Alberta Health. Seasonal Influenza in Alberta Season. Analytics and Performance Reporting Branch

Transcription

1 Alberta Health Seasonal Influenza in Alberta Season Analytics and Performance Reporting Branch August 2016

2 For more information contact: Analytics and Performance Reporting Branch Health Standards, Quality, and Performance Division Alberta Health P O Box 1360 Stn Main Edmonton, AB T5J 2N3 Website: Health.Surveillance@gov.ab.ca Government of Alberta 2

3 Executive Summary The influenza season in Alberta was unusually delayed, starting in mid-december and peaking in mid-february. The epidemic spread slowly but steadily such that while this season had the largest number of lab-confirmed cases since the pandemic (and thus would be considered to have higher than usual activity), the health-care system did not experience the same types of pressures as the or seasons. Influenza A(H1N1)pdm09 was the predominant circulating strain, resulting in higher rates of illness in children, teenagers and working age adults, as well as more ICU-admissions. This season also featured significant influenza B activity; the majority of isolates were of the Victoria lineage, which was not in the trivalent vaccine. Influenza immunization coverage was 27 per cent. Introduction The influenza season in North America was delayed, peaking in early March in Canada and the United States The season was reported to be moderate in the United States, with fewer hospitalizations, deaths, and outpatient visits for influenza-like illness (ILI) than the previous three season 3. In Canada, different jurisdictions experienced different levels of activity. For instance, British Columbia experienced a mild season where influenza B predominated and peaked prior to influenza A(H1N1)pdm09 4, while Alberta had an unusually severe season with overlapping influenza A(H1N1)pdm09 and influenza B peaks. The predominant circulating strain in Canada was influenza A(H1N1)pdm09, however both lineages of influenza B co-circulated and there were a small number of influenza A(H3N2) cases. All influenza A isolates analyzed by the National Microbiology Laboratory (NML) were genetically or antigenically similar to the vaccine strains 5. Of the 1,249 influenza B isolates NML analyzed, 21 per cent were of the same lineage as the trivalent vaccine (Yamagata). The remaining 79 per cent were of the Victoria lineage, and thus mismatched to the trivalent vaccine. The components included in the northern hemisphere trivalent vaccine were: Influenza A/Switzerland/ /2013-like virus (H3N2), Influenza A/California/07/2009(H1N1)pdm09- like virus and Influenza B/Phuket/3073/2013-like virus (Yamagata lineage) 6. B/Brisbane/60/2008 was a component of the quadrivalent vaccine. Interim estimates of influenza vaccine effectiveness in the United States and Europe for influenza A(H1N1) ranged from 35 per cent to 44 per cent; Canadian interim estimates were 64 per cent (95% CI: per cent) This report describes the influenza season in Alberta. While influenza surveillance in Alberta occurs year round, this report includes surveillance of influenza activity from August 30, 2015 (Week 35) to June 11, 2016 (Week 23) (See Appendix 1 for weeks and date ranges for the season) Government of Alberta 3

4 Influenza Activity in Alberta The influenza season in Alberta was delayed, crossing the seasonal threshold in the second week of December (week 49) and peaking in the third week in February (week 7) (Figure 1), significantly later than recent years (Figure 2, 3, and 4). The epidemic curve rose gradually, resulting in a broad, sustained peak (Figure 5 and 6) with a large area under the curve, and thus, an unusually large number of cases. There were 5,311 lab-confirmed cases diagnosed in the season for a rate of per 100,000, the highest since the pandemic. In comparison, there were 4,862 cases diagnosed last season (rate per 100,000) and 3,913 diagnosed in the season (rate 97.6 per 100,000). The predominant circulating strain, with 61 per cent (n=3,230) of all of the lab-confirmed cases, was influenza A(H1N1)pdm09. However influenza B was also a significant contributor to influenza illness, accounting for 30 per cent of all lab-confirmed cases. The majority (83 per cent) of influenza B that circulated in Alberta was B/Brisbane/60/2008-like (of the B/Victoria lineage). This was a mismatch to the B/Phuket/3073/13-like strain (B/Yamagata lineage) of the 2015/16 vaccine strain in the 2015/16 vaccine 6. The remaining isolates were influenza A(H3N2) (n=159, 3 per cent) or were influenza A that were unable to be subtyped (n=321, 6 per cent). Figure 1: Laboratory-confirmed cases of influenza, by subtype and week of diagnosis, as compared to the five-year seasonal average*. Count *The average seasonal peak has been shifted to match this season s peak to aid comparisons. See Appendix 2 for data notes Government of Alberta 4

