Comment: Maternal vaccination with Tdap currently seems to be the most effective strategy to protect young infants against pertussis. All other strategies including ‘cocoon vaccination’ with Tdap have not been effective, as it relies on herd immunity, which is not sufficiently well induced by acellular pertussis (aP)-containing products. In fact, this is the only valid and effective indication of use of available Tdap vaccine now, considering unexpectedly faster waning of aP vaccine induced immunity. However, there are certain issues like optimum timing of the maternal Tdap vaccination, possible interference with first infant dose of pertussis-containing vaccine, and hyper-immunization with tetanus and diphtheria toxoid; these need to be sorted out.

Vaccine-induced immunity against pneumococcal infection relies on the generation of high concentrations of antibody and B cell memory. Both the 10- and the 13-valent pneumococcal conjugate vaccines (PCV-10 and PCV-13) effectively reduce disease caused by vaccine serotypes. It is unknown whether the generation of B cell memory requires several doses of the same vaccine or whether different PCVs are interchangeable.

Children in the United Kingdom (n=178) who had previously received PCV-13 at 2 and 4 months were randomized 1:1 to receive a PCV-13 or PCV-10 booster at age 12 months. Peripheral blood memory B cells (BMEM) were quantified before and at 1 and 12 months after vaccination using a cultured enzyme-linked immunospot assay for pneumococcal serotypes 1, 3, 4, 9V, 14, 19A, and diphtheria and tetanus toxoid. Correlations between BMEM frequencies and simultaneously measured antibody (IgG and opsonophagocytic assay) was also assessed.

A significant rise in post-booster BMEM frequency was seen for 5 out of 6 serotypes in the PCV-13 group and none in the PCV-10 group. In the PCV-13 group, there was a particularly large increase in serotype 3–specific BMEM associated with only a small increase in antibody. Post-booster BMEM responses correlated positively with antibody, but correlations between pre-booster BMEM and subsequent BMEM and antibody responses were inconsistent.

Comments: The scarcity of the vaccines – such as recently witnessed with IPV – would bring the issue of interchangebility of PCVs in the limelight. The above study suggests that immunizing with a PCV-containing polysaccharides conjugated to a novel carrier protein is not sufficient to generate a rapid and strong B-cell memory response, at least when primary vaccination with PCV-13 is followed by a booster dose of PCV-10.