An Advisory Committee Statement (ACS)
National Advisory Committee on Immunization (NACI)*

Update: Statement on Influenza Vaccination
for the 2003-04 Season

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Preamble

The National Advisory Committee on Immunization (NACI) provides Health Canada with ongoing and timely medical, scientific, and public health advice relating to immunization. Health Canada acknowledges that the advice and recommendations set out in this statement are based upon the best current available scientific knowledge and is disseminating this document for information purposes. People administering or using the vaccine should also be aware of the contents of the relevant product monograph(s). Recommendations for use and other information set out herein may differ from that set out in the product monograph(s) of the Canadian licensed manufacturer(s) of the vaccine(s). Manufacturer(s) have sought approval of the vaccine(s) and provided evidence as to its safety and efficacy only when it is used in accordance with the product monographs.

The influenza season in Canada generally occurs between November and April each year, and up to 25% of the population may be infected in non-pandemic years. Although serious complications of influenza are most likely to occur in persons with certain pre-existing medical conditions or those > 65 years of age, previously healthy young children may have hospitalization rates comparable to those among the elderly during influenza seasons⁽¹⁾. Although influenza-related morbidity (physician visits, otitis media, and lower respiratory tract disease) is high in children^(1,2), deaths from influenza in children are rare.

The Canadian 2003-04 influenza season began earlier than usual and involves a new variant of the A(H3N2) strain (A/Fujian/411/2002) that is not included in the current year vaccine. As with other H3N2 predominant seasons, this influenza season is expected to be more severe than average, although surveillance indicators to date are still within the range of past seasons. In this update, NACI summarizes the epidemiology of this year's influenza season to date (week ending 20 December, 2003) on the basis of data from the national FluWatch surveillance program (http://www.phac-aspc.gc.ca/fluwatch/index.html) and reaffirms its recommendations for annual immunization programs published in August 2003⁽³⁾.

Influenza Virus Surveillance

Influenza virus began circulating relatively early in Canada this year compared with past seasons. For the weeks spanning 24 August to 20 December, 2003, sentinel laboratories tested 30 217 clinical specimens for influenza; 4321 (14.3%) were positive. The weekly percentages of clinical specimens that were positive for influenza increased from 0.15% at the start of this period to a peak of 21.4% during the week ending 15 November, 2003. In previous years, peaks in positive laboratory identifications generally did not occur before mid-December. During the 2000-01, 2001-02, and 2002-03 influenza seasons, the peak percentages of specimens found to be positive for influenza ranged from 12.8% to 26.0%. During the 1999-2000 influenza season, a relatively severe season when influenza A (H3N2) viruses predominated, the peak weekly percentage of positive specimens was 25%. It is still relatively early in the current influenza season, and activity is just beginning in many areas. Further peaks in positive laboratory identifications for influenza are expected to occur.

To date, of the 4321 positive influenza identifications, 4309 (99.7%) were influenza A viruses, and 12 (0.3%) were influenza B viruses. Of the 4309 influenza A viruses, 374 (8.7%) have been antigenically characterized so far by the National Microbiology Laboratory; 347 (92.8%) are A/Fujian/411/0(H3N2)-like viruses, 25 (6.7%) are A/Panama/2007/99(H3N2)-like viruses, one (0.3%) is an A/New Caledonia/20/9/9-like (H1N1) virus, and one (0.3%) is an influenza A(H1N2) virus. As of 20 December, all provinces and territories except Newfoundland and Labrador have reported laboratory-confirmed influenza.

Over 50% of laboratory-confirmed influenza infections reported to Health Canada this season have been reported in children < 15 years of age. This pattern in age distribution is expected, since as influenza viruses change over time new strains are more likely to infect young children, who are less likely to have had previous exposure to influenza viruses and, therefore, have limited protective immunity. The relatively mild influenza seasons in recent years may have contributed to the lack of natural immunity in young children.

The A/Fujian/411/2002(H3N2)-like virus was the predominant strain circulating in Australia and New Zealand during the recent 2003 southern hemisphere influenza season, and activity during this season was relatively high in both countries⁽⁴⁾. Many countries in the Northern hemisphere, including the United States, are experiencing the same predominance of A/Fujian/411/2002(H3N2)-like viruses during the current season⁽⁵⁾. While influenza seasons in which A(H3N2) viruses predominate are typically associated with more severe illness and deaths, there is insufficient evidence to date to determine whether the A/Fujian-like viruses are more virulent that other influenza A(H3N2) viruses⁽⁵⁾.

Influenza-Like Illness (ILI) Surveillance

For the weeks spanning 24 August to 20 December, the weekly proportion of patient visits to approximately 200 sentinel providers nationwide for ILI increased from 13 to 52 per 1000 patients seen. This is consistent with typical influenza seasons, apart from the early onset this season. During the 2000-01, 2001-02, and 2002-03 influenza seasons, the peak weekly percentage of patient visits for ILI ranged from 40 to 60 per 1000. During the 1999-2000 season, the peak weekly percentage of patient visits for ILI was 150 per 1000.

