Influenza immunization timing

Influenza immunization timing Technical report March 2017

Public Health Ontario Public Health Ontario is a Crown corporation dedicated to protecting and promoting the health of all Ontarians and reducing inequities in health. Public Health Ontario links public health practitioners, frontline health workers and researchers to the best scientific intelligence and knowledge from around the world. Public Health Ontario provides expert scientific and technical support to government, local public health units and health care providers relating to the following: communicable and infectious diseases infection prevention and control environmental and occupational health emergency preparedness health promotion, chronic disease and injury prevention public health laboratory services Public Health Ontario's work also includes surveillance, epidemiology, research, professional development and knowledge services. For more information, visit www.publichealthontario.ca How to cite this document: Ontario Agency for Health Protection and Promotion (Public Health Ontario). Influenza immunization timing. Toronto, ON: Queen's Printer for Ontario; 2017. ISBN 978-1-4606-9162-5 [PDF] ISBN 978-1-4606-9161-8 [Print] Public Health Ontario acknowledges the financial support of the Ontario Government. Queen s Printer for Ontario, 2017 Technical report: Influenza immunization timing ii

Authors Hannah Chung Epidemiologist Institute for Clinical Evaluative Sciences Dr. Jeff Kwong Senior Scientist Institute for Clinical Evaluative Sciences and Public Health Ontario Christina Renda Health Analyst Communicable Diseases Public Health Ontario Dr. Bryna Warshawsky Public Health Physician Communicable Disease Prevention and Control Public Health Ontario Technical report: Influenza immunization timing iii

Acknowledgements We gratefully acknowledge the following individuals who provided very helpful and informative advice and support: Dr. Shelly Bolotin, Scientist, Applied Immunization Research, Public Health Ontario Dr. Natasha Crowcroft, Chief, Applied Immunization Research, Public Health Ontario Dr. Ian Gemmill, Medical Officer of Health, Kingston, Frontenac, Lennox & Addington Public Health Dr. Jonathan Gubbay, Medical Microbiologist, Public Health Ontario Laboratories Karin Hohenadel, Senior Program Specialist, Communicable Diseases, Public Health Ontario Emily Karas, Manager, Communicable Disease, Public Health Ontario Dr. Jeff Kwong, Public Health Ontario and Institute for Clinical Evaluative Science Dr. Allison McGeer, Director of Infection Control, Mount Sinai Hospital Stacy Recalla, Nurse Consultant, Ministry of Health and Long-Term Care Lauren Ramsay, Research Assistant, Public Health Ontario Christina Renda, Health Analyst, Public Health Ontario Dr. Beate Sander, Scientist, Public Health Ontario Dr. Doug Sider, Medical Director, Communicable Disease Prevention and Control, Public Health Ontario Dr. Rob Stirling, Senior Medical Advisor, Public Health Agency of Canada Dr. Bryna Warshawsky, Public Health Physician, Public Health Ontario - Chair Anne-Luise Winter, Epidemiologist Specialist, Public Health Ontario Dr. Barbara Yaffe, Director, Communicable Disease Control & Associate Medical Officer of Health, Toronto Public Health A special thank you to the external reviewers who dedicated their time and expertise: Dr. Rodica Gilca, Médecin-conseil, Institut national de santé publique du Québec Dr Helen K Green, Specialty Registrar in Public Health, England, formerly Influenza Epidemiologist, Public Health England Technical report: Influenza immunization timing iv

Dr. Judy MacDonald, Medical Officer of Health - Calgary Dr. Susan Tamblyn, former Medical Officer of Health, Perth District Health Unit The assistance of Matt Root, Public Health Ontario for his administrative support is also greatly appreciated. Disclaimer This document was developed by Public Health Ontario (PHO). PHO provides scientific and technical advice to Ontario s government, public health organizations and health care providers. PHO s work is guided by the current best available evidence. PHO assumes no responsibility for the results of the use of this document by anyone. This document may be reproduced without permission for non-commercial purposes only and provided that appropriate credit is given to Public Health Ontario. No changes and/or modifications may be made to this document without explicit written permission from Public Health Ontario. Technical report: Influenza immunization timing v