5 Figure 2: Laboratory-confirmed cases of influenza, by subtype, season and week of diagnosis*. * See Appendix 2 for data notes. Figure 3: Number of individuals diagnosed with influenza in general practitioner offices, by season. Count Source: Supplemental Enhanced Service Event (Physician Claims) 2016 Government of Alberta 5

6 Figure 4: Number of individuals diagnosed with influenza in emergency departments by season. Source: Supplemental Enhanced Service Event (Physician Claims) Figure 5: Per cent of patient visits due to influenza-like illness as reported by sentinel physicians, by week and season. Percent ILI Count Source: TARRANT 2016 Government of Alberta 6

7 Figure 6: Number of antiviral prescriptions dispensed by pharmacists, by week. Source: Pharmaceutical Information Network Alberta Health Service Zone This season, Calgary Zone had the highest number of reported cases (1,693) and North Zone had the most cases per capita with a rate of reported cases per 100,000 (Table 1). Every zone experienced higher rates of influenza this season compared to last season or the season before. The epidemic curves and relative proportions of subtypes by zone were similar to the overall pattern for Alberta. Table 1: Rate of laboratory-confirmed influenza by Zone (per 100,000 population) Count Rate Count Rate Count Rate Northern Zone Edmonton Zone 1, , , Central Zone Calgary Zone , , South Zone Alberta Overall 3, ,862* , * Includes two isolates where zone is unavailable. Source: Communicable Disease reporting System (CDRS), Alberta Health 2016 Government of Alberta 7

8 Age The rate of illness was high in children, low in seniors and the elderly, and slightly elevated in the younger age groups, this is the expected pattern for an influenza A(H1N1) predominate season. The rate was highest in babies under one year old (410.5 per 100,000), followed by children between the ages of 1-4 years (277.0 per 100,000) and children age 5-9 (184.3 per 100,000) (Figure 7, Table 2). The rate in the elderly was six times lower than last season (an Influenza A(H3N2)-predominant season), which was characterized by low vaccine effectiveness and high numbers of outbreaks in long-term care facilities. Rates were similar to the season. Figure 7: Rate of laboratory-confirmed influenza infections by age and season (per 100,000). Rate Table 2: Rate of laboratory-confirmed influenza infections by age and season (per 100,000) Count Rate Count Rate Count Rate 0-11 months years years years years years years years 1, , years years years ,317 1, Government of Alberta 8

9 Outbreaks Influenza outbreaks that occur in group settings such as hospitals, residential institutions, schools, and child care facilities are reported to Alberta Health. There were 59 influenza outbreaks reported to Alberta Health this season, four times fewer than the 243 influenza outbreaks reported in the H3N2-predominant season, but only slightly more than the previous H1N1-predominant season in (Figure 8, Table 3). The high number of outbreaks last season was likely due to the poor effectiveness of the influenza vaccine against the predominant circulating H3N2 strain. Figure 8: Number of lab-confirmed influenza outbreaks reported by week. Count 2016 Government of Alberta 9

