Administration of Protein-Conjugate Pneumococcal Vaccine to Patients Who Have Invasive Disease after Splenectomy Despite Their Having Receiv
Infectious Disease Section, Michael E. DeBakey Veterans Affairs Medical Center, Ben Taub General Hospital, and Departments of Medicine and Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas
Centers for Disease Control and Prevention, Atlanta, Georgia
Patients who undergo splenectomy are at greatly increased risk for overwhelming pneumococcal bacteremia and death. Twenty-threevalent pneumococcal polysaccharide vaccine (PPV-23), which contains capsular polysaccharides (PSs) from 23 common serotypes of Streptococcus pneumoniae, is strongly recommended for such patients. The capacity to respond to PPV-23 by producing immunoglobulin (Ig) G is genetically regulated. Some proportion of adults do not respond and, despite postsplenectomy administration of PPV-23, may remain susceptible to recurrent pneumococcal sepsis. Here, we describe 2 patients who had recurring pneumococcal bacteremia after undergoing splenectomy despite having received numerous doses of PPV-23. Heptavalent protein-conjugate pneumococcal vaccine (PCPV-7) was then administered, and it induced high levels of IgG to all 7 PSs; in one of the patients, functional activity against 5 of the 7 PSs was demonstrable, both in vitro and in vivo. Recurrent pneumococcal bacteremia in patients who have undergone splenectomy may indicate a genetically regulated failure to respond to PPV-23; PCPV-7 may stimulate production of IgG to PSs in such patients.
Patients who undergo splenectomy are at greatly increased risk for overwhelming pneumococcal bacteremia and death [13]. Twenty-threevalent pneumococcal polysaccharide vaccine (PPV-23), which contains capsular polysaccharides (PSs) from 23 common serotypes of Streptococcus pneumoniae, is strongly recommended for such patients [4], because it stimulates the production of anti-capsular antibody and has wide serotype coverage. Although administration of PPV-23 greatly reduces the incidence of pneumococcal infection in patients who have undergone splenectomy [5], bacteremia caused by a vaccine serotype still occurs; most [69], but not all [812], of these cases have occurred in patients with conditions (such as Hodgkin disease or lymphoma) that are associated with immune deficiency.
The immunologic response to PS is genetically regulated in an autosomal, codominant inheritance pattern; 5%10% of healthy adults fail to produce antibody to most or all PSs contained in PPV-23 [13]. Such persons may generate appropriate levels of IgG after receipt of protein-conjugate pneumococcal vaccine (PCPV) [14]. We here describe 2 patients who had overwhelming postsplenectomy pneumococcal sepsis despite having received PPV-23. Both lacked detectable antibody to any PS contained in PPV-23, even after repeated administration of this vaccine, but showed excellent IgG responses when heptavalent PCPV (PCPV-7) was subsequently administered.
RESULTS
Vaccination and IgG Response
Patient 1.
This 57-year-old man underwent splenectomy in 1978 because of a complication of intestinal bypass surgery. He was first administered PPV-23 in 1999. In 2001, he developed septic shock; S. pneumoniae serotype 4 was isolated from his blood. Serum obtained after he had recovered and after he had received an additional dose of PPV-23 lacked detectable antibody to any of the 7 PSs contained in PCPV-7, including serotype 4 PS (table 1). PCPV-7 (Prevnar; Wyeth Lederle Vaccines) was then administered (0.5 mL of vaccine into each deltoid muscle, for a total volume of 1.0 mL). Six weeks later, this patient had IgG to all of the PSs contained in PCPV-7, including serotype 4 PS (table 1).
Patient 2.