Contents Executive summary... 1 Purpose and methods... 1 Results... 1 Implications of the findings... 1 Influenza immunization timing... 2 Introduction... 2 Methods... 2 Results... 4 Discussion... 5 Conclusions... 7 Reference... 16 Technical report: Influenza immunization timing 1

Executive summary Purpose and methods This technical report assesses the timing of influenza immunization to determine whether delayed immunization negatively impacts the effectiveness of influenza immunization programs. The primary analysis assesses the timing of influenza immunization in relation to the circulation of influenza over five influenza seasons (2010-2011 to 2014-2015). Information on influenza circulation was obtained from influenza-confirmed specimens tested at the 16 Ontario laboratories that report to the Public Health Agency of Canada. Information on the timing of influenza vaccination for physicians and pharmacists was obtained based on administrative billing data held by the Institute for Clinical Evaluative Sciences (ICES). A secondary analysis outlines the missed opportunities resulting from delayed immunization, using composite data from the five influenza seasons. This information can be used to determine a target date for completion of influenza immunization. To maximize the impact of influenza immunization, immunization could be promoted to take place prior to that target date. Results The primary analysis found that in the four influenza seasons that started in mid-november and early December (2010-2011, 2012-2013, 2013-2014 and 2014-2015) between 21% and 52% of influenza vaccinations were given too late to achieve maximum benefit. In 2011-2012, the influenza season began in late January to mid-february (depending on the definition used), by which time the vast majority of people had been vaccinated. The secondary analysis identified potential missed opportunities to prevent influenza due to delayed vaccination for each week of a composite influenza season; the results can be found in Table 2. As an example, immunization after the first week in December occurs too late to potentially prevent an estimated 10% of cases in an average influenza season. Implications of the findings Late vaccination results in missed opportunities to prevent influenza infections and can decrease the potential impact of influenza immunization programs. Health care providers should consider strategies to improve the timeliness of influenza vaccination. Technical report: Influenza immunization timing ǀ 1

Influenza immunization timing Introduction This technical report assesses the timing of influenza immunization to determine whether delayed immunization negatively impacts influenza immunization programs. The primary analysis assesses the timing of influenza immunization in relation to the circulation of influenza over five influenza seasons (2010-2011 to 2014-2015). The timing of immunization provision by physicians offices and pharmacists by age of recipient is also reviewed. A secondary analysis outlines the missed opportunities that result from delayed immunization, using amalgamated data from the five previously indicated influenza seasons. This information can be used to determine a target date for completion of influenza immunization. To maximize the impact of influenza immunization, immunization could be promoted to take place prior to that target date. Methods The primary analysis involves determining the percent of individuals who were immunized by physicians offices or pharmacists before the start and peak of five influenza seasons (out of all the individuals immunized by these providers during those seasons). It should be noted that the analysis does not include influenza vaccines given by other providers, such as public health units or workplaces, as details on the timing of vaccine administration are not available for these providers. Health administrative data Health administrative data from September 1, 2010 to August 13, 2015 were used to determine the administration of influenza vaccines by physicians offices and pharmacists. Counts of physician billings claims from the Ontario Health Insurance Plan (OHIP) database a1 and pharmacist claims from the Ontario Drug Benefits (ODB) database b2 were extracted and grouped by week, based on the service date of the claim. Only the first claim for each person per influenza season was included. Over this five-year period, there were 13,027,715 claims (OHIP and ODB combined) submitted for 11,666,644 courses of influenza immunization (as some individuals require two doses during the same influenza season to be considered immunized). Some of these individuals were included multiple times over the five influenza seasons. In total, 4,745,534 unique individuals were vaccinated one or more times during the five-year period. a OHIP fee codes G590 and G591 b Drug Information Numbers 02015986, 02223929, 02346850, and 02362384 Technical report: Influenza immunization timing 2

