Volume 25-22
15 November 1999

[Table of Contents]

INFLUENZA IN CANADA - 1998-1999 SEASON

Introduction

The Laboratory Centre for Disease Control (LCDC) maintains a national influenza surveillance program, FluWatch. The objective of FluWatch is to provide a national picture of influenza activity across Canada during the influenza season. This program has three main elements which include (1) laboratory-based influenza virus identification, (2) influenza-like illness (ILI) surveillance, and (3) reporting of influenza activity level by provincial and territorial epidemiologists. In addition, international reporting on influenza activity, by the United States Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), is reviewed weekly and reported through FluWatch.

A number of mechanisms were used to disseminate information on influenza activity to public-health professionals and the public, including the LCDC FAXlink (dial 613-941-3900 from a telephone-equipped fax machine), fax, electronic mail, and the Division of Disease Surveillance, LCDC, Website <http://www.hc-sc.gc.ca/hpb/lcdc/bid/dsd/index.html>. FluWatch reports were made available weekly or biweekly during the influenza season and summaries of laboratory surveillance data were made available weekly throughout the year. Summaries of influenza activity worldwide were included periodically in the weekly Infectious Diseases News Brief which was also available on the Division of Disease Surveillance Website. Surveillance reports on influenza virus activity were published periodically in the Canada Communicable Disease Report.

This report summarizes case-by-case data on laboratory-confirmed influenza infection, reports of ILI for the 1998-1999 season, and influenza activity level reporting by provinces and territories. Comparison is made with previous seasons^(1-5).

Methods

Laboratory-confirmed influenza: Laboratories participating in the case-by-case surveillance program were asked to report the numbers of isolations and identifications made by direct antigen detection and seroconversion, i.e. > fourfold rise in antibody titre by any method, to LCDC. Laboratory-confirmed case-by-case data were presented by the province from which the specimen originated (some laboratories received out-of-province samples), and were analyzed by week of onset of illness and the age of the case.

Influenza-like illness reported by sentinel physicians: The College of Family Physicians of Canada (CFPC) National Research System (NaReS) was responsible for much of the recruitment of sentinel physicians. The objective was to recruit at least one physician from each of the census divisions across Canada. The exceptions were in British Columbia, the greater Calgary area and Edmonton, Saskatchewan, and Newfoundland, where sentinel physicians were already involved in local surveillance programs. For one clinic day per week, between 30 September 1998 to 20 April 1999, physicians were asked to complete a report form with the number of patients seen and the number of patients meeting a standard definition for ILI. The case definition for ILI was "acute febrile respiratory illness (fever and/or chills) characterized by one or more of the following: cough, sore throat, arthralgia, myalgia, or prostration which in the opinion of the attending physician could be due to influenza virus." Both groups of patients were broken down by age category. Report forms were either faxed, or the information was conveyed via electronic mail or by telephone to LCDC on a weekly basis. LCDC would then collate the data and prepare a report which was distributed once every 2 weeks, or weekly when influenza activity was considered to be high, to participating physicians and provincial, territorial, federal, and international health authorities.

Influenza activity assessed by the provincial and territorial epidemiologists: On a weekly basis, provincial and territorial epidemiologists or influenza surveillance representatives assessed the influenza activity level in their respective jurisdictions using a variety of sources of information which may have included laboratory confirmation of influenza, sentinel physician ILI surveillance, reports of outbreaks, school and work-site absenteeism, and emergency department and hospital admission data. Most provinces and territories were subdivided into influenza surveillance regions as defined by the provincial or territorial epidemiologist. For the 1998-1999 influenza season, there were a total of 46 surveillance regions: British Columbia (4), Alberta (3), Saskatchewan (3), Manitoba (9), Ontario (5), Quebec (1), New Brunswick (7), Nova Scotia (4), Prince Edward Island (1), Newfoundland (6), Yukon (1), and Northwest Territories (2). Influenza activity level were defined as: no activity reported, sporadic, localized, and widespread activity.*

Results

Laboratory-confirmed influenza: During the 1998-1999 laboratory surveillance period (1 September 1998 to 4 June 1999), a total of 4,203 case-by-case records were reported to LCDC by 16 laboratories in nine provinces (Table 1). This compared with 3,802 cases reported by 15 laboratories in eight provinces for the previous season (1997-1998). The variation in numbers of confirmed cases and distribution of virus type and subtype among provinces should be interpreted with caution; these numbers are likely to reflect differences in population size and distribution, reporting practices and criteria, and the availability of diagnostic services.

Table 1 Laboratory-confirmed cases of influenza reported to LCDC, by laboratory, Canada, 1998-1999

Table 2 shows laboratory-confirmed case-by-case data, by province and influenza type and subtype. The largest number and proportion of cases were recorded in Ontario, 1,329 cases (32%); followed by Quebec, 927 cases (22%); Alberta, 850 cases (20%); and Saskatchewan, 513 cases (12%). The majority of isolates, 3,622 (86%), were of type A virus, and 581 (14%), were of type B. These results represent an increase in the reporting of influenza B virus infections when compared with the previous season⁽⁵⁾. Ontario reported the greatest number of influenza B isolates. Of the 3,622 influenza A virus identifications, 446 were further characterized; 443 were of the H3N2 subtype and three were of the H1N1 subtype.