This 28-year-old HIV-infected man underwent splenectomy for idiopathic thrombocytopenic purpura in November 2000; PPV-23 was administered before the procedure. At the end of that month and again in March 2001, he had overwhelming pneumococcal bacteremia (serotypes were unknown) that was complicated by meningitis on one occasion and by peripheral gangrene on the other. PPV-23 was administered again in March 2001. At that time, his CD4+ cell count was 441 cells/L, and his viral load was 253 copies/mL. In January 2002, he developed septic shock. S. pneumoniae serotype 6B was isolated from cerebrospinal fluid and blood; his CD4+ cell count was 475 cells/L, and his viral load was 25,900 copies/mL. His convalescent serum had no detectable IgG to any of the PSs tested (including serotype 6B PS), either before or after a third dose of PPV-23 was administered. After receiving three 0.5-mL doses of PCPV-7 at monthly intervals, he had IgG to all of the PSs contained in PCPV-7, including serotype 6B PS (table 1).
Biological Activity of IgG
The biological activity of IgG was determined in an in vitro assay of opsonic activity [15] and in in vivo mouse protection experiments [16]. Both sets of experiments compared the effect of serum obtained before vaccination with PCPV-7 (hereafter, "prePCPV-7 serum") and after vaccination with PCPV-7 (hereafter, "postPCPV-7 serum") and used known positive and negative controls. In the opsonophagocytosis assay (table 2), postPCPV-7 serum from patient 1 exhibited good opsonic activity against serotypes 6B, 9V, 14, and 23F PSs but not against serotypes 4, 19F, or 18C PSs. PrePCPV-7 serum exhibited no opsonic activity. Serum from patient 2 contained antibiotics and, therefore, could not be tested.
In vivo, we studied the capacity of serum from both patients to protect mice against infection with S. pneumoniae serotype 4, the only vaccine serotype contained in PCPV-7 that reliably infects adult outbred mice; this was also the serotype that infected patient 1. Groups of 6 mice were injected intraperitoneally (ip) with 0.1 mL of saline that contained serum from each patient, diluted to deliver a range of concentrations of IgG to serotype 4 PS. One hour later, inocula of varying sizes (generally 550 × LD50) of S. pneumoniae serotype 4 were injected ip. Time to death was recorded by observation at 8-h intervals [16]. PrePCPV-7 serum that lacked IgG to serotype 4 PS was not protective (figure 1). Pooled serum from 6 healthy young adults who had been vaccinated with PPV-23 was highly protective against the challenge doses used in these experiments. PostPCPV-7 serum from patients 1 and 2 provided an intermediate degree of protection. Each experiment was repeated 3 times, and similar results were observed.
DISCUSSION
The risk of invasive pneumococcal disease in patients who have undergone splenectomy is 1225 times greater than that in the population at large [2, 17, 18], and the fatality rate is also higher, approaching 50%; these facts reflect the overwhelming nature of such infection [2, 19, 20]. Notwithstanding the controversy over the efficacy of PPV-23 in the population at large, this vaccine is regarded as a mainstay of preventive therapy for patients who have undergone splenectomy, and vaccination followed by repeated revaccination is recommended [2]. Studies from other countries and anecdotal evidence in the United States suggest that such treatment effectively reduces the risk of serious pneumococcal infection in this population. For example, in one county in Denmark in which compliance with vaccination was high, only 1 of 38 cases of postsplenectomy sepsis was caused by pneumococcus, and that case occurred in an unvaccinated patient [21].
In the present study, we have described 2 patients who failed to respond serologically and clinically to PPV-23. Despite the administration of repeated doses of this vaccine, neither patient had IgG to PS, and each had at least 1 bout of overwhelming pneumococcal sepsis caused by a vaccine serotype. Interestingly, natural infection also failed to stimulate production of IgG. In contrast, immunization with PCPV-7 resulted in high levels of IgG to all of the PSs included in the vaccine. To our knowledge, this is the first report of patients who failed to generate detectable IgG to an infecting serotype of S. pneumoniae and yet developed such antibody after vaccination; it is a finding that raises interestingand unansweredquestions on the biological nature of antigen processing and the immune response.
Serum from only 1 of our patients could be studied in an in vitro assay of opsonic activity; his serum was shown to be functional against 4 of 7 PSs. The in vivo mouse protection experiments documented modest protection against challenge with a fifth serotype, S. pneumoniae serotype 4. The protection was not as great as that observed with pooled serum from healthy young adults who had been vaccinated with PPV-23. This difference could be a result of the genetically governed failure to process and respond normally to PS that characterized our 2 patients [13], or it could reflect differences in antibody activity between older adults who have underlying conditions and healthy young adults [15].