Seasonal influenza laboratory data The start and peak of each influenza season was determined using Ontario-specific laboratory data from the Respiratory Virus Detection Surveillance System. These data were compiled by the Public Health Agency of Canada s Centre for Immunization and Respiratory Infectious Diseases (CIRID). Ontariospecific data are based on submissions from 16 Ontario laboratories, consisting of the 11 Public Health Ontario Laboratories and five hospital-based laboratories. The start of influenza season was defined as the week when influenza percent positivity exceeded 10%. A sensitivity analysis was also conducted by defining the start of influenza season as 5% or 15% positivity. The peak of influenza activity was defined as the week when percent positivity was the highest for that season. The data were analyzed as follows: 1) The cumulative percentage of people immunized by week in physicians offices and by pharmacists combined (out of all the people immunized by these providers), compared to the influenza percent positivity by week. 2) The cumulative percentage of people immunized by week in physicians offices and by pharmacists combined (out of all the people immunized by these providers), compared to influenza percent positivity by week based on an adjusted analysis in which: the percentages of influenza positivity per week are shifted one week earlier to account for the approximate timing of influenza exposure (i.e., to take into account the elapsed time from infection to reporting of laboratory test results to the submitter, including symptom onset, seeking medical attention and performing laboratory testing); and the percentages of people immunized are shifted two weeks later to account for the maximum time period it takes for an immune response to develop. 1 3) Based on the original, unadjusted data, the weekly number of people immunized in physicians offices and by pharmacists are presented separately. This analysis is further stratified by age group as follows: 6 months to 19 years, 20 to 64 years, and 65 years. Secondary analysis To identify the potential missed opportunities that result from delayed influenza immunization and to determine possible target dates before which it is optimal to be immunized, a secondary analysis was conducted. In this secondary analysis, the numbers of laboratory-confirmed specimens from the Respiratory Virus Detection Surveillance System from 2010-2011 to 2014-2015 were combined by week of reporting into one composite influenza season. The cumulative percentage of influenza cases occurring in or before each week in this composite season was calculated. For the purposes of interpretation, each positive specimen is considered to represent one influenza case, although there is the possibility for specimens to be double-counted (e.g., more than one positive specimen reported per case and/or more than one positive result reported per specimen). Technical report: Influenza immunization timing 3

The results are presented showing the cumulative percentage of cases that occur in or before the influenza reporting week and the corresponding target vaccination week. The target vaccination week is three weeks prior to the reporting week in order to account for the following: The estimated one-week time lag from exposure to reporting of the laboratory-confirmed case; and The two-week maximum time period for an immune response to develop after vaccination 1. The percentage associated with each influenza reporting week is the cumulative percentage of cases reported in or before that week. The corresponding target vaccination week can be interpreted as representing the percentage of cases potentially missed (i.e., missed opportunity to prevent cases due to delayed immunization) if vaccination occurred after that week. This information can be used by policy makers and health care providers to establish a target vaccination date for promotion of influenza immunization. Results Cumulative vaccination rates compared to influenza positivity unadjusted data: Using a 10% positivity cut-off to define the start of the influenza season, four of the five seasons from 2010-2011 to 2014-2015 began in mid- November or early December and peaked in either late December or early January. The other season (2011-2012) was mild and only began in mid-february, peaking in mid-march. In the four seasons that began in mid-november or early December, 73.6% to 92.5% of influenza vaccinations were given before the start of the influenza season. In the 2011-2012 season, as expected because of its delayed start, 99.7% of influenza vaccinations were given before the start of the influenza season (see Table 1 and Figure 1). In the sensitivity analyses, using a more conservative estimate of the start of influenza season based on a weekly percent positivity of 5%, four of the five seasons began in November or early December. Using the 5% cut-off for the start of the season, 58.8% to 85.3% of influenza vaccinations in these four seasons were given before the start of the season. Using a less conservative cut-off for the start of the season of 15%, 82.9% to 92.5% of influenza vaccinations in these four seasons were given before the start of the season. For the four seasons with onset in November or December, 89.5% to 96.8% of vaccinations were given before the peak week of influenza positivity (see Table 1). Cumulative vaccination rates compared to influenza positivity adjusted data: In the four seasons that began in mid-november or early December, based on the 10% positivity definition and using the adjusted analysis, 47.7% to 79.3% of influenza vaccinations were given before the start of the influenza season. Using the 5% positivity to define the start of the season for these four seasons, 23.7% to 67.7% were given before the start of the season in the adjusted analysis; using 15% positivity as the start of the season, the percentages were 67.3% to 82.7%. Using the adjusted analysis Technical report: Influenza immunization timing 4

