Current guidelines of the American Society of Transplantation state that a single dose of PPV should be given to transplant recipients, with subsequent revaccination at 3–5 years 
. However, at present there is no evidence that PPV protects against pneumococcal infections in adults who are immunosuppressed 
It has been suggested by several experts that the strategy of priming with 7vPnC and boosting with PPV may be beneficial for immunocompromised patients, who may have suboptimal immunological responses to PPV alone 
We performed a randomized, single-blind, parallel-group controlled trial of a sequential vaccination strategy involving administration of PPV after primary vaccination with either 7vPnC or PPV. Overall, both vaccination strategies were well tolerated, adverse events were generally mild, and there was no evidence of a “triggering” effect or acute rejection.
In our study, the observed immune responses to the two sequential vaccination schedules were similar. Moreover, there was no benefit in either of the sequential regimens (7vPnC/PPV, PPV/PPV) when compared with single-dose PPV vaccination as recommended by the guidelines. Significantly greater increases in antibodies to serotypes 7F and 9V were seen in the PPV/PPV group when compared with the 7vPnC/PPV group, whereas only for serotype 7F we detected a significant difference in the number of patients with seroconversion. The response to at least 1 serotype was high in both groups (7vPnC/PPV 87.5%, PPV/PPV 87.1%) and was similar to the response after primary vaccination (7vPnC 77.1%, PPV 93.1%). The previous study by Kumar et al. in liver transplant recipients showed no additional benefit in sequential pneumococcal vaccination with 7vPnC followed by PPV 
. Further, we could not detect any benefit in an additional dose of PPV 6 weeks after 7vPnC in heart and lung transplant recipients 
To our knowledge there are no data on sequential vaccination in renal transplant patients. Kumar et al showed that 7vPnC is immunogenic in such patients: a response to at least 1 serotype was seen in 22 (73.3%) of 30 patients who received 7vPnC and 16 (53. 3%) of 30 patients who received PPV (P
. This is in contrast to our present findings (77.1% response after 7vPnC vs. 93% response after PPV; P
Several studies, however, have suggested that 7vPnC elicits a superior immune response in adults when compared with PPV 
. In the majority of studies a double dose (1 mL instead of 0.5 mL pediatric dose) of the conjugated vaccine was used, thus the available data suggest that the 7vPnC vaccine, particularly when administered at twice the usual pediatric dose, elicits a more robust functional immune response in adults 
With regard to vaccine response to at least 1 serotype, our finding of reduced response after a single dose of 7vPnC vaccine, compared with the PPV group, may be due to the low dose (0.5 mL) of 7vPnC used in the present study. The dose of serotype-specific antigen in PPV is 6–10 times greater than that contained in 7vPnC (25 µg vs. 2 µg or 4 µg) 
, and the greater antigen dose may contribute to the higher antibody concentrations observed. Nevertheless, the quality of the antibodies, such as avidity or opsonic functional capacity, could not be determined by ELISA and thus we do not know whether patients with increased antibody concentrations show greater clinical protection from invasive pneumococcal disease.
Some controlled studies in the non-transplantation context have shown the potential benefit of a combined vaccination schedule, mainly in children. In adults, however, the results are more conflicting.
In agreement with our results, other recent studies using a second dose of vaccine in adults have failed to show that the initial dose of 7vPnC induces a boostable or memory response. It is unclear why we are unable to demonstrate boostability after a dose of 7vPnC in adults as we do in infants 
. Previously, Baxendale et al 
showed that on day 7 after immunization of healthy young adults with 7vPnC or PPV, pneumococcal-specific B cells with all the characteristics of memory B cells could be isolated, suggesting that memory had been established before vaccination 
. It is possible that pneumococcal antigen-experienced adults who have encountered S. pneumoniae
in the nasopharynx develop immunologic memory and, when they are immunized with a first dose of 7vPnC or PPV, existing memory cells are stimulated.
The time period between the prime and the boost vaccine may be important. In a study by de Roux et al in elderly patients ≥70 years of age, a combined vaccination schedule of 7vPnC followed 1 year later by PPV induced a significant increase in immune response after the revaccination with PPV 
. Similarly, in a study by Chan et al, 39 patients with Hodgkin lymphoma, who were first immunized with 7vPnC and revaccinated with PPV 1 year later, were able to mount significantly higher serotype-specific antibody concentrations than patients who received PPV alone 
. In contrast, in the study by Goldblatt et al no benefit could be observed after a second vaccination with either PPV or 7vPnC in older patients receiving a 7vPnC 6 months previously. Further, Miernyk et al found no improved immune response in native Alaskan adults given 7vPnC 2 or 6 months before PPV 
, and neither Kumar et al nor Gattringer et al could demonstrate any benefit of a booster vaccine 6–8 weeks after the prime vaccine in liver, heart or lung transplant recipients 
. Moreover, Musher et al recently reported that subjects who received PPV within a year of prior vaccination had almost no response to the revaccination, although IgG levels increased in proportion to the time elapsed after the first vaccination 
. On the basis of those results we used a time period of 1 year between the first and second vaccinations in the present study and did not observe any benefit of a second vaccination with PPV 1 year after priming with 7vPnC.
