvdi-divr/index.html>. Summaries of worldwide influenza activity are included periodically in the CIDPC Infectious Diseases News Brief <http://www.hc-sc.gc.ca/ pphb-dgspsp/bid-bmi/dsd-dsm/nb-ab/index.html> and periodic updates on influenza surveillance in Canada are published in the Canada Communicable Disease Report <http://www.phac-aspc.gc.ca/publicat/ccdr-rmtc/03vol29/index.html>.

Twenty of the 31 laboratories that report weekly aggregate data through the Respiratory Virus Detection system also report additional laboratory and epidemiologic information on all their confirmed cases (i.e. viral culture, direct antigen detections, and positive detections from serologic testing) through the case-by-case surveillance system. This information, though available on a less timely basis, includes variables such as age, sex, and virus type and subtype, which allows for a more detailed analysis of the annual distribution of influenza viruses in the population.

This report provides a summary of influenza activity in Canada during the 2001-2002 influenza season through an analysis and interpretation of case-by-case epidemiologic data as well as a final summary and interpretation of the 2001-2002 weekly aggregate data (total laboratory detections of influenza from sentinel laboratories, genetic analysis of circulating influenza viruses by the National Microbiology Laboratory (NML), ILI consultations from sentinel physician reporting, and provincial and territorial activity level reporting). Comparisons are made with previous seasons throughout^(1-7). 

* Laboratory-based influenza virus detections are reported through the Respiratory Virus Detections system, which continues year round (August-August). 
** Strain identifications are reported by the National Microbiology Laboratory.

Methods 

Respiratory Virus Detections (aggregate data) 

Thirty-one laboratories across Canada participated in the FluWatch program during the 2001-2002 season through weekly reporting of total influenza virus detections to the Respiratory Virus Detections system. Although the influenza season commonly runs from October to May, laboratory detections are performed year round, and analyzed and reported from August to August (total detections for 2001-2002 are reported for the period 26 August, 2001, to 24 August, 2002). All participating laboratories are asked to report the total number of influenza tests performed by viral culture and direct antigen detection as well as the total number of tests positive for influenza infection to the Division of Disease Surveillance, CIDPC, on a weekly basis. Laboratory detection data were presented as aggregate data by testing laboratories (territorial samples were tested by laboratories in nearby provinces) and analyzed by the province responsible for testing according to various parameters, including the type of influenza. 

Case-by-Case Surveillance Data 

On a less timely basis (bi-monthly to monthly), 20 of the 31 laboratories that report weekly influenza virus detections across Canada provided additional epidemiologic and laboratory information to CIDPC. These data represent a subset of the cumulative weekly detections, supplemented with a small proportion of positive detections by seroconversion (i.e. >= fourfold rise in antibody titre by any method). Laboratory-confirmed case-by-case data were presented by the province/territory from which the specimen originated (some laboratories received out-of-province samples) and were analyzed by week of onset of illness, age and sex of the case, and influenza type and subtype. Methods used in the detection of influenza included viral culture, direct antigen detection, and seroconversion.

Influenza Virus Strain Identification 

The NML conducts national surveillance on human influenza virus strains in collaboration with provincial laboratories and other Canadian hospital and university-based laboratories. Approximately 10% of the total weekly influenza detections across Canada were referred to the NML for strain identification. NML virologic surveillance detects and describes antigenic changes in the circulating strains of influenza virus. Canadian influenza virus surveillance information and actual representative strains are shared with the WHO's collaborating centres for influenza to contribute to global influenza monitoring and decision-making for vaccine composition recommendations for the upcoming season.