for these four seasons, 82.9% to 92.5% of vaccinations were given before the peak week (see Table 1 and Figure 2). See Table 1 and Figure 2 for the results of the adjusted analysis, and Table 1 and Figure 3 to compare the unadjusted and adjusted analyses. Vaccination rates by provider type and age unadjusted data: Figure 4 indicates the numbers of influenza vaccinations given in physicians offices and by pharmacists. Vaccinations by pharmacists began in the 2012-2013 influenza season for those aged 5 years. The number of vaccinations provided by pharmacists increased progressively since 2012-2013. Vaccination by pharmacists tended to start slightly later than vaccinations by physicians. During all three seasons when vaccinations by pharmacists were available, those aged 19 years and 65 years were more likely to be vaccinated in a physician s office. Unlike the earlier two seasons when physician vaccinations were more common, in the 2014-2015 season, those aged 20 to 64 years were almost equally likely to be vaccinated by a pharmacist or in a physician s office (see Figures 5, 6 and 7). Composite season analysis by specific cut-off dates: Over this five-year period (2010-2011 to 2014-2015), there were 40,021 laboratory-confirmed influenza A and B positive specimens reported. Table 2 displays the percentage of cases that occurred in or before each influenza reporting week and corresponding vaccination target week in the composite five year season. As an example, 10.3% of cases occurred in or before influenza reporting week 51 (December 19 to December 25). Influenza reporting week 51 corresponds to vaccination target week 48; therefore, influenza vaccination after vaccination target week 48 (November 28 to December 4) occurred too late to prevent the 10.3% of cases in the composite season that were diagnosed in or prior to week 51. However, it should be noted that not all of these cases would have been prevented by earlier vaccination since the influenza vaccine is not 100% effective. Discussion Although there is variability in when influenza activity begins each year, in four of five years from 2010-2011 to 2014-2015, activity began in mid-november or early December using a 10% positivity rate to define the beginning of the season. In these four seasons, 73.6% to 92.5% of influenza vaccinations were given before the start of the influenza season. However, using an adjusted analysis, only 47.7% to 79.3% of influenza vaccinations were given before the start of the influenza season in the four seasons that started in November or December. In other words, 20.7% to 52.3% of vaccinations were given too late to achieve maximum benefit. Technical report: Influenza immunization timing 5

There are several limitations to the analyses related to timing of influenza immunization. The extent to which laboratory test results can be relied on to indicate the start of the season is uncertain as the proportion of a season s cases diagnosed each week may not be representative of influenza activity due to variations in testing patterns over the course of the influenza season (i.e., since more testing is generally done earlier in an influenza season, a disproportionate number of cases will have a labconfirmed diagnosis earlier rather than later during a particular influenza season). Furthermore, the populations tested may not be representative of the general population with influenza, as those tested may be more likely to have underlying medical conditions or have complications such as hospitalizations. The validity of the use of a cut-off of 10% positivity of laboratory-confirmed samples to indicate the start of the season is arbitrary; therefore a sensitivity analysis was performed using a 5% and 15% cut-off. The percent positivity rates presented in this analysis are for influenza A and B combined. Generally influenza A occurs earlier in the influenza season, with influenza B activity occurring afterwards. Therefore later vaccination is less likely to prevent influenza A cases but more likely to be effective in the prevention of influenza B cases, assuming an appropriate match between vaccine and circulating strains. This analysis was based on billings by physicians offices and pharmacists and excluded other vaccine providers such as public health units and workplaces, as exact dates of administration are not available for these providers. Based on data from the Ministry of Health and Long Term Care for 2012-2013 to 2014-2015, these other providers give only 9.2% to 16% of the total number of doses administered each year, with the majority of doses being given by pharmacists (7.9% to 26.9%) and in physicians offices (76.1% to 63.9%). Vaccination by pharmacists occurs somewhat later than in physicians offices, possibly due to when pharmacists receive the vaccine and when clients choose to present for vaccination at these locations. Vaccination by pharmacists is becoming increasingly popular, especially for those 20 to 64 years of age. In contrast, vaccination by public health has been decreasing from 8.7% in 2012-2013 to 4.0% in 2014-2015 (based on data from the Ministry of Health and Long Term Care). Using a composite season based on five years of laboratory-confirmed data, vaccination after the first week of December potentially misses the opportunity to prevent 10.3% of cases, recognizing that earlier vaccination would not prevent all of these cases given that vaccine effectiveness varies by year and is generally approximately 50% to 60% effective in healthy adults. 1 As expected, later vaccination dates miss the opportunity to prevent even more cases. As the timing of influenza seasons varies from year to year, this represents an average estimate only. Additionally, if laboratory testing tends to occur more frequently earlier in the influenza season than later on, this may inflate the percentage of cases that represent missed opportunities for prevention. Based on the information provided in Table 1 for the seasons that began in November and December, 83% to 92% of influenza vaccinations were provided by Technical report: Influenza immunization timing 6