10 Table 3: Count of outbreaks by setting and season Count Per cent Count Per cent Count Per cent Acute Care Facility 11 21% 20 8% 11 19% Camp 1 2% 0 0 Child Care Facility 5 10% 0 1 2% Correctional Facility 6 12% 5 2% 4 7% Group Home % 0 Long Term Care 20 38% % 21 36% Other, Specify % School (K-12) 2 4% 13 5% 5 9% Shelter % 0 Supportive Living/Home Living Sites 7 13% 90 37% 15 26% Total Hospitalized Cases There were 1,698 hospitalizations, 271 ICU admissions and 53 fatalities due to influenza this season, for a rate of 41.2 per 100,000, 6.6 per 100,000 and 1.3 per 100,000 respectively (Table 4). The rate of hospitalizations was highest in children under the age of two (Figure 9). Compared to last season, the rate of hospitalizations and fatalities was lower, but the rate of ICU admissions was higher (Table 4, Figure 10 and 11). While the rate of ICU admissions was the highest this season since the pandemic, the broad, sustained peak of the influenza outbreak prevented the types of utilization pressures seen in the ICU in the season (Figure 11). The difference in the rates of fatalities, ICU admissions, and hospitalizations is likely due to the different subtypes of influenza that circulated during the two seasons. H3N2 viruses are known to cause increased severity of disease in the elderly 11, while the elderly have residual immunity as H1N1 influenza viruses were the predominant circulating strains between 1918 and the mid-1950s 12. Younger individuals are more likely to be sickened by influenza A(H1N1); those that become ill enough to be hospitalized are more likely to be admitted to ICU and then recover, while elderly individuals are more likely to pass away and less likely to be admitted to ICU. This can be seen in difference in age distribution of cases between the two seasons (Figure 10); the median age of hospitalized cases this season was 51.0 as compared to 74.0 in the season (Table 5) Government of Alberta 10

11 Table 4: Rate (95% CI) of lab-confirmed influenza hospitalizations, ICU admissions, and hospitalized deaths by season (per 100,000). Rate (95% CI) Hospitalizations 30.4 ( )* 47.0 ( )* 41.2 ( ) ICU admissions 5.6 ( ) 4.1 ( )* 6.6 ( ) Deaths (in hospitalized cases) 0.7 ( ) 2.3 ( )* 1.3 ( ) *Statistically significant different than (p< 0.05). Fisher s exact test utilized where appropriate. Figure 9. Rate of hospitalized influenza cases by age group and season (per 100,000) Government of Alberta 11

12 Figure 10: Number of influenza fatalities by season. Figure 11: Number of ICU-admitted influenza cases, by week and season Government of Alberta 12

13 Table 5: Characteristics of hospitalized influenza cases, by season No. Hospitalizations 1,220 1,936 1,698 Median age (IQR) 51.0 ( ) 74.0 ( )* 51.0 ( ) No. cases with one or more chronic conditions (per cent) 982 (80%)* 1,720 (89%)* 1,181 (70%) No. ICU Admissions in Hospitalized Cases (per cent) 222 (17%) 168 (9%)* 271 (16%) Median age (IQR) 53.0 ( ) 65.0 ( )* 51.0 ( ) No. cases with one or more chronic conditions (per cent) 201 (90%)* 153 (91%)* 205 (76%) No. Deaths in Hospitalized Cases (per cent) 30 (3%) 95 (5%) 53 (3%) Median age (IQR) 59.0 ( ) 86.0 ( )* 58.0 ( ) No. cases with one or more chronic conditions (per cent) 30 (100%) 93 (98%) 47 (89%) *Statistically significant different than (p< 0.05). Fisher s exact test utilized where appropriate. Antiviral Resistance As of July 28, 2016, the NML had tested 2,126 influenza viruses (249 from Alberta) for resistance to the antiviral drugs oseltamivir, zanamivir, and amantadine 13,14,15. Similar to last season, all influenza isolates were sensitive to zanamivir, 99 per cent of isolates were sensitive to oseltamivir, and all influenza A isolates were resistant to amantadine, except one isolate from Alberta. Ten isolates were resistant to oseltamivir; of these, one was isolated in Alberta. Influenza Immunization The seasonal influenza vaccination program is universal in Alberta. 1,146,569 influenza immunizations were given to Albertans in the season; influenza immunization coverage is 27 per cent (Table 6). Coverage is highest for children under the age of 2 (60 per cent), seniors (52 per cent) and the elderly age 80 and over (60 per cent). The majority of influenza immunizations were given by pharmacists (41 per cent) and AHS public health (39 per cent) (Figures 12 and 13). Conclusion The influenza season had a delayed start with slow, steady spread, resulting in a larger than usual number of lab-confirmed cases, but with fewer pressures on the health-care system. Influenza A(H1N1)pdm09 was the predominant strain, resulting in higher rates of illness in children and working age adults, and higher rates of ICU-admissions but lower fatalities due to influenza as compared to the season Government of Alberta 13

