Sizing up the new pneumococcal vaccine
"Does the new 7-valent protein-conjugated pneumococcal vaccine confer lifelong immunity to Streptococcus pneumoniae? How does it differ from the older 23-valent pneumococcal vaccine, and who might benefit the most from it?" wonders H. BOUMAN, MD, of Bragg Creek, AB.
Until recently, all pneumococcal vaccines were polysaccharide products that contain purified capsular components of multiple common serotypes. The utility of these vaccines is limited by poor immunogenicity in infants younger than two years old and unpredictable reactivity in children under five. In addition, antibody levels wane within a few years in the populations most at risk for pneumococcal disease, and the response to individual serotype components varies. The protein-conjugated pneumococcal vaccine was developed and licensed in an effort to address these deficiencies. The seven serotypes in this vaccine make up over 80% of invasive pneumococcal disease (IPD) in children less than five years of age, but only one-half to two-thirds of IPD in adults. The vaccine is highly immunogenic in children, including those under two years, and has demonstrated safety and efficacy in preventing IPD in children younger than five years. We don't know yet how long the protection lasts in kids, but immunologic memory does persist for at least 18 months. There's also data to suggest longer-lasting immunity (a true "boost") when this immunization is followed by vaccination with the 23-valent polysaccharide preparation. So far, the vaccine appears to be safe and immunogenic in adolescents and adults, but preliminary studies suggest that levels of antibodies aren't any higher than those induced by the standard 23-valent polysaccharide preparation. In addition, the current protein-conjugated vaccine doesn't include nearly as many serotypes relevant to adult IPD, though an 11-valent version will likely be available in two to three years. The National Advisory Committee on Immunization (NACI) recently developed Canadian recommendations for the use of pneumococcal conjugate vaccine (January 15, 2002 Canada Communicable Disease Report; www.hc-sc.gc.ca/pphb-
dgspsp/publicat/ccdr-rmtc). Since S. pneumoniae is the leading cause of invasive bacterial infections, pneumonia and otitis media in children (with the highest rate occurring in infants), NACI recommends that all children under the age of two be routinely vaccinated: * two to five months of age: three doses, eight weeks apart; booster at 12-15 months; *Seven to 11 months of age: two doses, eight weeks apart; booster at 12-15 months; *12-24 months of age: two doses, eight weeks apart; no booster. Unfortunately, a four-dose course costs about $250, so no health jurisdictions in Canada have decided to offer the conjugated vaccine for free yet. It's being widely used, however, in the U.S. and parts of Europe. In addition, NACI recommends the conjugate vaccine for all children with a higher-than-usual risk of pneumococcal disease, including those with functional or anatomic asplenia, human immunodeficiency virus (HIV), and underlying cardiopulmonary conditions, as well as aboriginal children. Such patients should be vaccinated as above if under two years, while two- to five-year olds require two doses, six to eight weeks apart. It's suggested they receive two "boosters" with the 23-valent polysaccharide vaccine: *< two years when initially vaccinated: first "booster" at age two, second three to five years later; *> two years: first "booster" six to eight weeks after the second conjugate vaccine dose, second three to five years later. Although controversial, NACI also recommends that healthy children aged two to five years be considered for conjugate vaccine, particularly if they attend daycare centres. Adolescents and adults with an indication for immunization should continue to receive the 23-valent polysaccharide vaccine. DM