ILI Consultations Reported by Sentinel Physicians 

The College of Family Physicians of Canada, National Research System (NaReS), was responsible for recruiting sentinel physicians in 10 out of 13 provinces and territories across Canada. In the other three provinces (British Columbia, Alberta, and Saskatchewan), sentinel recruitment and reporting was managed by independent provincial programs. FluWatch maintained a network including all of these provincial surveillance systems. The FluWatch objective was to have at least one physician recruited from each of the census divisions across Canada or, in the case of Quebec, where there are 99 census divisions, one or more sentinels recruited for each of the 18 health regions. For census divisions/ health regions with large populations, the objective was to have at least one sentinel physician recruited per 250,000 population. The case definition for ILI was “acute onset of respiratory illness with fever and cough and with one or more of the following - sore throat, arthralgia, myalgia, or prostration which could be due to influenza virus. In children under 5 years of age, gastrointestinal symptoms may also be present. In patients under 5 years or 65 years and older, fever may not be prominent.” For 1 clinic day each week, between 30 September, 2001, and 4 May, 2002, sentinel sites were asked to complete a report form, including the total number of patients seen for any reason (denominator) and the total number of patients meeting the standard case definition for ILI (numerator). Age group information was collected for all patients (for both the numerator and denominator) seen by sentinel physicians recruited through NaReS, and for patients seen through provincial surveillance systems in British Columbia and Saskatchewan. In Alberta, age group information was collected only on numerator data; age group for denominator data was generated by applying the Canadian population distribution. Sentinel report forms were either returned by fax, or the information was conveyed via e-mail or telephone to CIDPC on a weekly basis for data collation, analysis and dissemination.

Regional Influenza Activity Levels Assessed by Provincial and Territorial Epidemiologists 

Most provinces and territories are subdivided into influenza surveillance regions as defined by the provincial or territorial epidemiologist. For the 2001-2002 influenza season, there were 53 surveillance regions: British Columbia (4), Alberta (3), Saskatchewan (3), Manitoba (12), Ontario (5), Quebec (1), New Brunswick (7), Nova Scotia (4), Prince Edward Island (1), Newfoundland (10), Yukon (1) Northwest Territories (1) and Nunavut (1). Provincial and territorial FluWatch representatives assessed the influenza activity level in their respective jurisdictions on a weekly basis, using a variety of sources of information, which included laboratory reports of influenza detection, sentinel physician reports of ILI surveillance, and reports of outbreaks. In addition, school and work-site absenteeism, and emergency department and hospital admission data may also have been be used in assessing the level of influenza activity. Influenza activity levels^† were reported as 1) no activity reported, 2) sporadic activity, 3) localized activity, and 4) widespread activity. 

† For the 2001-2002 influenza surveillance season, FluWatch program activity levels were defined as follows:
1 = No activity reported
2 = Sporadic: sporadically occurring ILI and confirmed influenza† with no outbreaks detected within the surveillance region
3 = Localized: sporadically occurring ILI and confirmed influenza† and outbreaks of ILI in < 50% of the surveillance region(s)
4 = Widespread: sporadically occurring ILI and confirmed influenza† and outbreaks of ILI in >=50% of the surveillance region(s) 

† confirmation of influenza within the surveillance region at any time within the prior 4 weeks. 

Results 

Respiratory Virus Detections (aggregate data) 

Between 26 August, 2001, and 24 August, 2002, a total of 58,010 influenza tests were performed by 31 laboratories receiving samples from all provinces and territories across Canada. Approximately 12% (6,771) of tests were positive. Of these, 5,905 (87%) were influenza A, and 866 (13%) were influenza B. Although influenza A infections predominated in all provinces and territories, Ontario reported a significantly greater proportion of influenza B infections (30% versus 0% to 12% reported by the other provinces and territories). Of the 866 influenza B identifications, Ontario reported 796 (78%). 

The number of tests performed during 2001-2002 is comparable with the previous 3 years. The overall percentage of positive tests in 2001-2002 (12%) was higher than in the previous season (7.6%) but comparable with the 3 previous years of laboratory surveillance (range 10% to 12%). Both the number of tests performed and the percentage of positive test results have increased considerably since 1996-1997, the first year of the FluWatch program, and earlier^(1-7). The variation in the numbers of confirmed cases and their distribution by provinces/territories should be interpreted with caution, as these numbers are likely to reflect differences in population size and distribution, testing and reporting practices and criteria, and availability of diagnostic services. These factors vary among the regions and have changed from year to year.