14 Table 6: Doses of influenza vaccine administered and coverage, by season. Season Number of Immunizations Percent of Albertans Immunized 2011/12 874,046 23% 2012/13 919,348 24% 2013/14 1,157,550 29% 2014/15 1,254,950 30% 2015/16 1,146,569 27% Figure 12. Number of dose of influenza vaccine administered by provider and season. Figure 13. Proportion of influenza immunizations administered by provider in the season Government of Alberta 14

15 Acknowledgements The data presented in this report is from the Provincial Laboratory for Public Health (ProvLab), Alberta s influenza like illness (ILI) sentinel physician system (TARRANT), the National Microbiology Laboratory (NML), Supplemental Enhanced Service Event (SESE) physician claims data, the Pharmacy Information Network (PIN), as well as outbreak reports and hospitalized case report forms from Alberta Health s Communicable Disease Reporting System (CDRS). We would like to thank ProvLab, Alberta Health Services (AHS), First Nations Inuit Health Branch (FNIHB), the National Microbiology Laboratory (NML), and TARRANT sentinel physician system for their partnership in influenza surveillance in Alberta Government of Alberta 15

16 Appendix 1: 2015/16 Influenza Season Reporting Weeks Week Start End Aug-15 5-Sep Sep Sep Sep Sep Sep Sep Sep-15 3-Oct Oct Oct Oct Oct Oct Oct Oct Oct Nov-15 7-Nov Nov Nov Nov Nov Nov Nov Nov-15 5-Dec Dec Dec Dec Dec Dec Dec Dec-15 2-Jan Jan-16 9-Jan Jan Jan Jan Jan Jan Jan Jan-16 6-Feb Feb Feb Feb Feb Feb Feb Feb-16 5-Mar Mar Mar Mar Mar Mar Mar Mar-16 2-Apr Apr-16 9-Apr Apr Apr Apr Apr Apr Apr May-16 7-May May May May May-16 Week Start End May May May-16 4-Jun Jun Jun Jun Jun Jun Jun Jun-16 2-Jul Jul-16 9-Jul Jul Jul Jul Jul Jul Jul Jul-16 6-Aug Aug Aug Aug Aug Aug Aug Government of Alberta 16

17 Alberta Health, Surveillance and Assessment Seasonal Influenza in Alberta, Summary Report August 2016 Appendix 2: Data Notes Defining Baselines and Thresholds for Lab-Confirmed Influenza Surveillance An important function of influenza surveillance is to determine whether the timing or magnitude of the influenza season is unusual compared to previous seasons. The World Health Organization recommends comparing current-season influenza activity to the average epidemic curve, and to create two thresholds one to determine when the influenza season has begun (seasonal threshold) and to determine at what point current influenza activity would be unusually high (alert threshold) 1. The average epidemic curve for lab-confirmed influenza surveillance was estimated using data collected between the 2010/11 and 2014/15 seasons. Lab-confirmed influenza became routinely reported in 2009; however, the 2009/10 pandemic season was excluded. The peaks of each season were aligned, and the average number of cases reported per aligned week was calculated as well as the 90 per cent confidence limit. The 90 per cent confidence limit acts as the Alert Threshold. If the number of cases reported in a week passes this threshold it is considered to be unusually high. Until the current season has peaked, the best estimate of the peak is utilized to align the current season to the average epidemic curve. The first estimate utilized was the median of the previous five seasons, week 2. The seasonal threshold was estimated as the average number of cases reported per week, in weeks considered outside of the influenza season (i.e. pre-season weeks). The start of the influenza epidemic was defined as the week in which the percent positivity of influenza A laboratory tests at ProvLab were 10 per cent of all respiratory laboratory tests ordered that week 2. The end of the influenza epidemic was defined as the week in which the percent positivity of influenza B was < 10 per cent of all respiratory laboratory tests ordered that week 2. All weeks in between these time periods were considered part of the influenza epidemic. Percent positivity is an accepted method of determining the influenza season 1, however visual inspection was also utilized to ensure face validity. Figures 1 & 2: Laboratory-confirmed cases of influenza, by subtype and week of diagnosis. To ensure consistency in graphing between seasons, cases diagnosed in week 53 in the season (December 28, 2014 January 3, 2015) were placed into week 52 or week 1. This makes the peak of the influenza season appear to be week 52, although the number of lab-confirmed cases was slightly higher in week 51 than week World Health Organization 2013 Global Epidemiological Surveillance Standards for Influenza. Geneva. 2 Provincial Laboratory for Public Health (ProvLab) Weekly Respiratory Summary 2016 Government of Alberta 18

