Pneumococcal Serotypes before and after Introduction of Conjugate Vaccines, United States, 1999–2011

Serotyping data for pneumococci causing invasive and noninvasive disease in 2008–2009 and 2010–2011 from >43 US centers were compared with data from preconjugate vaccine (1999–2000) and postconjugate vaccine (2004–2005) periods. Prevalence of 7-valent pneumococcal conjugate vaccine serotypes decreased from 64% of invasive and 50% of noninvasive isolates in 1999–2000 to 3.8% and 4.2%, respectively, in 2010–2011. Increases in serotype 19A stopped after introduction of 13-valent pneumococcal vaccine (PCV13) in 2010. Prevalences of other predominant serotypes included in or related to PCV13 (3, 6C, 7F) also remained similar for 2008–2009 and 2010–2011. The only major serotype that increased from 2008–2009 to 2010–2011 was nonvaccine serotype 35B. These data show that introduction of the 7-valent vaccine has dramatically decreased prevalence of its serotypes and that addition of serotypes in PCV13 could provide coverage of 39% of isolates that continue to cause disease.

serotype 6A in the past were likely 6C or 6D.Monitoring of pneumococcal serotypes causing disease provides insight into pathogenesis and guidance for vaccine composition.
Although the effect of pneumococcal vaccine on invasive disease has been documented (7,8,13,14), the effect on noninvasive disease is unclear.CDC has conducted longitudinal surveillance for invasive pneumococcal disease by using 10 selected sites since the 1990s, but noninvasive disease has been excluded (2).Collection of diagnostic specimens that confirm the etiologic agent causing pneumonia, bronchitis, sinusitis, and otitis media can be difficult.Although isolation of S. pneumoniae from a normally sterile site is always considered a pathogen, recovery from a respiratory site may represent colonization.Despite these limitations, the role of surveillance that is not limited to isolates causing invasive disease is shown by recent studies reporting high case-fatality rates for noninvasive serotypes (15)(16)(17).
The purpose of this study was to examine changes in pneumococcal serotypes causing invasive and noninvasive disease in all age groups in the United State from 1999-2000 through 2010-2011.The data examined reflect longitudinal surveillance from 4 periods before and after conjugate vaccine use was implemented.

Materials and Methods
Clinical isolates of S. pneumoniae were collected from 45 medical centers throughout the United States during November 1, 2008-April 30, 2009, and from 43 US centers during October 1, 2010-March 31, 2011, as part of a longitudinal surveillance program.For each study period, 50 unique, consecutive pneumococcal isolates considered by the submitting laboratory to have a major clinical role were requested from each center.Isolates were not limited by patient age or specimen source.Identification of isolates was confirmed by using the bile solubility test after receipt at the central reference laboratory.
Susceptibility testing was performed on 1,750 isolates obtained during 2010-2011 and 1,946 isolates obtained during 2008-2009 by using the Clinical Laboratory and Standards Institute broth microdilution method and interpretive criteria (18,19).The results of susceptibility testing were analyzed by all 3 categories of the Clinical Laboratory and Standards Institute penicillin interpretative criteria (oral, meningitis parenteral, nonmeningitis parenteral) that have been defined since 2008.Clinical laboratories are instructed to report all 3 interpretations for penicillin results on all specimen types, with 1 exception.For cerebrospinal fluid (CSF) isolates, penicillin results are only interpreted according to the meningitis parenteral breakpoints.Therefore, CSF isolates were excluded from the analysis when nonmeningitis parenteral breakpoints were applied.
The capsular serotype of all isolates was determined by using the Quellung reaction with antisera from the Statens Serum Institut (Copenhagen, Denmark).Identity of nontypeable isolates was confirmed by using DNA probes.Serotype distributions during 2008-2009 and 2010-2011 were compared with those during 2 earlier surveillance periods representing the preconjugate vaccine (1999)(2000) and the postconjugate vaccine (2004)(2005) periods.This comparison required serotyping to be performed on archived penicillin-susceptible and penicillin-intermediate isolates from the 1999-2000 period (4).Serotyping was repeated on serogroup 6 isolates from 1999-2000 (penicillin-resistant isolates) and from 2004-2005 to detect 6C and 6D in collections for which serotyping results were published (20,21).Susceptibility data have been reported for the [1999][2000][2004][2005], and 2010-2011 surveillance periods (4,21,22).The statistical significance of differences in serotype distribution for time periods, age groups, and specimen sources was assessed by using the Fisher exact test (2-tailed).This research was approved by the human subjects research review board at the University of Iowa.

Results
Patient age and specimen source distributions for the pneumococcal isolates obtained during the 4 surveillance periods were similar, with 2 exceptions (Table 1).The percentage of isolates from patients <5 years of age decreased from 29% in the preconjugate vaccine period to 20%-21% in the postconjugate period (p<0.001).The percentage of isolates from blood cultures decreased from 31% during 1999-2000 to 23%-25% during the postconjugate vaccine period (p<0.01).