Currently, there is no clear evidence demonstrating that, compared with how they respond to PPV-23, adults respond to PCPV-7 by producing higher levels of antibody or antibody that is biologically more active [22]. However, some adults who, as a result of genetic regulation, fail to respond to PPV-23 develop IgG after receipt of protein-conjugate PS [13, 14], and it seems likely that this was true of our 2 patients. The emergence of IgG to all of the PSs contained in PCPV-7 in patients who had failed to mount an antibody response after natural infection or after administration of PPV-23 supports the notion that protein conjugation of PS alters antigen presentation and leads to effective processing and production of IgG in genetically determined nonresponders.
It seems reasonable to conclude that, after administration of PCPV-7, our 2 patients are at reduced risk for recurring life-threatening pneumococcal infection. The limitations of the present study are (1) only 12 years have elapsed without serious infection and (2) it is, of course, not known whether these findings can be generalized to a larger population. For patients who develop invasive pneumococcal disease despite having received PPV-23, it might be reasonable to document the failure by measuring serum antibody levels before administering PCPV-7 or to simply administer the conjugate vaccine if such assays are not available. Before substituting a pneumococcal conjugate vaccine for the currently used PPV-23 vaccine, wider serotype coverage is needed. A vaccine based on pneumococcal proteins may eventually provide an alternative solution.
References
1. Styrt B. Infection associated with asplenia: risks, mechanisms, and prevention. Am J Med 1990; 88(5N):N3342. First citation in article
2. Fedson DS, Musher DM. Pneumococcal vaccine. In: Plotkin SA, Orenstein WB, eds. Vaccines. Philadelphia: WB Saunders, 2003. First citation in article
3. Norris RP, Vergis EN, Yu VL. Overwhelming postsplenectomy infection: a critical review of etiologic pathogens and management. Infect Med 1996; 13:77983. First citation in article
4. Centers for Disease Control and Prevention. Prevention of pneumococcal disease: recommendations of the Advisory Committee on Immunization Proctices (ACIP). MMWR Morb Mortal Wkly Rep 1997; 46:118. First citation in article
5. Konradsen HB, Henrichsen J. Pneumococcal infections in splenectomized children are preventable. Acta Paediatr Scand 1991; 80:4237. First citation in article
6. Overturf GD, Field R, Edmonds R. Death from type 6 pneumococcal septicemia in a vaccinated child with sickle-cell disease [letter]. N Engl J Med 1979; 300:143. First citation in article
7. Sumaya CV, Harbison RW, Britton HA. Pneumococcal vaccine failures: two case reports and review. Am J Dis Child 1981; 135:1558. First citation in article
8. Schlaeffer F, Rosenheck S, Baumgarten-Kleiner A, Greiff Z, Alkan M. Pneumococcal infections among immunised and splenectomised patients in Israel. J Infect 1985; 10:3842. First citation in article
9. Schutze GE, Mason EO Jr, Barson WJ, et al. Invasive pneumococcal infections in children with asplenia. Pediatr Infect Dis J 2002; 21:27882. First citation in article
10. Herer B, Brivet F, Fremaux A, et al. [Failure of anti-pneumococcal vaccination and prophylactic antibiotic therapy in 2 splenectomized subjects]. Nouv Rev Fr Hematol 1986; 28:3716. First citation in article
11. Giebink GS, Schiffman G, Krivit W, Quie PG. Vaccine-type pneumococcal pneumonia: occurrence after vaccination in an asplenic patient. JAMA 1979; 241:27367. First citation in article
12. Shetty N, Aurora P, Ridgway GL. Failure of anti-pneumococcal vaccine and prophylactic penicillin in a splenectomized patient. J Infect 1998; 37:878. First citation in article
13. Musher DM, Groover JE, Watson DA, et al. Genetic regulation of the capacity to make immunoglobulin G to pneumococcal capsular polysaccharides. J Investig Med 1997; 45:5768. First citation in article
