Within a defined geographical area in Kilifi District, Kenya, we compared the incidence of invasive Hib disease in hospitalized children in the 2 years before and the 4 years after Hib vaccine was introduced into the childhood immunization schedule. As the effectiveness of the program in the first few years would be strongly influenced by the age distribution of invasive Hib disease cases we also defined the age frequency curve for invasive Hib disease as precisely as possible by using data from hospitalized cases during the 8 years preceding vaccine introduction.
The definition of the denominator population was adapted from the study area of the Kilifi Demographic Surveillance Study (DSS). At its inception the boundaries of the DSS were chosen to represent the smallest area that would include the residence of 80% of children admitted to Kilifi District Hospital. This comprised 14 administrative locations and half of a 15th location, a total area of 891 km2. The Hib vaccine effectiveness study area was confined to the 14 locations covered completely (869 km2). In the year 2000 the area was mapped by fieldworkers on motorcycles and on foot and every building structure was registered by its global positioning system (GPS) coordinates and categorized into residential household units. A census in September 2000-October 2001 defined the resident population and all subsequent births, deaths and migration events were monitored by fieldworker visits to every participating household on 8 subsequent occasions at approximately 6-monthly intervals. At each re-enumeration round the housing register was also updated by remapping and participation rates were high. For example, of 23,389 households identified in the Hib vaccine study area in the last re-enumeration round, 19 declined to participate. At the mid point of the study, 1st January 2003, the population of children aged <5 years under surveillance in this area was 37,614.
There are 10 government funded health centers within the study area and a similar number of private clinics; none of these has inpatient facilities. Sick children are normally referred to Kilifi District Hospital (KDH) for admission. Since 1994 all admissions to the 42-bed pediatric ward have been recorded and investigated in a standardized manner. Approximately 5,000 children are admitted annually from 20,000 outpatient visits. Since July 1998 all children, except non-emergency patients, have been investigated with blood cultures on admission2
. From August 1998 the clinical indications for lumbar puncture (LP) were impaired consciousness or meningism in children <5 years, prostration in children <3 years, seizures (other than febrile seizures) in children <2 years and suspicion of sepsis in children <60 days old. In March 2004, the criterion of prostration (inability to sit or drink/suck) was replaced by coma (inability to localize a painful stimulus). Sensitivity of the new LP criteria for bacterial meningitis was 79%9
Tetanus-toxoid-conjugated Hib vaccine was introduced into the Kenya EPI as part of a pentavalent formulation in which lyophilized Hib vaccine (Hiberix) was resuspended in diphtheria/tetanus/whole-cell-pertussis/hepatitis B virus vaccine (Tritanrix, SKB). Vaccine was distributed from KDH to 10 government clinics throughout the study area during October 2001 and all stocks of trivalent DTP vaccine were withdrawn, effecting the switch to Pentavalent vaccine, by 1st November 2001.
Pentavalent vaccine was targeted at children aged 6, 10 and 14 weeks. The first children eligible to receive a 6-week dose of pentavalent vaccine on 1st November 2001 were born on 20th September 2001 and would have received their 3rd dose at the end of December 2001. For population-based analyses we designated 1st January 2000–31st December 2001 the pre-vaccine period, and 1st January 2002–31st December 2005 the post-introduction period. To analyze the change in vaccine effectiveness over time we split the post-introduction period into 2 observation periods of 2 years each. To describe the age distribution of invasive Hib disease prior to vaccine introduction we analyzed all cases of invasive Hib disease admitted to KDH between January 1994 and December 2001 irrespective of their geographical residence.
Data were analyzed using STATA v8.2 (College Station, TX). The incidence of invasive Hib disease was calculated as the number of culture-confirmed cases of Hib disease admitted to KDH among residents of the study area divided by the resident population at the midpoint of each observation period. The resident population at the mid-point of each observation period was estimated from a linear regression line of the logarithmically-transformed population counts at the original census and each of the 8 re-enumeration rounds10
. Each population count was considered to have taken place on the date of the median enumeration date for the entire round and ages were calculated for the day each individual was enumerated. Vaccine effectiveness was calculated as 1-RR (rate-ratio) expressed as a percentage. Incidence rate-ratios were calculated for each of the two post-introduction periods compared with the pre-vaccine period.
