Population-based surveillance of Hib invasive infections in children in British Columbia Alberta and Ontario - 1995 to 1997

Abstract

OBJECTIVE:

To assess vaccine effectiveness through enhanced disease surveillance following the change in childhood immunization programs in 1995, when all provinces and territories chose to use polyribosyl ribitol phosphate-tetanus protein (PRP-T) Haemophilus influenzae type b (Hib) conjugate vaccine, generally in combination with diphtheria-pertussis-tetanus inactivated polio vaccine (DPT-IPV) (as PENTA vaccine) because the protective efficacy of this regimen had not been directly measured.

DESIGN:

Prospective, active, laboratory-based Hib case surveillance was implemented in British Columbia and Alberta, and enhanced, stimulated laboratory surveillance in Ontario during 1995 to 1997, centred on invasive infections in children. Case details and immunization histories were uniformly collected and centrally collated.

MAIN RESULTS:

Thirty-eight Hib cases were detected, but only 12 cases arose among PENTA-eligible children, an attack rate of 0.85 cases/100,000 child-years of observation. Annual case totals declined from 20 in 1995 to seven in 1997, when only one to three cases were encountered in each province and the incidence rate in children under age five years was 0.6/100,000. Only four cases occurred after primary immunization with PENTA, a failure rate of 0.28 cases/100,000 child-years of observation. Three cases among PENTA-eligible children reflected parental refusal of infant vaccinations, accounting for 25% of cases in eligible children.

CONCLUSIONS:

PRP-T conjugate vaccine was highly effective when given in combination with DPT-IPV vaccine. Provincial programs that used this regimen resulted in the near elimination of invasive Hib disease in children, but unimmunized children remain at risk.

Haemophilus influenzae type b (Hib) was, until recently, the principal cause in children of purulent meningitis, and its sequelae of deafness and mental impairment (1,2). Hib also accounted for most cases of epiglottitis and a substantial proportion of the cases of pneumonia, bacteremia, cellulitis and septic arthritis (3,4).

The control of Hib infections through immunization has been one of the major public health advances of the past decade (5,6). Disease control was achieved with a series of increasingly effective vaccines, starting with a plain capsular polysaccharide vaccine licensed in 1986 (7). The polysaccharide vaccine, containing polyribosyl ribitol phosphate (PRP), was able to protect by eliciting bactericidal and opsonizing antibodies but could not do so in children younger than 24 months of age, the group at greatest risk of Hib infection.

In 1988, a novel PRP Hib diphtheria toxoid conjugate vaccine (PRP-D) (ProHIBit, Pasteur Mérieux Connaught, Toronto, Ontario) was licensed for use in children from 18 months of age (8,9). It was incorporated into routine childhood immunization programs in all provinces and territories except Manitoba. In 1992, the current generation of Hib conjugate vaccines was licensed in Canada (10), including PRP-tetanus (PRP-T) (Act-HIB, Pasteur Mérieux Connaught), PRP-meningococcal (PRP-OMP) (PedvaxHIB, Merck Frosst Canada Inc, Kirkland, Quebec), and CRM₁₉₇ oligosaccharide (HbOC) (HibTITER, Wyeth-Ayerst Canada Inc, Ste-Laurent, Quebec) Hib conjugate vaccines, all of which were immunogenic enough to use in infants as young as two months of age. Each product initially found a market in one or more jurisdictions, but in 1995, all provinces and territories chose to use PRP-T vaccine, generally in combination with diphtheria-pertussis-tetanus inactivated polio vaccine (DPT-IPV) (as PENTA, Pasteur Mérieux Connaught) (11). Manitoba opted to use a combination of DPT and PRP-T with oral poliomyelitis vaccine.

When PRP-T vaccine was selected for use by all provinces and territories, evidence for its protective efficacy was indirect, based on studies of immune responses (12,13). Efficacy was assumed because responses elicited by PRP-T matched or exceeded responses elicited by other Hib vaccines with demonstrated protective efficacy. Whether protection would be reduced by combining PRP-T with DPT-IPV was uncertain because serum anti-PRP levels were somewhat lower after the combination than after the separately injected vaccines (13). Also of concern was the observation that anti-PRP levels in serum declined substantially following completion of the three-dose primary series at six months of age. By the time of the recommended booster dose at 18 months of age, anti-PRP levels were undetectable or low (less than 0.15 μg/mL) in 27% to 45% of children (14,15). The possibility existed that breakthrough infections might occur during this serum antibody nadir, although the minimum antibody level required for protection has not been determined for Hib conjugate vaccines. Some experts argued that because such children responded strongly to booster immunization, evidence of persistent immunological memory, susceptibility to infection was unlikely (16).