Discussion
This longitudinal study demonstrates the effectiveness of PCV7 in children <5 years of age by a decrease in PCV7 serotypes from 70% of isolates in during 1999-2000 to only 1.9% during 2010-2011.Among other age groups, for which routine PCV7 use is not recommended, an indirect vaccine effect is apparent; the percentage of PCV7 serotypes decreased from 48% to 4.9%.Although PCV13 (PCV7 plus serotypes 1, 3, 5, 6A, 7F, and 19A) has been licensed for administration to children and adults >50 years of age, the Advisory Committee on Immunization Practices has not yet issued guidance for use in adults (23).Fewer cases and lower cost have been projected as potential benefits if PCV13 were given to the older US population (24).
As PCV7 serotypes decreased, serotype 19A strains began causing a higher percentage of invasive and noninvasive disease (21,25).Serotype 19A was the predominant serotype in our study during the post-PCV periods and accounted for 20% of isolates and 41% of PNSP during 2010-2011.The prevalence of serotype 19A strains was similar for invasive and noninvasive disease.Populationbased CDC surveillance of invasive pneumococcal disease in the United States during 2007 reported that 40% of PNSP were serotype 19A (13).Recovery of serotype 19A isolates during 2010-2011 from middle ear fluid was much higher in our study than for other specimen sources (36.4% vs. 18.9%).A lower nasopharyngeal carriage rate of serotype 19A in PCV13-vaccinated children in France with acute otitis media suggests a decrease in serotype 19A disease will follow (26).Our study showed a slight decrease in the relative number of serotype 19A clinical isolates since introduction of PCV13 that should be apparent as a major trend by the 2012-2013 respiratory season.
The prevalence of serotype 3 strains in this study was fairly constant among invasive and noninvasive specimen types, and there was no evidence of change apparent since PCV13 introduction.High mortality rates have been associated with invasive disease caused by serotype 3 (27,28).The lower occurrence of serotype 3 isolates among children observed in the present study has also been reported in Germany (29).An increase in 7F strains from 1.4% during 1999-2000 to 5.8% during 2008-2009 (p<0.001) did not change in the post-PCV13 period.A lower rate of nasopharyngeal colonization with 7F strains in children in France vaccinated with PCV13 is predictive of a future decrease in 7F disease (26).Serotype 7F was the only serotype recovered in more invasive than noninvasive specimen types in the current study (12.4% vs. 4.9% during 2010-2011).A study in Germany of invasive pneumococcal disease in children <16 years of age demonstrated the highest risk for severe and fatal outcomes for infection with serotype 7F (27).However, a meta-analysis associating serotypes with deaths from bacteremic pneumonia, including a small number of cases in children, reported a decreased risk for death for serotype 7F (28).
Serotypes 1 and 5 were included in PCV13 because these strains are major causes of invasive disease in children outside the United States (30).In our study, only a small number of these serotypes were obtained.During 2010-2011, the 2 serotype 1 isolates were from blood cultures, and the only serotype 5 isolate was from a lower respiratory tract specimen.Although serotype 6A is included in PCV13, steady decreases in the number of 6A and 6B isolates observed since 1999-2000 suggest cross-reactivity of the 6B PCV7 component against 6A strains.The newly recognized serotype 6C strains increased from 0.8% during 1999-2000 to 8.5% of all isolates during 2010-2011, and a similar change (from 0.5% to 8.7%) was observed in children <5 years of age.Intermediate resistance to penicillin was noted in 50% of the serotype 6C isolates obtained from all age groups and in 72% of 6C strains recovered from the youngest patient age group.Opsonophagocytic killing studies comparing responses of serum from PCV7 and PCV13 recipients to serotype 6C showed minimal response with PCV7, but a strong response was elicited by PCV13 (31).Responses to serotype 6A and 6B were strong for immune serum samples from PCV7 and PCV13 recipients (31).In the current study, lack of a major increase in the frequency of isolates with serotype 6C during 2010-2011 (8.5%) compared with 2008-2009 (7.3%; p = 0.17) suggests PCV13 cross-reactivity to serotype 6C.The trend of a relative increase in serotype 35B strains observed in this study is likely to continue because it is not included in PCV13.Serotype 35B isolates were more commonly recovered from children <5 years of age and from noninvasive specimens.Most (83%) of the 35B isolates obtained in 2010-2011 were PNSP.CDC reported that 51% of serotype 35B strains causing invasive disease during 1995-2001 were from older patients (>60 years of age) and 69% were PNSP (32).
A limitation of this study is the lack of incidence data.Only relative changes among serotypes causing disease are documented.The proximity in time of 2010-2011 data to PCV13 introduction limited our assessment of the effect of this vaccine.
On the basis of this 2010-2011 surveillance data, PCV13 could provide coverage for 48% of PNSP and 39% of all isolates causing disease.Continued monitoring of pneumococcal serotypes causing invasive and noninvasive disease will be crucial for assessing the full effect of PCV13.

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serotype distributions of the 2008-2009 and 2010-2011 isolates in comparison with those during 1999-2000 and 2004-2005 are shown in