We compared two sequential vaccine regimes, 7vPnC/PPV and PPV/PPV, with single PPV vaccination. The immune response to capsular polysaccharides is T cell independent and should have a limited duration. If waning antibody levels are associated with decreased clinical protection, then it is reasonable to conclude that revaccination may be appropriate.
The American Advisory Committee on Immunization Practices (ACIP) of the Centers for Disease Control and Prevention recommends that all persons ≥65 years old receive PPV and that a single revaccination be administered if ≥5 years have elapsed since the initial dose and if they were <65 years old at time of primary vaccination. Repeated vaccination has been shown to be relatively free of adverse events 
Elderly persons who have been revaccinated with PPV show significant antibody responses to most serotypes studied 
. However, the magnitude of the antibody response in some studies has been lower than after initial vaccination, raising questions about whether these adults may experience hyporesponsiveness or immune tolerance to repeated doses of these polysaccharide antigens 
In recent studies by Musher et al and Manoff et al, primary vaccination and revaccination within 3–5 years with PPV induced antibody response in middle-aged and older adults 
without evidence of significant hyporesponsiveness with revaccination. Nonetheless, as mentioned above, Musher et al reported that subjects who had received PPV within a year of prior vaccination had almost no response to revaccination with PPV, indicating that this time interval may be too short and may induce hyporesponsiveness. In contrast, Landgren et al reported that splenectomized individuals with Hodgkin lymphoma showed improved immune responses after repeated PPV vaccination beginning 1 year after the initial PPV dose 
. In our study lower response after revaccination for serotypes 4 and 6B in PPV/PPV group and for serotypes 4, 9V, 18C and 23F in 7vPnC/PPV group was detected. It is unclear why hyporesponse to second vaccination occurs for these serotypes.
There are limited data on longitudinal follow-up of pneumococcal antibody levels in organ transplant recipients. McCashland et al vaccinated patients with chronic liver disease who went on to receive a liver transplant. Pneumococcal antibody levels declined to baseline levels rapidly in the first few months post-transplant 
. In a study by Blumberg et al 
, 7 heart transplant recipients showed a decline in antibody titer to 50–80% over 2 years. In renal transplant recipients, Kumar et al found a significant decline of vaccine response 3 years after vaccination and use of conjugate vaccine did not improve the durability of response 
. However, it was not shown how quickly the antibodies decline in such high risk patients. In an earlier study in 33 renal transplant recipients, patients were found to have significantly waning antibody levels 2 years post-polysaccharide vaccine 
In the present study we measured antibody concentrations 1 year after vaccination with 7vPnC or PPV and found that serotype-specific IgG titers decreased after both vaccine regimes. However, in comparison with baseline, no significant difference was detected in antibody response 8 weeks and 1 year after vaccination with single dose of 7vPnC or PPV.
On the basis of the above data, in the present study we also revaccinated the PPV group 1 year after the initial PPV dose to evaluate whether this approach might show a benefit in immune response. Our results showed that the PPV/PPV regimen leads to no significant improvement of the immune response when compared with single PPV vaccination.
Although many of the diseases that routine immunization prevents are rare in recipients of solid organ transplants, it is important that these patients remain up to date with their immunizations. As result of immunosuppression, antibody titers achievable in transplant candidates are often suboptimal and, associated with the underlying liver or renal failure, antibody responses are usually even worse when vaccines are given after transplantation. However, immunosuppressive therapy is continually being improved and in most cases little is known about the impact of the more recently adopted regimes on durability of antibody response. We found that use of corticosteroid or use of triple therapy comprising glucocorticoids (prednislone), calcineurin inhibitors (cyclosporine or tacrolimus), and purine synthesis inhibitors (mycophenolate mofetil, azathioprine) did not affect the immune response. The 7vPnC vaccine is T cell dependent and stimulates production of memory B cells. In the present study, although we hypothesized that memory B cells may be further stimulated by a polysaccharide booster to produce higher antibody titers than observed with administration of the standard single-dose PPV, no benefit of revaccination could be detected.
A limitation of our study is that we used anti-pneumococcal IgG antibodies measured by ELISA as a surrogate marker for vaccine efficacy. However, there is no consensus regarding the antibody level that provides protection against infection in adults, nor what defines an appropriate vaccine response 
. Although older adults develop antibody titers that are similar to those in their younger counterparts, these antibodies have reduced function 
and ELISA cannot distinguish between functional and nonfunctional antibodies 
. A further limitation of our study is that we did not measure in vitro opsonophagocytic killing (OPK) activity of serum antibodies with a phagocytic cell line. OPK has been shown to correlate with immune protection in animal studies 
but there have been no studies correlating OPK assay results with protection in adults. Furthermore, unlike ELISA, the available 7-serotype opsonophagocytic assay is not standardized or validated.
We were also limited by the relatively small sample size of patients and by 18 of the 80 (22.5%) patients dropping out of the study, a higher drop-out rate than expected. The logistic regression analysis should therefore be regarded as exploratory, since the power to detect variables of significance is limited.
In conclusion, although the strategy of revaccination 1 year later after primary vaccination did not cause any significant adverse events, such as rejection, our results suggest no additional benefit of sequential pneumococcal vaccination with 7vPnC followed by PPV in adult recipients of renal transplants. Although a stronger immune response was seen in the PPV/PPV group than in the 7vPnC/PPV group, significant increase in antibody response was seen for two serotypes only.