In 1994, Pasteur Mérieux Connaught sponsored several postmarketing surveillance projects to gauge the effectiveness of PRP-T vaccine given as the PENTA combination product. The study described here involved an innovative collaboration between three provinces and the Canadian Paediatric Society/Laboratory Centre for Disease Control Immunization Monitoring Program, ACTive (IMPACT) (17). The latter had been conducting Hib case surveillance at paediatric tertiary care centres across Canada since 1992 (6). The purpose of this study was to assess program and vaccine effectiveness during the initial three years of PENTA vaccine use in a large population.

PATIENTS AND METHODS

Population-based surveillance was accomplished by enlisting all relevant hospital laboratories into surveillance networks, tailored to circumstances in each participating province.

Each province carried out surveillance from January 1, 1995 to December 31, 1997, using the same case definitions. A case was considered definite if Hib was isolated from blood, cerebrospinal fluid (CSF) or other normally sterile body fluid, or from the surface of an inflamed epiglottis. In probable cases, Hib was isolated from tracheal secretions obtained through an endotracheal tube in children with pneumonia, or PRP antigen was detected in urine or CSF, except in recently vaccinated individuals. The age range was set between birth and 14 years. A vaccine failure was defined as the onset of Hib infection 28 or more days after the completion of age-appropriate primary immunization.

Isolates were identified by hospital laboratories in a routine fashion. Smaller centres referred isolates to a regional or provincial laboratory for confirmatory testing and serotyping.

In British Columbia and Alberta, specific laboratory-based surveillance networks were set up and monitored from the IMPACT hospitals in Vancouver, Edmonton and Calgary.

In British Columbia, 38 hospitals and the provincial laboratory were included in the network, encompassing all regional hospitals and all hospitals staffed by a paediatrician or having four or more paediatric beds. IMPACT program staff at BC's Children's Hospital in Vancouver contacted laboratory representatives by fax at one to three month intervals, depending on centre size. The fax reply form asked if a case had been diagnosed since the previous contact. If so, basic case information was included to enable the IMPACT monitor to request a case summary and detailed immunization history from the attending physician. Information was abstracted using a specific case report form, a copy of which was sent to the IMPACT data centre for review and collation. All replies from laboratories were tracked to avoid discontinuity.

In Alberta, all hospital laboratories in the newly regionalized health care system were included, along with the two provincial laboratories. Northern centres were contacted by the IMPACT monitor at the Health Sciences Centre in Edmonton, southern centres by the monitor at Alberta Children's Hospital in Calgary. Surveillance was carried out as above.

In Ontario, five laboratories located in tertiary care centres carried out serotyping of H influenzae isolates identified at their facility. All remaining Ontario laboratories submitted isolates to the public health laboratory for confirmation of serotype. For the purpose of active Hib surveillance, private and hospital laboratories were notified and reminded of the requirement under legislation to report all cases of invasive Hib disease to the local medical officer of health, who in turn reports to the Ministry of Health. Additionally, as each case report was received at the ministry, a nurse epidemiologist based at the Ministry of Health contacted the medical officer of health, the regional and central public health laboratories, and IMPACT hospitals in Toronto and Ottawa, as appropriate, to confirm and complete data related to the case. The same case report form mentioned above was used. Each of the five self-sufficient serotyping laboratories were contacted once a year to verify that all confirmed cases of Hib had been reported centrally.

At the IMPACT data centre, reports were checked for completeness and eligibility, and then entered into an electronic database. Duplicates were identified and combined.

RESULTS

During the three-year surveillance period, approximately 706,000 children were born in the participating provinces. The period of observation of these children ranged from birth to 36 months, with a median of 18 months. Based on 1996 census data, the three provinces had approximately 1.18 million children under five years of age (at highest risk of Hib infection) and about 3.5 million children under 15 years of age, yielding a total of about 10.5 million child-years of exposure to Hib infection during the survey.

Compliance of hospitals and physicians with case reporting was excellent. In British Columbia, one medium-sized hospital declined to participate, whereas all relevant hospitals in Alberta participated. Responses of laboratories to information requests were generally timely. The system in Ontario appeared to work well, given the built-in redundancies for case notification. Key information was obtained for every reported case.

In total, 38 eligible Hib cases were reported, 35 definite and three probable (positive endotracheal culture). The latter included one case of croup with suppurative tracheitis, from which Hib was the only isolate. The male to female ratio was 24:14, with males predominating in each year. Table 1 shows the distribution of cases and average annual incidence rates per province. The annual case total declined each year to a low of seven in 1997, when only one to three cases were encountered in each province. The incidence rate of infection in 1997 per 100,000 children under five years of age (based on 1996 population census figures) was 0.4 in British Columbia, 0.4 in Ontario and 1.5 in Alberta, with an overall mean of 0.6.

