Are anticapsular antibodies the primary mechanism of protection against invasive pneumococcal disease?

Abstract

Background: Antibody to capsular polysaccharide has been the basis of several vaccines that offer protection against invasive disease from Streptococcus pneumoniae. The success of such vaccines has led to the inference that natural protection against invasive pneumococcal disease is largely conferred by anticapsular antibody. If this is so, one would expect that the decline in disease from different serotypes would vary significantly, and that the appearance of substantial concentrations of anticapsular antibodies would coincide temporally with the decline in age-specific incidence.

Methods and findings: Using incidence data from the United States, we show that, on the contrary, the decline in incidence with age is quite similar for the seven most important serogroups, despite large differences in exposure in the population. Moreover, only modest increases in antibody concentration occur over the second and third years of life, a period in which serotype-specific incidence declines to less than 25% of its peak. We also present detailed data on the distribution of antibody concentrations in Israeli toddlers, which are consistent with the United States findings. The same conclusion is supported by new data on age-specific incidence in Finland, which is compared with published data on antibody acquisition in Finnish toddlers.

Conclusion: We suggest some additional studies of the mechanisms of protection that could distinguish among potential alternative mechanisms, including acquired immunity to noncapsular antigens, maturation of nonspecific immune responses, or changes in anatomy or exposure.

Figure 1. Age-Specific Incidence of Invasive Pneumococcal Disease in the United States by Serogroup, Based on Data from Active Bacterial Core Surveillance

Serogroups 4 and 23 are shown only up to 48 mo, after which incidence is less than 1/100,000 person-years. All serogroups besides those in the heptavalent vaccine are shown combined as non-vaccine serogroups (NVG).

Figure 2. Age-Specific Incidence of Invasive Pneumococcal Disease in the United States by Disease Type, Based on Data from Active Bacterial Core Surveillance

Meningitis incidence is plotted only up to 30 mo, after which it remains at or below 1/100,000 person-years. “Pneumonia” indicates bacteremic pneumonia, while “bacteremia” indicates nonfocal bacteremia. “Total” includes other invasive diagnoses.

Figure 3. Box-Whisker Plot of Anti-Type-14 Polysaccharide Antibodies in Israeli Toddlers, by 6-Mo Age Groups

Central boxes indicate median and 25th and 75th percentiles; whiskers indicate upper and lower adjacent values.

Figure 4. Age-Specific Incidence of Invasive Pneumococcal Disease Caused by Serogroups 6 and 14 in Finland, Based on Active Laboratory-Based Surveillance