Influenza Activity Reported by Provincial and Territorial Epidemiologists

Increasing influenza activity began in late September and early October in Alberta, Saskatchewan, and the Northwest Territories. Seasonal activity began next in the Yukon, British Columbia, Ontario, and Nunavut. The Atlantic provinces and Quebec were the last to begin reporting influenza activity. Influenza activity appears to have peaked in Alberta and Saskatchewan by the week ending 22 November. During the week ending 20 December, widespread activity was reported throughout Ontario, while appearing to be on the decline in the Territories, British Columbia, and Saskatchewan. Activity is continuing to increase in Quebec and Nova Scotia.

Reports of Severe Illness and Deaths in Persons < 15 Years of Age

During annual influenza seasons, up to 10% to 25% of the population may be infected, and attack rates of over 30% have been estimated in children < 5 years of age⁽²⁾. Studies in the U.S. have shown that previously healthy children < 1 year may have hospitalization rates comparable to those in the elderly during influenza seasons^(1,2). However, death from influenza in young children is rarely reported.

At the end of October 2003, Health Canada requested that provinces and territories report all influenza-related deaths in children, after having received reports of deaths from influenza A in children in the United Kingdom and the United States at the start of the season^(6,7). To date, Health Canada has received four reports of deaths associated with laboratory-confirmed influenza A infection in children < 15 years of age (range 7 to 14 years). Of these, three had an underlying chronic illness and had received influenza vaccine this season (one child was vaccinated > 2 weeks before his/her presentation, one child was vaccinated 1 day before presentation, and the vaccination date of the other case is unknown). The fourth death was in a previously healthy child who had not been vaccinated against influenza.

There is no real-time national reporting for influenza-related hospitalizations or deaths in Canada. Based on a retrospective review, 700 to 1000 hospitalizations due to influenza are reported in children < 15 years of age each year, the majority of these in children < 5 years of age. The average number of reported deaths due to influenza in children < 15 years of age, based on a retrospective review of vital statistics (death certificate) data from 1991 to 2000, is 2 per year (range 0-5 per year). It is difficult to compare historical data that are likely to underestimate influenza-related deaths with the number of deaths reported prospectively this season. It is likely that there is increased awareness of severe complications of influenza and increased reporting of influenza-related deaths in children during the current season. Recent reports from the U.S. suggest that there is a wide range of influenza-associated complications resulting in serious illnesses and deaths in the pediatric population, and sudden deaths associated with influenza may occur in previously healthy children and adolescents^(6,8). Among the complications of influenza to date are pneumonia and invasive bacterial co-infection.

Influenza Vaccine Recommendations

NACI reaffirms its recommendation for annual immunization of persons at higher risk of serious illness from influenza, including those > = 65 years of age; those > 6 months of age with cardiac or pulmonary disease or chronic conditions such as diabetes mellitus, cancer, immunodeficiency, or immunosuppression (due to underlying disease and/or therapy); and children > 6 months of age receiving long-term acetylsalicylic acid (ASA) therapy. Vaccine is also recommended for health care workers or others (e.g. family members) in close contact with persons who have underlying medical conditions⁽³⁾.

While the priority for vaccine programs is persons at highest risk of serious morbidity associated with influenza, healthy adults and children can benefit from protection from influenza and, as previously stated by NACI⁽³⁾, should also be encouraged to receive vaccine. Children < 9 years of age require two doses (dose for children 3 to 8 years old: 0.5 mL; for children 6 to 35 months old: 0.25 mL) 1 month apart if they have not been immunized with influenza vaccine in a previous season. Children < 6 months of age and persons with severe allergy to eggs or to a previous dose of influenza vaccine should not be vaccinated.

The A/Fujian-like viruses are drift variants of the A/Panama/2007/99-like (H3N2) strain included in the current 2003-2004 vaccine and were detected by global surveillance early this year but too late for inclusion in the current influenza vaccine^(9-11). Experimental testing of ferrets using hemagglutination inhibition assays indicates that antibodies to the A/Panama vaccine virus cross-react with A/Fujian-like viruses; therefore, current influenza vaccines should provide some protection against A/Fujian-like viruses⁽⁵⁾. Early serologic studies to assess cross-reacting antibodies have demonstrated that adults and elderly persons immunized with vaccines containing A/Panama/2007/99 develop antibodies against A/Fujian/411/2002-like viruses⁽¹¹⁾. However, the level of protection against the A/Fujian-like viruses remains uncertain until vaccine effectiveness studies for the current year have been completed^(5,11). Since the current trivalent vaccines also contain A/New Caledonia/20/99 (H1N1)-like and B/Hong Kong/330/2001-like strains, they should offer protection against these viruses if they circulate during the rest of the season.

