Invasive Haemophilus influenzaeinfections in Germany: impact of non-type b serotypes in the post-vaccine era

Cases of invasive Hi infections in children younger than 10 years were collected from a hospital-based surveillance system (Hospital-ESPED) and a laboratory-based surveillance system (Laboratory-ESPED) over the period 1998–2005. Both systems are voluntary but require active reporting: the absence of cases has to be reported as well and reminders for missing reports are included on a monthly basis.

In Hospital-ESPED, physicians from all paediatric hospitals in Germany report incident cases of a number of rare conditions on a monthly basis since 1992 to a central study office[22]. Reporting on Hi was included since 1998. These reports provide information on an identification number for Hospital-ESPED, location of the reporting hospital, date of birth, age at disease onset, sex, date of disease onset, bacteriologic serotype, detection material, potential predisposing conditions and outcome as well as vaccination information.

For isolation of respective pathogens paediatric hospitals send specimen of body tissue to microbiological laboratories. From 1998–2005, all laboratories throughout Germany serving the paediatric hospitals were asked to report any detection of Hi in physiologically sterile materials in children to Laboratory-ESPED. Collected data were comparable to Hospital-ESPED: identification number for the sample as given by the laboratory, patient's month and year of birth, 3-digit postal code of patient's residence, sex, date of sample arrival at laboratory, bacteriologic serotype, material, and whether an isolate was sent to the National H. influenzae Consulting Laboratory for capsule typing, and bacteriologic serotype if available. From 2001 onwards, possible predisposing conditions and clinical information, were also collected by tracing each patient back to the reporting hospital [23].

Cultural confirmation was performed in the local laboratories participating in the surveillance programme according to their routine procedures. Local laboratories also performed PCR testing for capsule type on recovered isolates when available. Laboratories were encouraged to send their specimens to the National H. influenzae Consulting Laboratory at the Department of Paediatric Infectious Diseases, Johannes-Gutenberg-University, Mainz, Germany. Capsular typing was performed by slide agglutination using a commercial kit (Haemophilus influenzae Agglutinating Sera (a-f); Murex Biotech Ltd., Dartford, UK). The capsular type was confirmed by molecular typing with the primers and methods recommended by the European Haemophilus Reference Unit [24]. First, the capsular gene bexA and the b-specific capsular gene were detected by PCR in order to identify b^--mutants. If bexA was present the specific gene for the capsular types a-f was detected by PCR. The biotype was determined as described by Kilian [25]. If slide agglutination and PCR results were discordant, PCR results were considered final. If samples for the case were not sent to the reference laboratory but local typing results were available, they were considered final; if samples for the case were sent to the reference laboratory, the reference laboratory results are considered final. If no typing was performed, the case is considered "untyped".

The linkage between the two sources was done by hand applying defined matching rules and using the following patient characteristics: month and year of birth, sex, region of residence (ie, county and federal state vs. address of the hospital), and date of disease onset.

To describe annual trends in the different Hi serotypes we used all cases detected in the two surveillance systems from January 1998 to December 2005. Trends in cases with Hib, non-type b and untyped Hi infections were analysed using a Poisson model. Differentiation in capsulated and uncapsulated non-type b cases was not performed due to the low number of capsulated cases. Since information on potential predisposing conditions and clinical outcome was only available from January 2001 onwards for both sources, we restricted the description of the clinical characteristics for the different serotypes to those cases. For comparison between children with Hib, capsulated non-b and uncapsulated infections, a global test of difference was performed. If results were significant (p < 0.001), cases with capsulated and uncapsulated non-type b infections were pairwise compared to cases with Hib infections.

The study received the positive vote by the ethics committee of the medical faculty of the Ludwig-Maximilians University, Munich.

The total number of reported cases decreased from 51 children in 1998 to 32 in 2005 (Fig. 1). The number of untyped serotypes decreased from 13 cases in 1998 to 6 cases in 2005 (p < 0.001). Overall, type b cases decreased from 28 in 1998 to 12 in 2005 (p < 0.001). There was no trend for non-type b cases (p = 0.771). In most years non-b Hi serotypes outnumbered Hib serotypes, with the exception of 1998, 1999 and 2003. In each year, non-capsulated serotypes outnumbered capsulated non-type b ones (0–5 capsulated non-type b vs. 10–24 non-capsulated per year).

In total, 192 cases of invasive Hi in children were detected from January 2001 to December 2005. Serotyping was performed in 157 of the 192 reported cases (82%). Overall 64 cases were determined to be type b and 93 to be non-type b. Of the 93 non-type b cases, 18 were capsulated, 70 were non-capsulated (a-f negative) and 5 cases were not further specified.

Table 1 summarises the possible predisposing conditions, diagnostic characteristics, and vaccination status of the different Hi serotypes.

We found no increase of invasive diseases in children caused by non-type b Hi serotypes in a nationwide active surveillance system covering eight years of the post-vaccine-period. This is in contrast to a recently published Canadian study conducted between 1996 and 2001 during the era of universal immunisation against Hib showing that annual numbers of non-type b invasive disease increased over the years [16]. Further literature on the impact of Hib vaccination on the emergence of non-type b isolates as invasive pathogens in children is scarce, but almost all of them showed no increase of non-b serotypes in young children [14, 15, 17, 18, 20, 21]. An exception is a study done by Ribeiro and colleagues [19], who described an increase of type a Hi invasive infections from 0.02 per 100,000 person years in the year before Hib vaccine introduction to 0.16 per 100,000 person years one year after introduction. Since this 'trend' is based on one single year after vaccine introduction, the interpretation of these data should be done with caution.

Unlike the Canadian study, which was based on 12 paediatric tertiary care centres, our study was based on a population-wide active surveillance system including all levels of care hospitals. Additionally, we used two independent surveillance systems assuring a high completeness of case detection. Capture-recapture estimates for three surveillance systems reporting Hi in Germany and covering years 2001–2005 showed that the two sources, Laboratory- and Hospital-ESPED, detected 83% of all Hi cases [30].

In many surveillance systems, it is very difficult to ensure consistency of case ascertainment over long periods of time because reporting and typing of cases changes over time. In the data presented, response rates to both surveillance systems were high [12, 22] and remained relatively constant, but typing of cases changed between 1998 and 2005. A decrease in typing rates could mask a "true" increase of non-type b serotypes. That was not the case: typing rates increased between 1998 and 2005.

We have covered a post-vaccine period of eight years in our study. A potential limitation is the unavailability of detailed data on the situation before the introduction and following the first years after the introduction of Hib vaccines in Germany. Nevertheless, McConnell and co-workers [16] found increasing numbers of non-b serotypes 10 years after the licensure of the first polysaccharide Hib vaccine and 4 years after the implementation of Hib conjugate vaccines in infancy in all territories of Canada.