Case-by-case Surveillance Data 

Twenty laboratories in 10 provinces reported a total of 6,258 case-by-case records, including epidemiologic and laboratory details, to CIDPC (Table 1). This compared with 3,935 laboratory-confirmed case-by-case records reported by 21 laboratories in 10 provinces for the previous season (2000-2001). As with the larger RVD aggregate dataset, the variation in the numbers of confirmed cases in the case-by-case dataset and the distribution of virus type and subtype among provinces/territories should be interpreted with caution. 

Table 1. Laboratory-confirmed cases of influenza reported to CIDPC, by laboratory, Canada, 2001-2002
Province	Laboratory	Number of cases
Newfoundland	Newfoundland Public Health Laboratory	78
Prince Edward Island	Queen Elizabeth Hospital	8
Nova Scotia	Queen Elizabeth II Health Science Centre - Victoria General Site, Halifax	105
New Brunswick	G.L. Dumont Regional Hospital, Moncton	172
Quebec	Laboratoire de santé publique du Québec (Sainte-Anne-de-Bellevue)	2,278
Ontario	Kingston Public Health Laboratory	148
	Central Public Health Laboratory, Toronto	988
	Hospital for Sick Children, Toronto	125
	Thunder Bay Public Health Laboratory	40
	Windsor Public Health Laboratory	30
	Peterborough Public Health Laboratory	106
	Sault Ste. Marie	41
	Timmins Public Health Laboratory	73
	Hamilton Public Health Laboratory	161
Manitoba	Cadham Provincial Laboratory,Winnipeg	105
Saskatchewan	Department of Health, Regina	562
	Saskatoon Public Health Laboratory	114
Alberta	Provincial Laboratory of Public Health for Northern Alberta, Edmonton	492
	Provincial Laboratory of Public Health for Southern Alberta, Calgary	317
British Columbia	Division of Laboratories, Health Branch, Vancouver	315
Total		6,258

The majority of influenza cases (88.6%) were laboratory confirmed by virus isolation. Less commonly reported methods of laboratory confirmation included direct antigen detection (11.3% of cases) and serologic testing (0.1% of cases). The same three methods of laboratory confirmation were used in previous seasons; however, use of virus isolation has been increasingly reported (compared with 54% in 1997-1998 and 78% in 1999-2000). 

Table 2 shows the case-by-case, laboratory-confirmed data by province/territory, and influenza type and subtype for cases reported during the 2001-2002 season. The largest number and proportion of cases were reported by Quebec (2,274 cases, 36%), Ontario (1,710 cases, 27%), Alberta (788 cases, 13%), and Saskatchewan (679 cases, 11%). As in previous years, the distribution of influenza types for laboratory-confirmed case-by-case data was the same as that for the more timely RVD data: 87% (5,462/6,258) of 2001-2002 case-by-case records were confirmed as influenza type A, and 13% (796/6258) were confirmed as influenza type B. As with the RVD data, Ontario reported a higher proportion of influenza B cases (55%) than other provinces and territories; 608/796 (76%) of the influenza B case-by-case reports were from Ontario. The predominantly influenza A distribution pattern in 2001-2002 was in contrast to the predominantly influenza B season in 2000-2001.

Further subtyping of the influenza A viruses isolated during the 2001-2002 season showed considerable differences in the proportion of viruses subtyped as well as in the proportion of subtypes identified when compared with the previous season. Of the 5,462 influenza A identifications, only 3% were subtyped (163/5,462), as compared with 28% subtyped in the previous year. Of the 163 influenza A subtypes identified, 5% (8) were of the H1NI subtype and 95% (155) were of the H3N2 subtype. The opposite subtype distribution was seen in the previous season, when 99% of viruses were identified as H1N1 and only1% as H3N2. 