Table 2 Laboratory-confirmed cases of influenza, by province and influenza type and subtype, Canada, 1998-1999

Figure 1 shows laboratory-confirmed case-by-case data, by type and week of onset, for Canada and the five regions: Atlantic (Newfoundland, Prince Edward Island, Nova Scotia, New Brunswick), Quebec, Ontario, the Prairies (Manitoba, Saskatchewan, Alberta), and British Columbia. Although confirmed cases were reported earlier in the Prairies, most of the cases in Canada were reported between late January to mid-March. Marked peaks in influenza A laboratory-confirmed cases were evident in the Prairies, Ontario, and Quebec.

Figure 1 Laboratory-confirmed cases of Influenza by region, type and week of onset, Canada, 1998-1999

Figure 2 shows the proportionate distribution of laboratory-confirmed case-by-case infections, by age group, reported to LCDC. During the 1998-1999 season, most were recorded in persons aged >= 65 years (42%) and in children < 5 years of age (19%). This is similar to the 1997-1998 season⁽⁵⁾.

Figure 2 Proportionate distribution of laboratory-confirmed cases of influenza, by age group, Canada, 1998-1999

Laboratory confirmations: Virus isolation, 2,771 reports (66%), and direct antigen detection, 1,248 reports (30%), were the most commonly recorded methods for laboratory confirmation of case-by-case influenza infection. This represented an increase of confirmation by virus isolation compared to the previous season⁽⁵⁾. The remaining cases, 184 reports (4%), for which information was available, were confirmed by serology.

Types of influenza virus circulating during the 1998-1999 season: Figure 3 compares the seasonal distribution of laboratory-confirmed case-by-case influenza infections for the 1998-1999 season with the previous four seasons. Only one period of peak activity was observed for the most recent influenza season and, as already mentioned, the majority of isolates were of type A.

Strain characterization by the National Laboratory for Respiratory Viruses, Bureau of Microbiology, LCDC, was completed on 263 influenza A isolates and 108 influenza B isolates. Of 263 influenza A, 262 were identified as A/Sydney/5/97(H3N2)-like and one as A/Johannesburg/82/96(H1N1)-like. All of 108 influenza B isolates were characterized as B/Beijing/184/93-like. The provincial distribution of the A/Sydney/5/97(H3N2)-like isolates was British Columbia (14), Alberta (25), Saskatchewan (31), Manitoba (9), Ontario (120), Quebec (45), New Brunswick (9), Nova Scotia (6), and Newfoundland (3). The one A/Johannesburg/82/96(H1N1)-like isolate was from Ontario. The provincial distribution of the B/Beijing/184/93-like isolates was British Columbia (2), Alberta (1), Saskatchewan (16), Ontario (76), Quebec (6), and New Brunswick (7).

Figure 3 Seasonal distribution of laboratory-confirmed influenza infections, Canada, 1994-1999

Influenza-like illness reported by sentinel physicians: One hundred and eighty-six sentinel physicians were recruited in 113 of the 288 census divisions across Canada; most of the well-populated urban and rural divisions were represented, with the exception of those in Quebec. However, the distribution of sentinel physicians across the country was not uniform; some provinces were underrepresented (e.g. Quebec and Ontario). The physician response rate in 1998-1999 was very good. All 186 physicians submitted at least one report. For all physicians, the mean response rate was 75% from mid-October to the end of the third week of April. The individual weekly response rate for the season was > 66% for 140 physicians, and 100% for nine physicians (across the country).

Figure 4 shows the standardized rates of ILI across Canada by reporting week, from 30 September 1998 to 20 April 1999. The curve obtained was smoothed using the technique of Hamming and Tukey⁽⁶⁾. The ILI rates increased in early January (reporting week 1, 1999),  remained elevated for 2 months and then decreased in early March (reporting week 10, 1999). Over the ILI surveillance period, a total of 5,065 cases of ILI were diagnosed from 152,611 patients seen - an ILI rate of 33 per 1,000 patients seen, compared to a rate of 56 per 1,000 patients seen in 1997-1998. The highest rates of ILI were in the 5- to 9-year-old age group (56 per 1,000 patients seen) and the 0- to 4-year-old age group (53 per 1,000 patients seen).

Figure 4 Influenza-like illness, Canada, weekly, standardized reporting rates, 1997-1998 and 1998-1999

Influenza activity level assessment: Figure 5 shows the number of influenza surveillance regions reporting widespread or localized influenza activity by week and year from 20 October 1998 through 27 April 1999. Manitoba was the first province to report regional influenza activity in the week ending 17 November 1998 (week 46). Widespread activity was first reported in the first week of January 1999. Each week, during the weeks ending 12 January to 16 March 1999, influenza activity was reported as widespread or localized in >= 20% of the 46 influenza surveillance regions.