There has been an increase in demand for influenza vaccine this season, over 10.2 million doses having been distributed to provinces and territories by the end of December 2003. To date, vaccine manufacturers have been able to meet the demands of publicly funded vaccine programs. However, the supply in the private sector is likely to be low as available vaccines are deemed to be a priority for the public programs.

In addition to immunization, infection control measures are an effective means to interrupt transmission of influenza virus. In particular, careful hand hygiene and staying at home for those who are febrile and unwell with respiratory symptoms will prevent spread of infection^(12,13). In the health care setting the use of Routine Practices and Additional Respiratory and Contact Precautions with symptomatic patients is an effective means of ensuring that respiratory infections are contained⁽¹⁴⁾. Use of the antivirals amantadine, oseltamivir and zanamivir may be appropriate in some clinical settings⁽³⁾.

References

1. Neuzil KM, Zhu Y, Griffin MR et al. Burden of interpandemic influenza in children younger than 5 years: a 25-year prospective study. J Infect Dis 2002;185(2):147-52.
2. Neuzil KM, Mellen BG, Wright PF et al. The effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children. N Engl J Med 2000;342(4):225-31.
3. National Advisory Committee on Immunization. Statement on influenza vaccination for the 2003-2004 season. CCDR 2003;29(ACS-4):1-20.
4. WHO Collaborating Centre for Reference & Research on Influenza, Melbourne, Australia. Outbreak of influenza reported throughout Australia. URL: <http://www.influenzacentre.org/index.htm>. Accessed 7 November, 2003.
5. Centers for Disease Prevention and Control. Update: influenza activity - United States, 2003-04 season. MMWR 2003;52(49):1197-1202.
6. Centers for Disease Prevention and Control. Update: influenza-associated deaths reported among children aged < 18 years - United States, 2003-2004 influenza season. MMWR 2003;52(Dispatch):1-2. URL: < http://www.cdc.gov/mmwr/preview/mmwrhtml/mm52d1219a1.htm>.
7. Health Protection Agency. Current influenza activity in the UK. Commun Dis Rep Wkly 2003;13(45). URL: < http://www.hpa.org.uk/infections/topics_az/influenza/seasonal/uk_data_sources.htm >. Accessed 30 December, 2003.
8. Centers for Disease Prevention and Control. Severe morbidity and mortality associated with influenza in children and young adults - Michigan, 2003. MMWR 2003;52:837-40.
9. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2003-2004 influenza season. Wkly Epidemiol Rec 2003;78:58-62. URL: <http://www.who.int/wer/2003/en/wer7809.pdf>.
10. World Health Organization. Addendum to the recommended composition of influenza virus vaccines for use in the 2003-2004 influenza season. Wkly Epidemiol Rec 2003;78:77. URL: <http://www.who.int/wer/2003/en/wer7811.pdf>.
11. World Health Organization. Influenza vaccine for the northern hemisphere 2003-2004: additional information. URL: < http://www.who.int/csr/disease/influenza/vaccine2003/en/index.html>. Accessed 30 December, 2003.
12. Hammond B, Ali Y, Fendler E et al. Effect of hand sanitizer use on elementary school absenteeism. Am J Infect Control 2000;28(5):340-46.
13. White C, Kolble R, Carlson R et al. The effect of hand hygiene on illness rate among students in university residence halls. Am J Infect Control 2003;31(6):364-70.
14. Health Canada. Routine practices and additional precautions for preventing the transmission of infection in health care. CCDR 1999;25(S4):1-142.

___________________________________

Members: Dr. M. Naus (Chairperson), Dr. A. King (Executive Secretary), Dr. I. Bowmer, Dr. G. De Serres, Dr. S. Dobson, Dr. J. Embree, Dr. I. Gemmill, Dr. J. Langley, Dr. A. McGeer, Dr. P. Orr, Dr. B. Tan, A. Zierler.

Liaison Representatives: S. Callery (CHICA), Dr. J. Carsley (CPHA), Dr. T. Freeman (CFPC), Dr. A. Gruslin (SOGC), A. Honish (CNCI), Dr. B. Larke (CCMOH), Dr. B. Law (ACCA), Dr. V. Lentini (DND), Dr. A. McCarthy (CIDS), Dr. J. Salzman (CATMAT), Dr. L. Samson (CPS), Dr. D. Scheifele (CAIRE), Dr. M. Wharton (CDC).

Ex-Officio Representatives: Dr. A. Klein and Dr. H. Rode (BREC), Dr. R. Ramsingh (FNIHB), Dr. T. Tam (CIDPC).

This statement was prepared by Drs. T. Tam and J. Langley, and approved by NACI.

[Canada Communicable Disease Report]