18 References 1 Public Health Agency of Canada (PHAC), FluWatch Report, Weeks (May 22 to June 18, 2016). Accessed August 4, surveillance-influenza/alt/pub-eng.pdf 2 Public Health Agency of Canada (PHAC), FluWatch Report, Week 6 (February 7 to February 13, 2016). Accessed August 4, surveillance-influenza/alt/fluwatch surveillance-influenza-eng.pdf 3 Davlin, S.L. et al Influenza Activity United States, Season and Composition of the Influenza Vaccine. MMWR 65(22): British Columbia Centre for Disease Control (BCCDC) British Columbia Influenza Surveillance Bulletin Influenza Season , Number 20, Weeks 17-19, April 24 to May 14, National Medical Laboratory (NML), 2016 NML Strain Characterization Completed on Influenza Isolates in Canada September 1, 2015 to July 28, Influenza and Respiratory Viruses Section, National Medical Laboratory, Public Health Agency of Canada. 6 WHO Recommended Composition of Influenza Virus Vaccines for Use in the Northern Hemisphere Influenza Season. Accessed August 4, Chambers, C. et al Interim estimates of 2015/16 vaccine effectiveness against influenza A(H1N1)pdm09, Canada, February Euro Surveill. 2016;21(11):pii= DOI: 8 Kissling, E. and M. Valenciano Early influenza vaccine effectiveness results : I-MOVE multicenter casecontrol study. Euro Surveill. 2016;21(6):pii= DOI: 9 Emberg, H. et al Influenza vaccine effectiveness in adults 65 years and older, Denmark, 2015/16 a rapid epidemiological and virological assessment. t. Euro Surveill. 2016;21(14):pii= DOI: 10 Pebody R. et al Effectiveness of seasonal influenza vaccine in preventing laboratory-confirmed influenza in primary care in the United Kingdom: 2015/16 mid-season results. Surveill. 2016;21(13):pii= DOI: ES Widdowson M. A. and Monto A. S Epidemiology of Influenza Textbook of Influenza, 2 nd edition. Editors: Webster et al. John Wiley & Sons, UK. 12 Monto, A.S. et al.influenza pandemics: History and lessons learned. Textbook of Influenza, 2 nd edition Editors: Webster et al. John Wiley & Sons, UK. 13 National Medical Laboratory (NML), 2016 NML Oseltamivir Susceptibility Assay Completed on Influenza Isolates in Canada September 1, 2015 to July 28, Influenza and Respiratory Viruses Section, National Medical Laboratory, Public Health Agency of Canada. 14 National Medical Laboratory (NML), 2016 NML Zanamivir Susceptibility Assay Completed on Influenza Isolates in Canada September 1, 205 to July 28, Influenza and Respiratory Viruses Section, National Medical Laboratory, Public Health Agency of Canada. 15 National Medical Laboratory (NML), 2016 NML Amantadine Susceptibility Assay Completed on Influenza Isolates in Canada September 1, 2015 to July 31, Influenza and Respiratory Viruses Section, National Medical Laboratory, Public Health Agency of Canada Government of Alberta 19