14. Musher DM, Groover JE, Watson DA, Rodriguez-Barradas MC, Baughn RE. IgG responses to protein-conjugated pneumococcal capsular polysaccharides in persons who are genetically incapable of responding to unconjugated polysaccharides. Clin Infect Dis 1998; 27:148790. First citation in article
15. Romero-Steiner S, Musher DM, Cetron MS, et al. Reduction in functional antibody activity against Streptococcus pneumoniae in vaccinated elderly individuals highly correlates with decreased IgG antibody avidity. Clin Infect Dis 1999; 29:2818. First citation in article
16. Musher DM, Johnson B Jr., Watson DA. Quantitative relationship between anticapsular antibody measured by enzyme-linked immunosorbent assay or radioimmunoassay and protection of mice against challenge with Streptococcus pneumoniae serotype 4. Infect Immun 1990; 58:38716. First citation in article
17. Cullingford GL, Watkins DN, Watts AD, Mallon DF. Severe late postsplenectomy infection. Br J Surg 1991; 78:71621. First citation in article
18. Aavitsland P, Froholm LO, Hoiby EA, Lystad A. Risk of pneumococcal disease in individuals without a spleen. Lancet 1994; 344:1504. First citation in article
19. Bisharat N, Omari H, Lavi I, Raz R. Risk of infection and death among post-splenectomy patients. J Infect 2001; 43:1826. First citation in article
20. Waghorn DJ. Overwhelming infection in asplenic patients: current best practice preventive measures are not being followed. J Clin Pathol 2001; 54:2148. First citation in article
21. Ejstrud P, Kristensen B, Hansen JB, Madsen KM, Schonheyder HC, Sorensen HT. Risk and patterns of bacteraemia after splenectomy: a population-based study. Scand J Infect Dis 2000; 32:5215. First citation in article
22. Eskola J, Black S, Shinefield H. Pneumococcal conjugate vaccines. In: Orenstein WA, ed. Vaccines. Philadelphia: WB Saunders, 2003. First citation in article, http://www.100md.com(Daniel M. Musher, Heather)
Centers for Disease Control and Prevention, Atlanta, Georgia
Patients who undergo splenectomy are at greatly increased risk for overwhelming pneumococcal bacteremia and death. Twenty-threevalent pneumococcal polysaccharide vaccine (PPV-23), which contains capsular polysaccharides (PSs) from 23 common serotypes of Streptococcus pneumoniae, is strongly recommended for such patients. The capacity to respond to PPV-23 by producing immunoglobulin (Ig) G is genetically regulated. Some proportion of adults do not respond and, despite postsplenectomy administration of PPV-23, may remain susceptible to recurrent pneumococcal sepsis. Here, we describe 2 patients who had recurring pneumococcal bacteremia after undergoing splenectomy despite having received numerous doses of PPV-23. Heptavalent protein-conjugate pneumococcal vaccine (PCPV-7) was then administered, and it induced high levels of IgG to all 7 PSs; in one of the patients, functional activity against 5 of the 7 PSs was demonstrable, both in vitro and in vivo. Recurrent pneumococcal bacteremia in patients who have undergone splenectomy may indicate a genetically regulated failure to respond to PPV-23; PCPV-7 may stimulate production of IgG to PSs in such patients.
Patients who undergo splenectomy are at greatly increased risk for overwhelming pneumococcal bacteremia and death [13]. Twenty-threevalent pneumococcal polysaccharide vaccine (PPV-23), which contains capsular polysaccharides (PSs) from 23 common serotypes of Streptococcus pneumoniae, is strongly recommended for such patients [4], because it stimulates the production of anti-capsular antibody and has wide serotype coverage. Although administration of PPV-23 greatly reduces the incidence of pneumococcal infection in patients who have undergone splenectomy [5], bacteremia caused by a vaccine serotype still occurs; most [69], but not all [812], of these cases have occurred in patients with conditions (such as Hodgkin disease or lymphoma) that are associated with immune deficiency.