Nationally, the week of onset of 65% of case-by-case reports was during the 9-week period from mid-January through mid-March, and for 27.5% of all cases it was during the 3-week period from 26 January to 9 February. Regional peaks in laboratory-confirmed cases were also evident, except in the Territories, where relatively few cases were reported. The first peak in confirmed cases occurred in Quebec, followed by Ontario and the Prairie provinces. Influenza B infections contributed to less than 12% of total RVD or total case-by-case infections reported by each of the regions except Ontario. In Ontario, the peak in reported influenza B cases occurred during mid-February (week 7) (Figure 1).

During the 2001-2002 season, the age group with the greatest proportion of cases (28%, 1,769/6,258) was the youngest one, 0 to 4 years, which is also the narrowest age grouping. A similarly young age distribution was seen during 2000-2001, with 24% of confirmed cases in the 0 to 4-year age group. However, this young age distribution of cases was not seen in the four earlier seasons (1996-1997 though 1999-2000), when the highest proportions occurred in the >= 65 year age group. The percentage of laboratory-confirmed cases reported for those in the >= 65 year age group has increased to 27% in the 2001-2002 season (Figure 2), which was dominated by influenza A (H3N2), from only 8% in the 2000-2001 season, which was dominated by influenza B. 

Figure 1. Laboratory-confirmed cases of influenza by region, type and week of onset, Canada, 2001-2002

Figure 2. Proportionate distribution of laboratory-confirmed influenza cases, by age group, Canada, 2001-2002

Influenza Virus Strain Identification 

During the period from 26 August, 2001, to 25 August, 2002, the NML antigentically characterized 575 influenza viruses (575/6,771 or 8.5% of all RVD of influenza A and B) received from provincial and hospital laboratories: 423 (74%) influenza A viruses and 152 (26%) influenza B viruses (Table 3). Of the 423 influenza A viruses tested, 347 (82%) were H3N2, 1 (0.2%) was H1N1, and 75 (17.7%) were new influenza A (H1N2) viruses, which resulted from gene reassortment of the co-circulating influenza A (H1N1) and A (H3N2) subtypes. Antigenic and genetic characterization revealed that the H3N2 viruses were similar to the corresponding vaccine strain A/Panama/2007/99 (H3N2); the H1N1, and the hemagglutinin of the H1N2 viruses were similar to the A/New Caledonia/20/99(H1N1) vaccine strain. Of the 152 influenza B isolates, 147 (96.7%) were B/Hong Kong/330/01-like viruses belonging to the B/Victoria lineage. The remaining five influenza B isolates characterized (3.3%) belonged to the B/Yamagata lineage and were antigenically similar to the vaccine strain, B/Sichuan/379/99. 

Figure 3. Seasonal distribution of laboratory-confirmed influenza infections by influenza type, Canada, 1997-2002

Drug Susceptibility Tests 

A rapid assay has been established at the NML to test influenza A viruses for resistance to amantadine, an antiviral drug that is currently available in Canada for the prevention and treatment of influenza A infection⁽⁸⁾. The polymerase chain reaction-restriction analysis method was used to analyze 451 influenza A isolates collected in Canada during the 2001-2002 season. Twenty-four of these isolates were obtained from influenza outbreaks in nursing homes where amantadine was used for treatment. The other 427 isolates were collected from non-institutional patients (field isolates). The results showed that three (0.7%) of 427 influenza A field isolates received during the 2001-2002 season were amantadine resistant. Of the 24 isolates collected from nursing home outbreaks, seven (29%) were found to have drug-resistant mutations.

ILI Reported by Sentinel Physicians 

In Quebec, where there are 99 census divisions, representative recruitment was accomplished by coverage of health regions (n = 18) rather than by census division. A total of 26 sentinel physicians (1/250,000 population) were recruited in 12 (67%) of the 18 health regions in Quebec. In all other provinces and territories, representative recruitment was accomplished by coverage of census divisions: 226 sentinel physicians and sentinel clinics (1 per 150,000 population) were recruited in 131 (70%) of the 189 census divisions outside of Quebec. Overall, recruitment represented most of the well-populated urban and rural regions across Canada. 

