Temporal Variations among Invasive Pneumococcal Disease Serotypes in Children and Adults in Germany (1992–2008)

Nationwide surveillance of invasive pneumococcal disease has been conducted in Germany since 1992. From 1992 to 2008, a total of 12,137 isolates from invasive pneumococcal disease were collected. Data on serotypes were available for 9,394 invasive isolates. The leading serotypes were serotypes 14 (16.5%), 3 (8.0%), 7F (7.6%), 1 (7.3%), and 23F (6.0%). Variations in serotype distribution over the years are particularly extensive, especially concerning serotype 14 (min 7.4%, max 33.5%) with the highest percentages among the isolates serotyped from around 1997 to 2006. Serotypes 1 and 7F increased over the last decade. No increase was observed concerning serotype 19A. Higher pneumococcal conjugate vaccine coverages were observed among children (7v, 57.3%; 10v, 72.8%; 13v, 83.5%) than among adults (7v, 39.9%; 10v, 55.5%; 13v, 73.5%). The temporal variations in serotype distribution have to be kept in mind when interpreting vaccine coverages reported in epidemiological studies.


Introduction
Streptococcus pneumoniae is one of the most important pathogens in bacterial pneumonia, sepsis, and meningitis worldwide [1]. Significant temporal [2][3][4] and regional [5][6][7] variations among pneumococcal serotypes have been described. While the "epidemic" serogroups (serogroups 1-3 and 5) decreased considerably in the United States during the last century, the serogroups included in the seven-valent pneumococcal conjugate vaccine (PCV7) increased clearly [2]. A similar trend has been described in Spain, where the prevalence of PCV7 serotypes increased significantly (except serotype 4) since the start of the study in the early 1980s but then decreased considerably in the 2000s for all PCV7 serotypes except serotype 23F. Among the "epidemic" serotypes, a significant decrease of serotypes 1 and 5 has been observed in the 1980s followed by a significant increase in the late 1990s, while serotype 3 decreased continuously during the observational period. Furthermore, serotypes 6A, 7F, and 19A increased significantly since the late 1990s [4].
The widespread use of antibiotics and the increasing application of pneumococcal conjugate vaccines (a general recommendation of pneumococcal conjugate vaccination for children <2 years in Germany was issued at the end of July 2006) will have an impact on future changes in serotype distribution.
The NRCS has conducted surveillance for invasive pneumococcal disease in Germany since 1992 [8]. Between 1992 and 1996 surveillance was based on a limited number of laboratories in Germany on a voluntary basis [9]. In 1996, nationwide population-based surveillance of IPD in children was started [10]. This surveillance system was extended to adults in one federal state (North-Rhine Westphalia) in 2001 [11] and successively broadened to nationwide surveillance until 2009.
The aim of this study was to evaluate the serotype distribution of S. pneumoniae among the isolates with invasive pneumococcal disease (IPD) that were sent to the German National Reference Center for Streptococci (NRCS) between 1992 and 2008, analyze temporal trends, and calculate the 2 International Journal of Microbiology Table 1 (a) Serotype distribution of IPD in Germany (1992-2008, n = 9,394) in all age groups.      Isolates ( vaccine coverages for the 7-valent, 10-valent, and 13-valent pneumococcal conjugate vaccines.

Study Design.
In this study data about invasive disease caused by Streptococcus pneumoniae in children (<16 years) and adults (≥16 years) in Germany were included using the data sources described above. Cases from January 1, 1992 to December 31, 2008 were included in this study. A case of IPD was defined by the isolation of S. pneumoniae from a normally sterile site.

Microbiological Investigations.
Isolates were identified using standard procedures including bile solubility and optochin sensitivity. As a control strain, Streptococcus pneumoniae ATCC 49619 was used. Pneumococcal isolates were serotyped by the Neufeld's Quellung reaction using type-and factor-specific antisera (Statens Serum Institut, Copenhagen, Denmark). Among isolates of adults, high levels of resistance were a main trigger for initiation of serotyping during the early years of this study. Since cross-reactive serotypes were not included in the calculation, coverage information strictly refers to the serotypes included in the vaccines. Serogroup 6 isolates were not tested with respect to serotype 6C.
Over the years the percentage of isolates serotyped increased continuously and in the last four years of the study almost all isolates were serotyped. Serotype 14 is considerably more frequent among children (22.5%) than among adults (13.7%). Overall, an increase of serotype 14 can be noticed from around 1997 to 2006 (Figure 2(a)). A high percentage of serotype 14 isolates remain in existence for a longer time among children (1999-2006, Figure 2 of serotypes 1 and 7F reach the highest values during this study in 2008, among adults they are high, but still within the range already noted before (Figures 3(a)-3(c)). Serotype 3 is far more common among adults (10.2%) than among children (3.3%) (Tables 1(b)-1(c)). The increase observed during the last years seems to be slightly higher among adults than among children (Figures 4(a)-4(c)). Concerning serotype 19A no clear change in frequency can be observed during the period under study (Figures 4(a)-4(c)).