The immunologic response to PS is genetically regulated in an autosomal, codominant inheritance pattern; 5%10% of healthy adults fail to produce antibody to most or all PSs contained in PPV-23 [13]. Such persons may generate appropriate levels of IgG after receipt of protein-conjugate pneumococcal vaccine (PCPV) [14]. We here describe 2 patients who had overwhelming postsplenectomy pneumococcal sepsis despite having received PPV-23. Both lacked detectable antibody to any PS contained in PPV-23, even after repeated administration of this vaccine, but showed excellent IgG responses when heptavalent PCPV (PCPV-7) was subsequently administered.
RESULTS
Vaccination and IgG Response
Patient 1.
This 57-year-old man underwent splenectomy in 1978 because of a complication of intestinal bypass surgery. He was first administered PPV-23 in 1999. In 2001, he developed septic shock; S. pneumoniae serotype 4 was isolated from his blood. Serum obtained after he had recovered and after he had received an additional dose of PPV-23 lacked detectable antibody to any of the 7 PSs contained in PCPV-7, including serotype 4 PS (table 1). PCPV-7 (Prevnar; Wyeth Lederle Vaccines) was then administered (0.5 mL of vaccine into each deltoid muscle, for a total volume of 1.0 mL). Six weeks later, this patient had IgG to all of the PSs contained in PCPV-7, including serotype 4 PS (table 1).
Patient 2.
This 28-year-old HIV-infected man underwent splenectomy for idiopathic thrombocytopenic purpura in November 2000; PPV-23 was administered before the procedure. At the end of that month and again in March 2001, he had overwhelming pneumococcal bacteremia (serotypes were unknown) that was complicated by meningitis on one occasion and by peripheral gangrene on the other. PPV-23 was administered again in March 2001. At that time, his CD4+ cell count was 441 cells/L, and his viral load was 253 copies/mL. In January 2002, he developed septic shock. S. pneumoniae serotype 6B was isolated from cerebrospinal fluid and blood; his CD4+ cell count was 475 cells/L, and his viral load was 25,900 copies/mL. His convalescent serum had no detectable IgG to any of the PSs tested (including serotype 6B PS), either before or after a third dose of PPV-23 was administered. After receiving three 0.5-mL doses of PCPV-7 at monthly intervals, he had IgG to all of the PSs contained in PCPV-7, including serotype 6B PS (table 1).
Biological Activity of IgG
The biological activity of IgG was determined in an in vitro assay of opsonic activity [15] and in in vivo mouse protection experiments [16]. Both sets of experiments compared the effect of serum obtained before vaccination with PCPV-7 (hereafter, "prePCPV-7 serum") and after vaccination with PCPV-7 (hereafter, "postPCPV-7 serum") and used known positive and negative controls. In the opsonophagocytosis assay (table 2), postPCPV-7 serum from patient 1 exhibited good opsonic activity against serotypes 6B, 9V, 14, and 23F PSs but not against serotypes 4, 19F, or 18C PSs. PrePCPV-7 serum exhibited no opsonic activity. Serum from patient 2 contained antibiotics and, therefore, could not be tested.
In vivo, we studied the capacity of serum from both patients to protect mice against infection with S. pneumoniae serotype 4, the only vaccine serotype contained in PCPV-7 that reliably infects adult outbred mice; this was also the serotype that infected patient 1. Groups of 6 mice were injected intraperitoneally (ip) with 0.1 mL of saline that contained serum from each patient, diluted to deliver a range of concentrations of IgG to serotype 4 PS. One hour later, inocula of varying sizes (generally 550 × LD50) of S. pneumoniae serotype 4 were injected ip. Time to death was recorded by observation at 8-h intervals [16]. PrePCPV-7 serum that lacked IgG to serotype 4 PS was not protective (figure 1). Pooled serum from 6 healthy young adults who had been vaccinated with PPV-23 was highly protective against the challenge doses used in these experiments. PostPCPV-7 serum from patients 1 and 2 provided an intermediate degree of protection. Each experiment was repeated 3 times, and similar results were observed.