Each week between late October and mid April, CIDPC received ILI data from an average of 64% (162/252) of FluWatch sentinels, including Quebec sentinels. This response rate was down from an average of 68% reporting each week in 1999-2000, but up from last season's response rate of only 50%. During the 2001-2002 season, 91% (230/252) of sentinels provided ILI data for at least 50% of the reporting weeks, and 64% (162/252) provided ILI data for at least 90% of the reporting weeks. Participation rates in 2001-2002 were considerably higher than during the previous seasons; in 2000-2001, 40% (91/231) of sentinels provided data for at least 50% of the reporting weeks, and only 4% (10/231) provided data for at least 90% of the reporting weeks. However, Quebec sentinels were not included in the 2000-2001 rates because sentinel participation occurred through an independent provincial system of Centres Locaux de Service Communautaires (CLSC) in that province. 

Over the 2001-2002 ILI surveillance period, 1.6% of patients (3,654/223,062) seen were given a diagnosis of ILI, with an overall ILI rate of 21 per 1,000 patients seen (as compared with 23/1,000 patients seen in 2000-2001 and 41 per 1,000 patients seen in 1999-2000). The highest rates of ILI were among children, at 30 cases of ILI per 1,000 patients seen in the 0 to 4-year age group and 35 per 1,000 patients seen in the 5 to 19-year age group. 

The peak in ILI reporting rates occurred during week 6 (week ending 9 February, 2001), at 58 per 1,000 patient visits. This rate is within the expected range for peak activity on the basis of data from the previous four seasons, but it represents a late peak,  since most seasons show peak activity in late December or early January. Furthermore, throughout most of the season, ILI rates remained well below the 1996-2001 mean rate (Figure 4). As in the 2000-2001 season, 2001-2002 peak activity was considerably lower and later than the 1999-2000 season peak of 149 cases per 1,000 during week 52. During week 6, in 2002 the age groups with the highest ILI rates were 5 to 19 years (155/1,000) and those > 65 years (42/1,000); patients aged 0 to 5 years had the lowest ILI rate (25/1,000). This age distribution differs from the previous season, when peak ILI rates were lower overall. During 2000-2001, although the highest ILI rate was also seen in 5 to 19 year olds (27/1,000), patients >= 65 years had the lowest rate (3/1,000) and those aged 0 to 5 years had the second highest rate (25/1,000). 

Figure 4. Census division weighted age-standardized ILI rates*, by influenza season and report week, Canada

Influenza Activity Level Assessment 

Ontario was the first province to report localized influenza activity, in the week ending 15 December, 2001 (week 50). The number of regions reporting localized activity increased very gradually over the following 10 weeks, with no widespread activity reported until week 9 (ending 2 March, 2002) in Prince Edward Island. After week 9, activity levels increased in most regions, and widespread activity was reported in Saskatchewan and Nova Scotia during weeks 11 through 13, and 12 through 17 respectively. The peak in activity level reporting occurred over a 5-week period, from week 10 to week 14, at which time 20 or more regions (up to 42% of the 53 influenza surveillance regions across Canada) reported localized or widespread activity each week (Figure 5).

Figure 5. Number of surveillance regions reporting widespread or localized influenza activity, by week and year, 6 October 2001 through 4 May 2002, Canada

Discussion 

Influenza activity in the Northern Hemisphere was initially reported during the 2nd week of October 2001, with increasing activity from November 2001 through March 2002. Worldwide, during the same period, influenza A (H3N2), A (H1N1), A (H1N2) and B viruses co-circulated. Influenza A viruses predominated in some countries and influenza B viruses in others. Countries in the Americas, Asia, Europe, and Oceania reported outbreaks of both influenza A (H3N2) and influenza B. Japan reported outbreaks of influenza A (H1N1) in late December, and A (H1N1) viruses were isolated sporadically in Asia, Europe, North America, and Oceania. 