approximately the first week in December, leaving some room for improvement in the timing of the provision of influenza vaccination if that week was chosen as a target. These data highlight the importance of minimizing late vaccination to optimize the impact of influenza immunization. Facilitation of early vaccination could include obtaining the vaccine from suppliers as early as possible, but this may be beyond provincial control. Therefore efforts should focus on distributing the vaccine to providers as soon as it is provincially available. Promotional efforts targeted at providers, including pharmacists, and the public could encourage influenza vaccination as soon as it is available. The missed opportunities to prevent influenza by delaying vaccination could be emphasized. Choosing a target date and promoting influenza vaccination prior to this date may decrease late vaccination and decrease missed opportunities to prevent influenza. Although waning immunity is potentially a concern later in the influenza season, its impact is not well understood. Further studies are required to assess the impact of waning immunity and its importance with respect to the timing of influenza vaccine administration. Conclusions Late vaccination misses opportunities to prevent influenza infections and can decrease the potential impact of influenza immunization programs. Strategies to improve the timeliness of influenza vaccination can improve the impact of influenza vaccination programs, including strategies targeted at pharmacists who are delivering increasing numbers of influenza vaccinations each year. Technical report: Influenza immunization timing 7

Table 1: Percentage of people vaccinated by the start and the peak of influenza season, using (i) original unadjusted data based on vaccination dates and laboratory-reported dates, and (ii) data adjusting for delays in immunity and influenza exposure dates; Ontario, 2010-2011 to 2014-2015 Season Influenza activity based on percent positivity b Start week Unadjusted data Cumulative % Vaccinated Adjusted data a Cumulative % Vaccinated >5% Nov 21, 2010 76.1% 48.1% 2010-11 >10% Dec 5, 2010 85.4% 69.3% >15% Dec 12, 2010 88.4% 76.1% Peak week Jan 2, 2011 92.6% 88.4% >5% Jan 29, 2012 99.2% 97.8% 2011-12 >10% Feb 19, 2012 99.7% 99.2% >15% Feb 26, 2012 99.8% 99.5% Peak week Mar 18, 2012 99.9% 99.8% >5% Nov 4, 2012 58.8% 23.7% 2012-13 >10% Nov 18, 2012 73.6% 47.7% >15% Dec 2, 2012 82.9% 67.3% Peak week Dec 23, 2012 89.6% 82.9% >5% Dec 1, 2013 82.7% 67.7% 2013-14 >10% Dec 8, 2013 85.3% 74.4% >15% Dec 22, 2013 87.8% 82.7% Peak week Dec 29, 2013 89.5% 85.3% >5% Nov 23, 2014 85.3% 58.8% 2014-15 >10% Dec 7, 2014 92.5% 79.3% >15% Dec 7, 2014 92.5% 79.3% Peak week Dec 28, 2014 96.8% 92.5% a Start week for adjusted data is 7 days before start week of original unadjusted data and vaccination date is shifted two weeks later to account for delays in mounting an immune response b Peak week = week when % positivity was the highest in the season Technical report: Influenza immunization timing 8

