Questions and answers on immunization

QUESTION: What are suitable figures to use for communicating the risk of meningococcal infection to parents? COMMENT: Meningococcal infections are rare, but their severity warrants preventive measures, such as immunization. Describing the disease-related risks to parents when counselling them about the benefits of vaccinating their child can be challenging. Standard epidemiological descriptors, such as disease incidence rates, have little meaning to nonexperts. More tangible risk descriptors are needed for this audience. We drew upon two contemporary data sources for the summary statements that follow. These included the most recent national disease statistics (2002 and 2003) (1) and data from the Immunization Monitoring Program Active (IMPACT) surveillance project spanning 2002 to 2005 (2). The IMPACT project involves active surveillance of 12 metropolitan centres across Canada, amounting to 58% of the total population. Both data sets mainly reflect the preimmunization baseline situation. Regarding meningococcal disease frequency: On average, four Canadians develop meningococcal infection each week, including: one child, zero to 14 years of age (actual figure 1.2). one adolescent, 15 to 19 years of age (actual figure 0.8). two adults, older than 20 years of age (actual figure 2.0). One Canadian dies of meningococcal infection every two weeks (recent annual totals 21 to 23 cases). The mortality rate for paediatric cases is 5% (five of 100) and is generally highest with group C cases (2). Approximately 15% of survivors (15 of 100) have chronic sequelae (3). The cumulative risks based on the IMPACT data, to 30 years of age (when rates settle to mid-life lows), for various ages at first vaccination are as follows: two months to 30 years of age: one in 3700 12 months to 30 years of age: one in 4700 10 to 30 years of age: one in 7900 lifetime risk, birth to 75 years of age: one in 2600 Less than one-half of paediatric cases are preventable with currently available group C or tetravalent conjugate vaccines (A, C, Y, W135). Serogroup B, for which none of the vaccines are effective, accounts for most cases younger than four years of age (70%) and many adolescents (40% to 50%). The preventable fraction increases with age (2): At zero to four years of age, 27% of cases are potentially preventable (C: 12%, Y and W: 15%). At 10 to 19 years of age, 50% of cases are potentially preventable (C: 26%, Y and W: 24%). Although uncommon, Neisseria meningitidis infection is associated with significant morbidity and mortality. Each year, approximately 220 Canadian adults and children develop meningococcal infection, with many developing complications such as meningitis (40%), meningitis and septicemia (40%), or septicemia alone (10% to 15%). An average of four Canadians per week develop infection; however, rates vary during the course of the year. Among children and adults with meningococcal infection, approximately 10% (one in 10) die despite prompt diagnosis and treatment (1,3). N meningitidis is a Gram-negative diplococcus with at least 13 serotypes, although only five (A, B, C, Y and W135) cause the majority of invasive disease worldwide. Since 1950, serotypes B and C have caused most cases of meningococcal disease in Canada (3). More recent reports, however, indicate that serogroup prevalence in Canada changes with time and age. For example, serogroup Y has become more common in children older than 10 years of age. In addition, hypervirulent clones, such as the group CET-15 clone in the late 1980s and the W135 ET-37 clone in 2000, have been identified (4). Such factors highlight the importance of ongoing meningococcal surveillance for directing vaccination programs. Due to the severity and rapid onset of meningococcal disease, protection against infection rests mainly with vaccination. Since the late 1960s, multiple meningococcal vaccines have been developed that vary in targeted serotypes (A, B, C, Y and W135), structure (conjugated or polysaccharide) and worldwide availability. In Canada, six meningococcal vaccines have been licensed. Meningococcal C conjugate vaccines are universally available through infant or childhood programs in all provinces and territories (5). Typically, one dose of the meningococcal C conjugate vaccine is given at one year of age through the universal programs, although these vaccines have been approved for use by the National Advisory Committee on Immunization starting at two months of age (4), when two to three doses are required for protection, depending on the product. One dose of vaccine, regardless of age, will not produce life-long protection. Thus, boosters will be required later in life. In the present report, we have translated standard epidemiological frequency measures, such as incidence and rates, into values that are accessible to the general public. Using values such as ‘cases per week’ or cumulative risks (eg, ‘1 in 100’) allows patients to appreciate complicated epidemiological data within a simpler framework. In particular, we have presented values that correlate with routine patient visits (eg, 12-month well child visit) so that they can be easily applied to current programs. We have provided summary data on the additional coverage provided by the new tetravalent conjugate vaccine at various ages. By using simple statistical tools for communicating infection risk, physicians will be prepared to communicate the risks and benefits of current and expanding vaccine programs in Canada.

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