In Canada, national surveillance data (weekly aggregate laboratory surveillance data and provincial and territorial activity level reporting) indicated that influenza activity reached a peak during the last 2 weeks of March 2002 (weeks 12 and 13). This is relatively late as compared with previous seasons - for example, during the previous two seasons peak activity occurred earlier, at the end of December (week 52) and the end of February (week 7). The season was also relatively mild, and only three provinces, Nova Scotia, Prince Edward Island and Saskatchewan, reported widespread influenza activity for >=1 week. At the same time, ILI activity remained within or below the mean weekly rates for the previous 5-year period (1996-2001) but peaked during the 2nd week of February (week 6), several weeks before the laboratory and activity level indicators. This earlier peak in ILI may be explained by co-circulation of RSV with influenza, contributing to increased clinical illness reporting. Nationally, RSV laboratory detections remained high (over 15% of total tests being positive) from the week ending 15 December, 2000, (week 50) through 23 February (week 8) with a peak in detections occurring during the week ending 5 January, 2001, (week 1) (<http://www.hc-sc.gc.ca/ pphb-dgspsp/bid-bmi/dsd-dsm/rvdi-divr/index.html>). Children had the highest ILI rates, a pattern comparable with previous years.

Influenza A predominated in the 2001-2002 season in all provinces and territories. In most regions, influenza B contributed very little to influenza activity, whereas in Ontario co-circulation of influenza A and influenza B occurred, influenza B contributing to an earlier peak in laboratory-confirmed influenza infections in mid-February (week 7), followed by a peak in influenza A infections 2 weeks later (week 9). Since 1993-1994, the first year of systematic weekly reporting through the RVD, there have been eight predominantly influenza A seasons (range 61.5% to 99.7% influenza A infections) and only one predominantly B season. Of eight influenza A seasons, only in 1995-1996 did the H1N1 subtype predominate (accounting for 91% of influenza A infections); in the remaining seven seasons, H3N2 subtypes predominated (range 94% to 100% of influenza A infections). 

A/Panama/2007/99 (H3N2)-like viruses accounted for most of the 2001-2002 season's influenza A isolates, the peak occurring from January to April. Similarly, most of B/Hong Kong/330/01-like viruses were received and identified from January to April, and an increasing number of the influenza A (H1N2) viruses were received and identified later in the season. The highest proportion of laboratory-confirmed cases was seen among children < 5 years and those > 65 years (28% and 27% respectively), which is the expected pattern for a predominantly A (H3N2) season. Nevertheless, caution should be used when interpreting age-specific data because of possible age-related biases in health care utilization and physician testing behaviour. 

Influenza A (H1N2) viruses have been identified in the past. Between December 1988 and March 1989, a number of these influenza A (H1N2) viruses were identified in China; however, the virus was not known to have spread further. The new H1N2 strain appears to have resulted from the reassortment of the genes of the co-circulating influenza A (H1N1) and A (H3N2) subtypes. The hemagglutinins of the new H1N2 viruses are antigenically and genetically similar to that of the A/New Caledonia/20/99 (H1N1) vaccine strain, while the neuraminidases are antigenically and genetically similar to that of the A/Panama/2007/99 (H3N2) vaccine strain. Therefore, the 2001-2002 influenza vaccines were expected to provide good protection against the new reassortant H1N2 viruses. Between September 2001 and February 2002, the reassortant influenza A (H1N2) viruses were also isolated in other countries, including Egypt, France, India, Israel, the United Kingdom, and the United States. 

Since the mid-1980s, influenza B viruses have evolved into two antigenically and genetically distinct lineages, represented by the reference strains B/Victoria/2/87 and B/Yamagata/16/88. The B component of the 2001-2002 influenza vaccine, B/Sichuan/379/99, belongs to the B/Yamagata lineage. Worldwide, viruses of both lineages have co-circulated. However, from 1992 to 2000, the B/Victoria-lineage viruses were virtually absent from North America. In March 2001, B/Victoria lineage viruses (the B/Hong Kong/330/01-like viruses) were identified for the first time in a decade in Canada, and these were followed by identifications in the United States and other countries, including Italy, Netherlands, Norway, Philippines, India, and Oman^(9,10). Phylogenetic analysis of the HA gene of 94 B/Victoria-like isolates by NML revealed two distinct clades, represented by the B/Hong Kong/330/01 reference strain and the previous reference strain B/Shandong/7/97. All the B/Shandong/7/97-like viruses were reassortants with a Victoria-lineage HA and a Yamagata-lineage NA. Given the re-emergence of the B/Victoria/2/87 lineage viruses worldwide, the lack of exposure of young children to these viruses in the past decade, and the expected poor coverage by the 2000-2001 B/Sichuan/ 379/99 vaccine strains, the WHO recommended the inclusion of a B/Victoria/2/87-like virus in the 2002-2003 Northern Hemisphere vaccines.