Figure 1. Cumulative percentage of people vaccinated versus weekly influenza percent positivity; Ontario, 2010-2011 to 2014-2015 Figure 2. Cumulative percentage of people vaccinated versus weekly influenza percent positivity (adjusted); Ontario, 2010-2011 to 2014-2015 Technical report: Influenza immunization timing 9

Figure 3. Cumulative percentage of people vaccinated versus weekly influenza percent positivity (original unadjusted and adjusted data superimposed); Ontario, 2010-2011 to 2014-2015 Figure 4. Number of people vaccinated at physicians offices and by pharmacists versus weekly influenza percent positivity; Ontario, 2010-2011 to 2014-2015 Technical report: Influenza immunization timing 10

Figure 5. Number of people 6 months to 19 years of age vaccinated at physicians offices and by pharmacists versus weekly influenza percent positivity; Ontario, 2010-2011 to 2014-2015 Figure 6. Number of people 20 to 64 years of age vaccinated at physicians offices and by pharmacists versus weekly influenza percent positivity; Ontario, 2010-2011 to 2014-2015 Technical report: Influenza immunization timing 11

Figure 7. Number of people 65 years of age and over vaccinated at physicians offices and by pharmacists versus weekly influenza percent positivity; Ontario, 2010-2011 to 2014-2015 Technical report: Influenza immunization timing 12

Table 2. Cumulative percentage of laboratory-confirmed influenza cases that occur in or before specific weeks. Ontario; composite from 2010-2011 to 2014-2015 Influenza reporting week number (approximate dates) Vaccination target week number (approximate dates) Cumulative percentage of cases that were diagnosed in or before the influenza reporting week or the vaccination target week 39 (Sept 26 Oct 2) 36 (Sept 5 Sep 11) 0.1% 40 (Oct 3 Oct 9) 37 (Sept 12 Sept 18) 0.2% 41 (Oct 10 Oct 16) 38 (Sept 19 Sept 25) 0.2% 42 (Oct 17 Oct 23) 39 (Sept 26 Oct 2) 0.3% 43 (Oct 24 Oct 30) 40 (Oct 3 Oct 9) 0.4% 44 (Oct 31 Nov 6 ) 41 (Oct 10 Oct 16) 0.5% 45 (Nov 7 Nov 13) 42 (Oct 17 Oct 23) 0.7% 46 (Nov 14 Nov 20) 43 (Oct 24 Oct 30) 1.1% 47 (Nov 21- Nov 27) 44 (Oct 31 Nov 6 ) 1.7% 48 (Nov 28 Dec 4) 45 (Nov 7 Nov 13) 2.6% 49 (Dec 5 Dec 11) 46 (Nov 14 Nov 20) 4.2% 50 (Dec 12 Dec 18) 47 (Nov 21- Nov 27) 7.1% 51 (Dec 19 Dec 25) 48 (Nov 28 Dec 4) 10.3% 52 (Dec 26 Jan 1 ) 49 (Dec 5 Dec 11) 20.2% 1 (Jan 2 Jan 8) 50 (Dec 12 Dec 18) 30.8% 2 (Jan 9 Jan 15) 51 (Dec 19 Dec 25) 41.1% 3 (Jan 16 Jan 22) 52 (Dec 26 Jan 1 ) 50.4% 4 (Jan 23 Jan 29) 1 (Jan 2 Jan 8) 57.1% 5 (Jan 30 Feb 5) 2 (Jan 9 Jan 15) 62.4% Technical report: Influenza immunization timing 13

