Immunogenicity of 3+1 Versus 2+1 Schedule for PCV7

Streptococcus pneumonia is the most common bacterial pathogen of community-acquired pneumonia in children [1]. It may also cause meningitis, bloodstream infections and acute otitis media [1]. In 2005, WHO estimated that 1.6 million people a year die from pneumococcal disease, including up to 1 million children less than 5 years old worldwide, in particularly for infants and children less than 2 years old [1]. Most of the death occur in developing countries [2, 3]. Case fatality ratios are highest for invasive infections and range from 5-20% for bacteremia to 40-50% for meningitis. Among meningitis survivors, long-term neurologic sequelae occur in 25-56% of cases [4].

Most pneumococcal infections can be treated effectively with antibiotics, although meningitis still often results in devastating outcomes. Over the last 20 years, the emergence of antimicrobial resistance among S. pneumoniae complicates treatment of infections. Penicillin and co-trimoxazole resistance are common in many parts of the world, including China [5-9]. Multidrug resistance has also emerged and is best documented in industrialized countries. Treatment failures due to resistance have been documented for acute otitis media, meningitis and bloodstream infections [5-6].

Currently two pneumococcal vaccines are licensed and available. One is the 23-valent pneumococcal polysaccharide vaccine (PPV23). However it is not recommended for children less than 2 year old. Another one is the 7-valent pneumococcal conjugate vaccine (PCV7) which can be used for children 6 weeks to 24 months of age. PCV7 includes the capsular polysaccharide of 7 serotypes (4,6B, 9V, 14, 18C, 19F, 23F), each coupled to a nontoxic variant of diphtheria toxin, CRM197. The vaccine contains 2 μg each of capsular polysaccharide from serotypes 4,9V, 14, 19F and 23F; 2 μg of oligosaccharide from 18C; 4 μg of capsular polysaccharide of 6B; 20 μg of CRM197; and 0.125 mg of aluminum/0.5 ml dose as an aluminum phosphate adjuvant [10].

Since the licensure of the PCV7 in 2000, it is being widely used for infants and toddlers in most of the developed countries. It has been demonstrated that PCV7 provide great protections for pneumococcal invasive disease (meningitis, bloodstream infections), pneumonia and otitis media [11, 12]. Routine use of PCV7 in the US has rapidly reduced rates of invasive pneumococcal disease in children. The impact of the vaccine was noted within 1 year of introduction. According to CDC's Active Bacterial Core Surveillance (ABCs) the incidence of invasive pneumococcal disease among children <5 years dropped 75% from 1998/1999 to 2005; disease caused by vaccine-type strains fell 94% from 80 to 4.6 per 100,000 [13, 14]. A multi-centre study of hospitalized patients found that 77% fewer cases in children <2 years were caused by vaccine serotypes in 2002 compared to the average number of cases during 1994-2000 [15]. Surveillance data on vaccine impact from outside the US are currently limited. Data from Calgary, Canada showed a 93% reduction in vaccine-type invasive disease in children <2 years of age [16]. In Australia, the rate of vaccine-type invasive pneumococcal disease reduced by 78% between 2002 and 2006 in children aged under 2 years [17]. In US, it was also found that one or more doses of PCV7 was 96% effective against invasive disease in healthy children, 81% effective in children with comorbid medical conditions and 76% effective overall against disease caused by strains resistant to penicillin [18]. PCV7 use also appears to be reducing non-invasive pneumococcal infections in the US, including otitis media and pneumonia [19-22].

Currently there are two immunization schedules: the 3+1 and the 2+1 schedules. In US, it is recommended to give three doses during infancy (scheduled at 2, 4, 6 month) plus one dose at 12-15 months (3+1 schedule). Since several studies have demonstrated that two doses may provide similar direct protection to three conjugate doses during infancy, it is recommended to give two doses during infancy plus a booster dose 12 months in some European countries including UK [23, 24].

In China, a recent serogroup distribution study in out-patient department (OPD) patients with acute upper respiratory infections showed that coverage with PCV7 was about 55% for nasopharyngeal carriage pneumococci isolates, and 75% for the penicillin-nonsusceptible pneumococci isolates from 2000 to 2005 [25], suggesting that PCV7 is effective for preventing pneumococcal infections. Since PCV7 was only licensed in China by May of 2008, there is no data for the effectiveness. For immunization schedule, the manufacturer of PCV7 (Wyeth Pharmaceuticals Inc) recommends to use 3+1 schedule in China as that in US. However, China may NOT have enough resources for mass vaccination as a developing country because PCV7 is very expensive. Therefore, generating our own data in China and developing an alternative immunization schedule, such as 2+1, may have great advantage to save more lives by using a limited resource.