DISCUSSION
The risk of invasive pneumococcal disease in patients who have undergone splenectomy is 1225 times greater than that in the population at large [2, 17, 18], and the fatality rate is also higher, approaching 50%; these facts reflect the overwhelming nature of such infection [2, 19, 20]. Notwithstanding the controversy over the efficacy of PPV-23 in the population at large, this vaccine is regarded as a mainstay of preventive therapy for patients who have undergone splenectomy, and vaccination followed by repeated revaccination is recommended [2]. Studies from other countries and anecdotal evidence in the United States suggest that such treatment effectively reduces the risk of serious pneumococcal infection in this population. For example, in one county in Denmark in which compliance with vaccination was high, only 1 of 38 cases of postsplenectomy sepsis was caused by pneumococcus, and that case occurred in an unvaccinated patient [21].
In the present study, we have described 2 patients who failed to respond serologically and clinically to PPV-23. Despite the administration of repeated doses of this vaccine, neither patient had IgG to PS, and each had at least 1 bout of overwhelming pneumococcal sepsis caused by a vaccine serotype. Interestingly, natural infection also failed to stimulate production of IgG. In contrast, immunization with PCPV-7 resulted in high levels of IgG to all of the PSs included in the vaccine. To our knowledge, this is the first report of patients who failed to generate detectable IgG to an infecting serotype of S. pneumoniae and yet developed such antibody after vaccination; it is a finding that raises interestingand unansweredquestions on the biological nature of antigen processing and the immune response.
Serum from only 1 of our patients could be studied in an in vitro assay of opsonic activity; his serum was shown to be functional against 4 of 7 PSs. The in vivo mouse protection experiments documented modest protection against challenge with a fifth serotype, S. pneumoniae serotype 4. The protection was not as great as that observed with pooled serum from healthy young adults who had been vaccinated with PPV-23. This difference could be a result of the genetically governed failure to process and respond normally to PS that characterized our 2 patients [13], or it could reflect differences in antibody activity between older adults who have underlying conditions and healthy young adults [15].
Currently, there is no clear evidence demonstrating that, compared with how they respond to PPV-23, adults respond to PCPV-7 by producing higher levels of antibody or antibody that is biologically more active [22]. However, some adults who, as a result of genetic regulation, fail to respond to PPV-23 develop IgG after receipt of protein-conjugate PS [13, 14], and it seems likely that this was true of our 2 patients. The emergence of IgG to all of the PSs contained in PCPV-7 in patients who had failed to mount an antibody response after natural infection or after administration of PPV-23 supports the notion that protein conjugation of PS alters antigen presentation and leads to effective processing and production of IgG in genetically determined nonresponders.
It seems reasonable to conclude that, after administration of PCPV-7, our 2 patients are at reduced risk for recurring life-threatening pneumococcal infection. The limitations of the present study are (1) only 12 years have elapsed without serious infection and (2) it is, of course, not known whether these findings can be generalized to a larger population. For patients who develop invasive pneumococcal disease despite having received PPV-23, it might be reasonable to document the failure by measuring serum antibody levels before administering PCPV-7 or to simply administer the conjugate vaccine if such assays are not available. Before substituting a pneumococcal conjugate vaccine for the currently used PPV-23 vaccine, wider serotype coverage is needed. A vaccine based on pneumococcal proteins may eventually provide an alternative solution.