Influenza reporting week number (approximate dates) Vaccination target week number (approximate dates) Cumulative percentage of cases that were diagnosed in or before the influenza reporting week or the vaccination target week 6 (Feb 6 Feb 12) 3 (Jan 16 Jan 22) 66.8% 7 (Feb 13 Feb 19) 4 (Jan 23 Jan 29) 70.2% 8 (Feb 20 Feb 26) 5 (Jan 30 Feb 5) 73.3% 9 (Feb 27 Mar 5) 6 (Feb 6 Feb 12) 76.2% 10 (Mar 6 Mar 12) 7 (Feb 13 Feb 19) 79.2% 11 (Mar 13 Mar 19) 8 (Feb 20 Feb 26) 82.3% 12 (Mar 20 Mar 26) 9 (Feb 27 Mar 5) 85.3% 13 (Mar 27 Apr 2) 10 (Mar 6 Mar 12) 87.8% 14 (Apr 3 Apr 9) 11 (Mar 13 Mar 19) 90.0% 15 (Apr 10 Apr 16) 12 (Mar 20 Mar 26) 92.5% 16 (Apr 17 Apr 23) 13 (Mar 27 Apr 2) 94.4% 17 (Apr 24 Apr 30) 14 (Apr 3 Apr 9) 96.0% 18 (May 1 May 7) 15 (Apr 10 Apr 16) 97.3% 19 (May 8 May 14) 16 (Apr 17 Apr 23) 98.3% 20 (May 15 May 21) 17 (Apr 24 Apr 30) 98.9% 21 (May 22 May 28) 18 (May 1 May 7) 99.2% 22 (May 29 June 4) 19 (May 8 May 14) 99.4% 23 (June 5 June 11) 20 (May 15 May 21) 99.5% 24 (June 12 June 18) 21 (May 22 May 28) 99.6% 25 (June 19 June 25) 22 (May 29 June 4) 99.7% 26 (June 26 July 2) 23 (June 5 June 11) 99.7% Technical report: Influenza immunization timing 14

Influenza reporting week number (approximate dates) Vaccination target week number (approximate dates) Cumulative percentage of cases that were diagnosed in or before the influenza reporting week or the vaccination target week 27 (July 3 July 9) 24 (June 12 June 18) 99.8% 28 (July 10 July 16) 25 (June 19 June 25) 99.8% 29 (July 17 July 23) 26 (June 26 July 2) 99.9% 30 (July 24 July 30) 27 (July 3 July 9) 99.9% 31 (July 31 Aug 6) 28 (July 10 July 16) 99.9% 32 (Aug 7 Aug 13) 29 (July 17 July 23) 100.0% 33 (Aug 14 Aug 20) 30 (July 24 July 30) 100.0% 34 (Aug 21 Aug 27) 31 (July 31 Aug 6) 100.0% 35 (Aug 28 Sept 3) 32 (Aug 7 Aug 13) 100.0% 36 (Sept 5 Sep 11) 33 (Aug 14 Aug 20) 100.0% 37 (Sept 12 Sept 18) 34 (Aug 21 Aug 27) 100.0% 38 (Sept 19 - Sept 25) 35 (Aug 28 Sept 3) 100.0% Interpretation note: Compared to the influenza reporting week, the vaccination target week takes into account the exposure time and time to mount an immune response. The percentages in the third column represent missed opportunities to prevent cases if influenza vaccination is delayed until after this target week. For example, immunization after vaccination target week 48 (column 2) is potentially too late to prevent 10.3% of cases. This is based on the percentage of cases that occurred in or before influenza reporting week 51 (column 1) in the composite five-year season (see highlighted week). Note that this is only an example and is not intended to suggest that this should be the target week for vaccination in Ontario. It should also be noted that earlier vaccination would not prevent all cases since influenza vaccine is not 100% effective. Technical report: Influenza immunization timing 15

Reference 1. National Advisory Committee on Immunization; Public Health Agency of Canada. An Advisory Committee Statement (ACS) National Advisory Committee on Immunization (NACI) statement on seasonal influenza vaccine for 2015-2016 [Internet]. Ottawa, ON; 2016 [updated 2015 Sep 30; cited 2016 Apr 4]. Available from http://www.phac-aspc.gc.ca/naci-ccni/flu-2015-grippe-eng.php#ii2 Technical report: Influenza immunization timing 16

Public Health Ontario 480 University Avenue, Suite 300 Toronto, Ontario M5G 1V2 647.260.7100 communications@oahpp.ca www.publichealthontario.ca Technical report: Influenza immunization timing 17