The FluWatch program provides an overall picture of influenza activity in Canada. Although each component of the program has its limitations, as a whole they complement each other. The main limitations include the following: (1) specimen collection and submission to the NML is subject to the individual practices of the attending physicians and the availability of the test within and among provinces/territories, (2) the background data used as a baseline for comparison of 2001-2002 ILI rates are based on only five previous seasons and are thus somewhat unstable as a result of the wide confidence intervals (the baseline will become more stable over time), (3) trends in ILI rates may also include reporting of acute respiratory illnesses not due to influenza and thus increases in other illnesses can mask the true seasonal trend of influenza, and (4) the activity level provided by the provincial/ territorial epidemiologists, although based on several standardized indicators, is somewhat subjective.

Acknowledgments 

We would like to thank the staff of the laboratories who participated in the Respiratory Virus Detection program during the 2001-2002 season and Donnie Bowness and Susan Normand of Influenza and Respiratory Viruses Section, National Microbiology Laboratory, for conducting influenza strain characterization and amantadine susceptibility testing. We also wish to thank all the physicians and nurse practitioners who contributed to the ILI surveillance program in association with the College of Family Physicians of Canada, NaReS, and the sentinel influenza surveillance programs in British Columbia, Alberta, and Saskatchewan. Finally, we wish to express our thanks to the provincial and territorial epidemiologists and FluWatch representatives for providing information about the influenza activity level in their jurisdictions. 

Laboratories wishing to participate in the FluWatch surveillance program should contact Mr. Peter Zabchuk, Division of Disease Surveillance, Centre for Infectious Disease Prevention and Control, at 613-952-9729. 

References 

1. Centre for Infectious Disease Prevention and Control. Influenza in Canada - 1999-2000 season. CCDR 2001;27:1-9. 

2. LCDC. Influenza in Canada - 1998-1999 season. CCDR 1999;25:185-92. 

3. LCDC. Influenza in Canada - 1997-1998 season. CCDR 1998;24:169-76. 

4. LCDC. Influenza in Canada - 1996-1997 season. CCDR 1997;23:185-92. 

5. LCDC. Influenza in Canada - 1995-1996 Season. CCDR 1996;22:193-99. 

6. LCDC. Influenza in Canada - 1994-1995 Season. CCDR 1995;21:205-12. 

7. LCDC. Influenza in Canada, 1993-1994 Season. CCDR 1994;20-21:185-91. 

8. Li Y. 1999-2000 influenza season: Canadian laboratory diagnoses and strain characterization. CCDR. 2000;26(22):185-89. 

9. Shaw MW, Xu X, Li Y et al. Reappearance and global spread of variants of influenza B/Victoria/2/87 lineage viruses in the 2000-2001 and 2001-2002 seasons. Virology 2002;303:1-8. 

10. CDC. Update: Influenza activity - United States and worldwide, June-September 2002. MMWR 2001;51(39):880-82. 

Source: JF Macey, MA, MSc, and TWS Tam, MD, FRCPC, Immunization and Respiratory Infections Division, CIDPC, Health Canada; Y Li, PhD, Influenza and Respiratory Virus Section, National Microbiology Laboratory, Public Health Agency of Canada, Health Canada, Winnipeg; B Winchester, MSc, Immunization and Respiratory Infections Division, CIDPC, Health Canada; and P Zabchuk, Division of Disease Surveillance, CIDPC, Health Canada. 

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