To assess the effect of secular trends in the presentation and filtering of invasive bacterial disease at KDH throughout the study period we evaluated the incidence of a control condition, invasive pneumococcal disease. No pneumococcal vaccine was used in this population during 2000–2004 and in 2005 the number receiving conjugate pneumococcal vaccine was less than 100 throughout the whole study area.
The indications for undertaking a lumbar puncture changed in March 2004 reducing the number of children being investigated with cerebrospinal fluid (CSF) cultures by approximately one-third. Therefore we were unable to compare the incidence rates of culture-confirmed meningitis before and after vaccine introduction to evaluate vaccine efficacy against Hib meningitis. Because Hib meningitis is clinically indistinguishable from other causes of bacterial meningitis we assumed that the change in clinical indications would affect our detection of all bacterial meningitis cases equally. Under this assumption the odds of Hib culture in cases of probable bacterial meningitis would remain constant in the absence of vaccine use and the odds ratio in the pre- and post-introduction periods would approximate the incidence rate-ratio for Hib meningitis in the presence of vaccine. For this analysis probable bacterial meningitis was defined by a CSF white cell count ≥50 × 106
/L or a CSF/plasma glucose ratio <0.111
and the effectiveness of the vaccine program against Hib meningitis was calculated as 1-OR (odds ratio) expressed as a percentage.
As the observed incidence of invasive Hib disease did not decline perceptibly in the 2 years following vaccine introduction we investigated whether this was caused by poor vaccine coverage or by failure of the vaccine to protect children because they were HIV infected. We estimated the immunization coverage for pentavalent vaccine doses 1–3 using vaccine cards and mothers’ histories in 204 children selected at random in March 2004 from the DSS register12
. We also investigated the vaccination and HIV infection status of children who presented to hospital with invasive Hib disease in the post-introduction period. Again vaccination status was determined by vaccine card or mother’s history. An effective dose of vaccine was defined as an immunization given ≥14 days before hospital admission for dose 1, or ≥7 days before admission for doses 2 and 3. Evidence from several studies suggest that 2 effective doses are protective and we categorized patients at this threshold13–15
Throughout the study blood was cultured in BACTEC® Peds-Plus media (Becton Dickinson, NJ) in a BACTEC® 9050 instrument for 4 days. Positive samples were subcultured on 7% horse blood and chocolate agar and incubated overnight in 5% CO2
. CSF was cultured on horse blood and chocolate agar. Haemophilus species were identified by colony morphology, Gram stain, X and V factor dependence and serotyping. External quality control for microbiological laboratory standards was provided by the UK National External Quality Assessment Service (www.ukneqas.org
). Serotype results for invasive H. influenzae
isolates were confirmed in England by PCR-based capsular genotyping using primers designed to amplify the type-specific regions of the cap
loci in each of the 6 (a-f) capsular types16
Between 1994 and 2000 latex agglutination tests for Hib antigen were done on CSF specimens with a white blood cell count >10 × 106/L or a CSF/plasma glucose ratio <0.67. In January 2001, the year that Hib vaccine was introduced, the CSF criteria for latex agglutination testing were changed. Hib antigen results were not included, therefore, in the case definition evaluating vaccine effectiveness but were used to describe the age distribution of Hib cases prior to vaccine introduction.
HIV antibodies were assayed by ELISA (Vironostica, BioMerieux, France) and rapid test (Determine, Abbott Laboratories, USA). Positive samples from children <18 months old and discordant samples were assayed by PCR for proviral DNA. After July 2003, HIV testing was offered as part of standard clinical care. For children admitted before July 2003, we invited the families of surviving children to voluntary counseling and testing; for children who had died we tested stored serum samples if available.
The surveillance evaluation was approved by the KEMRI national ethical review committee and the institutional review board of the Centers for Disease Control and Prevention.