Discussion
In this paper we present the results of 17 years of surveillance concerning serotypes of invasive pneumococcal disease in Germany.
In the present study serotype 14 was the most prevalent serotype among children, followed in frequency by the serotypes 1, 6B, 19F, 23F, and 7F, respectively. These data are similar to those published for German children previously [12,13] and in line with data from England [14], Belgium [15], and Denmark [16]. Among adults, the most frequent serotypes were 14, 3, 7F, 1, 4, and 23F (sorted in descending order). These results are similar to a previous study from the NRCS among adults in North-Rhine Westphalia, Germany, between 2003 and 2006 [17], and are generally in line with results reported from other countries [6,15,[18][19][20]; however, they deviate in part from older German data [12,21].
The variation of serotype 14 over the years is extensive in our study with an increased prevalence from about 1997 to 2006, reaching maximum values in 2000. Although the rate of serotyped isolates among adults was low from 1999 to 2001 (1999, 22.9%; 2000, 26.6%; 2001, 38.6%), the increase seems plausible. First, the increase among adults is paralleled by an increase among children, where nearly all isolates have  been serotyped. Second, the rate of isolates serotyped among adults is within the range serotyped in the years before 1999, where a similar increase of serotype 14 was not observed. Furthermore, similar data have been shown in a report from Spain [4]. Also, a small rise in frequency of serotype 14 has been reported from Denmark in 1995-1999 [16], and data from England demonstrated a rise in incidence in 2000 and 2001 [14]. The rise of serotypes 1 and 7F for nearly one decade is similar to results from Spain [4].
Concerning serotype 19A, the highest prevalences were observed in 1996 (8.3%) and 1998 (6.2%) among children, and in 1999 (7.5%) and 2000 (7.4%) among adults. Although there seems to be a slight upward movement during the years 2002 to 2008, the prevalences still are below the maximum values reported. The increase of serotype 19A reported from other countries [4,22,23] has not been found in Germany so far.
Although changes in the incidence of different serotypes have been reported, the reasons for this are not fully understood [14]. Potential reasons discussed are changes in socioeconomic conditions, antibiotic use and resistance levels, immunocompromised status of populations, precise age distributions of the populations studied and increased life expectancy, and blood-culturing rates [2,6,24]. Since the general recommendation of pneumococcal conjugate vaccination for children <2 years in Germany at end of July 2006 a reduction in the percentage of IPD caused by the 7-valent vaccine serotypes was observed [25]. This effect is more apparent among children, but also present among the adult population. Similar results have been reported from other countries [3,4,[26][27][28][29]. Logically, the percentage decrease in vaccine serotypes is paralleled by a percentage increase in the amount of IPD caused by nonvaccine serotypes, which was most prominent for serotypes 1 and 7F in children and serotype 3 in adults in this study. Serotype coverage for the 7-valent conjugate vaccine in Germany was 45.4% from 1992 to 2008. Calculated coverages for the 10valent vaccine and 13-valent vaccine are 60.9% and 76.6%, respectively. These data are comparable to those previously reported [5,14,30,31]. Coverages of all three vaccine formulations in our study are approximately 15% (7v, 17.4%; 10v, 17.3%; and 13v, 10.0%) higher among children than among adults. Differences in age distribution among study populations are known to have a major impact on vaccine coverage of the current and proposed vaccines for different age groups and, furthermore, comparable data have been published [14].
Nevertheless, some factors and limitations must be regarded when interpreting the results of this study. First of all, the isolates were sent by the participating laboratories on a voluntary basis, as participation in surveillance is not mandatory in Germany. Furthermore, the systematic sampling of invasive isolates from adults (1992) and children (1997)   (Bavaria and Saxony). Among isolates of adults, high levels of resistance were a main trigger for initiation of serotyping during the early years of this study. Therefore, the isolates may not be fully representative of all IPD in Germany over the last two decades.
Moreover, the structure of the surveillance project has been continousely improved over the 15 years and, in particular, after the general recommendation of pneumococcal conjugate vaccine for children <2 years in Germany at the end of July 2006 an increased disease awareness by both clinical micobiologists and pediatricians was observed [32].
Ongoing nationwide surveillance is necessary to observe further developments of pneumococcal serotype distribution in Germany.