References
1. Styrt B. Infection associated with asplenia: risks, mechanisms, and prevention. Am J Med 1990; 88(5N):N3342. First citation in article
2. Fedson DS, Musher DM. Pneumococcal vaccine. In: Plotkin SA, Orenstein WB, eds. Vaccines. Philadelphia: WB Saunders, 2003. First citation in article
3. Norris RP, Vergis EN, Yu VL. Overwhelming postsplenectomy infection: a critical review of etiologic pathogens and management. Infect Med 1996; 13:77983. First citation in article
4. Centers for Disease Control and Prevention. Prevention of pneumococcal disease: recommendations of the Advisory Committee on Immunization Proctices (ACIP). MMWR Morb Mortal Wkly Rep 1997; 46:118. First citation in article
5. Konradsen HB, Henrichsen J. Pneumococcal infections in splenectomized children are preventable. Acta Paediatr Scand 1991; 80:4237. First citation in article
6. Overturf GD, Field R, Edmonds R. Death from type 6 pneumococcal septicemia in a vaccinated child with sickle-cell disease [letter]. N Engl J Med 1979; 300:143. First citation in article
7. Sumaya CV, Harbison RW, Britton HA. Pneumococcal vaccine failures: two case reports and review. Am J Dis Child 1981; 135:1558. First citation in article
8. Schlaeffer F, Rosenheck S, Baumgarten-Kleiner A, Greiff Z, Alkan M. Pneumococcal infections among immunised and splenectomised patients in Israel. J Infect 1985; 10:3842. First citation in article
9. Schutze GE, Mason EO Jr, Barson WJ, et al. Invasive pneumococcal infections in children with asplenia. Pediatr Infect Dis J 2002; 21:27882. First citation in article
10. Herer B, Brivet F, Fremaux A, et al. [Failure of anti-pneumococcal vaccination and prophylactic antibiotic therapy in 2 splenectomized subjects]. Nouv Rev Fr Hematol 1986; 28:3716. First citation in article
11. Giebink GS, Schiffman G, Krivit W, Quie PG. Vaccine-type pneumococcal pneumonia: occurrence after vaccination in an asplenic patient. JAMA 1979; 241:27367. First citation in article
12. Shetty N, Aurora P, Ridgway GL. Failure of anti-pneumococcal vaccine and prophylactic penicillin in a splenectomized patient. J Infect 1998; 37:878. First citation in article
13. Musher DM, Groover JE, Watson DA, et al. Genetic regulation of the capacity to make immunoglobulin G to pneumococcal capsular polysaccharides. J Investig Med 1997; 45:5768. First citation in article
14. Musher DM, Groover JE, Watson DA, Rodriguez-Barradas MC, Baughn RE. IgG responses to protein-conjugated pneumococcal capsular polysaccharides in persons who are genetically incapable of responding to unconjugated polysaccharides. Clin Infect Dis 1998; 27:148790. First citation in article
15. Romero-Steiner S, Musher DM, Cetron MS, et al. Reduction in functional antibody activity against Streptococcus pneumoniae in vaccinated elderly individuals highly correlates with decreased IgG antibody avidity. Clin Infect Dis 1999; 29:2818. First citation in article
16. Musher DM, Johnson B Jr., Watson DA. Quantitative relationship between anticapsular antibody measured by enzyme-linked immunosorbent assay or radioimmunoassay and protection of mice against challenge with Streptococcus pneumoniae serotype 4. Infect Immun 1990; 58:38716. First citation in article
17. Cullingford GL, Watkins DN, Watts AD, Mallon DF. Severe late postsplenectomy infection. Br J Surg 1991; 78:71621. First citation in article
18. Aavitsland P, Froholm LO, Hoiby EA, Lystad A. Risk of pneumococcal disease in individuals without a spleen. Lancet 1994; 344:1504. First citation in article
19. Bisharat N, Omari H, Lavi I, Raz R. Risk of infection and death among post-splenectomy patients. J Infect 2001; 43:1826. First citation in article
20. Waghorn DJ. Overwhelming infection in asplenic patients: current best practice preventive measures are not being followed. J Clin Pathol 2001; 54:2148. First citation in article
21. Ejstrud P, Kristensen B, Hansen JB, Madsen KM, Schonheyder HC, Sorensen HT. Risk and patterns of bacteraemia after splenectomy: a population-based study. Scand J Infect Dis 2000; 32:5215. First citation in article
22. Eskola J, Black S, Shinefield H. Pneumococcal conjugate vaccines. In: Orenstein WA, ed. Vaccines. Philadelphia: WB Saunders, 2003. First citation in article, http://www.100md.com(Daniel M. Musher, Heather)