Figure 5 Number of influenza surveillance regions reporting widespread or localized influenza activity, Canada, by week and year, 20 October 1998 through 27 April 1999

Discussion

In the 1998-1999 influenza season, the Prairie provinces were the first to report increased laboratory isolates and reports of regional influenza activity. Nationally, there was a peak in the reporting of laboratory-confirmed cases from mid-January to early March, and increased rates of ILI were spread over a 2-month period, beginning in early January. Overall, influenza activity in 1998-1999 season was lower than in 1997-1998.

During the 1997-1998 and the 1998-1999 influenza seasons, the numbers of laboratory-confirmed case-by-case influenza infections reported to LCDC were higher than for any influenza seasons in the period 1978 to 1997^(1-5,7-9). This increase in cases may be partly explained by the increase in influenza surveillance activities and a small increase in the number of reporting laboratories, but likely also represented an increase in influenza activity.

For the 1998-1999 season, 86% of the isolates were influenza A; A/Sydney/5/97(H3N2)-like predominated and represented 71% of influenza strains characterized, and the remaining isolates were nearly all B/Beijing/184/93-like. During the 1997-1998 season, 99% of the influenza isolates submitted to LCDC were type A; A/Sydney/5/97(H3N2)-like viruses, isolated for the first time in Canada, represented 82% of influenza strains characterized. The trends observed in Canada were generally similar to those in the United States⁽¹⁰⁾.

The FluWatch program likely provides a good overall picture of influenza activity in Canada. While each component of the program has its limitations, they appear to complement each other well. The main limitations were (1) specimen collection and submission to the national laboratory were subject to the individual practices of the attending physicians and the availability of the test within and among provinces and territories, (2) the distribution of the sentinel physicians did not, in many instances, correlate with the population distribution, and (3) the activity level provided by the provincial or territorial epidemiologists, although based on many indicators, is somewhat subjective.

Influenza surveillance activities are critical to ensure early warning of epidemics and pandemics, identify the circulating influenza virus types and strains, monitor disease spread, and evaluate the control programs and interventions. A solid surveillance infrastructure should be in place during the interpandemic period, together with contingency plans for rapid expansion of surveillance activities in the event of a novel virus or a pandemic alert. The Canadian Contingency Plan for Pandemic Influenza was first drafted in 1988 and then revised in 1996. A federal/provincial/territorial working group has been meeting under the direction of the deputy ministers to further develop this plan. This working group is currently developing a memorandum of understanding to Cabinet, which will focus on (1) outlining the respective roles and responsibilities of the federal, provincial, and territorial governments, and (2) developing preferred options for vaccine and antiviral drug supply⁽¹¹⁾.

Laboratories wishing to participate in the FluWatch surveillance program should contact Mr. Peter Zabchuk, Division of Disease Surveillance, Bureau of Infectious Diseases, LCDC, at 613-952-9729.

Acknowledgments

We would like to thank the staff of the laboratories who participated in the respiratory virus surveillance program during the 1998-1999 season, and Dr. Yan Li and Ms. Carol Stansfield of the National Laboratory for Respiratory Viruses, Bureau of Microbiology, LCDC, for information regarding influenza virus strain characterization. We also wish to thank all the physicians who contributed to the ILI surveillance program in association with the CFPC, NaReS, and the sentinel influenza surveillance programs in Newfoundland, British Columbia, Saskatchewan, the Calgary area, and Edmonton, Alberta. Finally, we wish to express our thanks to the provincial and territorial epidemiologists for providing information about the influenza activity level in their jurisdictions.

References

1. LCDC. Influenza in Canada, 1993-1994 season. CCDR 1994;20:185-92.

2. LCDC. Influenza in Canada, 1994-1995 season. CCDR 1995;21:205-11.

3. LCDC. Influenza in Canada, 1995-1996 season. CCDR 1996;22:193-99.

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

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

6. Hamming RW, Tukey JW. Measuring color noise. Murray Hill, NJ: Bell Telephone Laboratory, 1949.

7. LCDC. Influenza in Canada, 1992-1993 season. CCDR 1993;19:152-57.

8. LCDC. Influenza in Canada, 1990-1991 and 1991-1992 seasons. CCDR 1992;18:137-41.

9. LCDC. Influenza in Canada, 1989-1990 season. CDWR 1990;16:185-89.

10. CDC. Update: influenza activity - United States and worldwide, 1998-99 season, and composition of the 1998-99 influenza vaccine. MMWR 1999;48:374-78.

11. LCDC. Report on the national influenza surveillance meeting. Ottawa; May 11-12, 1999.

Source: L Pelletier, MD, MPH, Chief, Division of Respiratory Diseases; P Buck, DVM, MSc, Field Epidemiologist, P Zabchuk, B Winchester, MSc, Division of Disease Surveillance, Bureau of Infectious Diseases; T Tam, MD, Acting Chief, Division of Respiratory Diseases, LCDC, Ottawa, Ont.

*For the 1998-1999 FluWatch program, activity levels were defined as:

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).

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