TABLE 1.

Haemophilus influenzae type b cases reported in three provinces from 1995 to 1997

Province	1995	1996	1997	Total	Average annual incidence rate*
British Columbia	7	5	1	13	0.6
Alberta	4	0	3	7	0.4
Ontario	9	6	3	18	0.3
Total	20	11	7	38	0.4

^*Per 100,000 persons, aged birth to 14 years

Among PENTA-eligible children (ie, those born during the survey period), 12 cases were reported, an attack rate of 0.85/100,000 child-years of observation. Provincial figures are presented in Table 2.

TABLE 2.

Haemophilus influenzae type b (Hib) cases among children eligible for PENTA vaccination from 1995 to 1997

Province	Birth cohort annual	Hib cases (total)	Attack rate/100,000 CYO	PENTA failures	Failure rate/100,000 CYO
British Columbia	48,880	3	1.0	1	0.3
Alberta	39,720	4	1.7	1	0.4
Ontario	146,835	5	0.6	2	0.2
Total	235,435	12	0.85	4	0.28

CYO Child-years of observation

The presenting syndromes are indicated in Table 3. Meningitis remained the most common syndrome, present in 37% of cases. Epiglottitis was a close second, accounting for 32% of cases. There were no deaths. Sequelae of meningitis were not determined.

TABLE 3.

Haemophilus influenzae type b syndromes reported from 1995 to 1997

Syndrome	Ontario	Alberta	British Columbia	Total
Meningitis	10	3	1	14
Epiglottitis	6	2	4	12
Pneumonia/tracheitis	1	2	4	7
Cellulitis	2*	0	3	3*
Other, bacteremia	0	0	3	3
Total	19*	7	13	39*

^*One child had both cellulitis and meningitis

The age distribution of cases and age incidence of infection are summarized in Table 4. Only seven cases (18.4%) occurred in children too young to have completed primary immunization; 81.6% occurred in children under five years of age. The oldest case was in a patient 10 years of age.

TABLE 4.

Age distribution of cases of Haemophilus influenzae type b at admission and three-year average age incidence rates

Age	1995	1996	1997	Total (%)	Age incidence rate*
Birth to 6 months	4	0	3	7 (18.4)	2.0
7 to 24 months	9	2	3	14 (36.8)	1.3
25 to 60 months	3	6	1	10 (26.3)	0.5
5 to 9 years	3	3	0	6 (15.8)	0.17
10 to 14 years	1	0	0	1 (2.6)	0.03
Total	20	11	7	38 (100)	0.4

^*Per 100,000

Immunization history was available for each case. Twenty children (52.6%) had completed appropriate primary Hib immunization, but two were severely immunocompromised at disease onset and two had missed recommended booster immunizations. The number of cases occurring after appropriate primary immunization declined progressively, from 11 in 1995 to seven in 1996 and to two in 1997. Products associated with immunization failure included polysaccharide given at 24 months of age (one case), ProHIBit (PRP-D) at 18 months (five cases), HibTITER (HBOC) at 18 months (two cases), unnamed product at 18 months (three cases), HibTITER primary series (four cases, with booster dose missed in two instances) and ActHib (PRP-T) primary series (five cases). Eleven of 20 failures followed single-dose regimens used for toddlers, and 15 instances (75%) involved products not currently used in Canada. Of the five cases that occurred after primary immunization with PRP-T, all occurred between the ages of 12 and 15 months and four followed the use of PENTA combination vaccine (Table 2). Of the PRP-T failures, only one to two instances occurred per year and per province. The only instance in British Columbia involved a severely immunocompromised child.

The preventability of the remaining 18 cases was assessed. Seven were too young to complete primary immunization, and another had just emigrated to Canada from a country where Hib vaccination was not offered. Two infants failed to complete the primary series on schedule. In five instances, the single dose meant for toddlers had been missed by the vaccination provider when other immunizations were given. Three cases occurred among PENTA-eligible children whose parents refused immunizations, accounting for 25% of cases among eligible children.

When the case experience of the IMPACT hospitals was compared with the total number of Hib cases in the corresponding province, the two sites in Alberta accounted for 100% of cases, the one site in British Columbia for 38.5% (five of 13) and the two sites in Ontario for 22.2% (four of 18).

DISCUSSION

This survey demonstrated remarkably low levels of Hib disease activity from 1995 to 1997 in three provinces using PRP-T vaccine, although the majority of cases encountered reflect on earlier programs using other Hib vaccines and schedules. Provincial case totals declined during the survey, reaching lows of three or fewer by 1997. In 1996, no cases were detected in Alberta. The incidence rate of Hib cases in 1997 among children under five years of age, the group at greatest risk, was 0.6/100,000 in the three provinces, in striking contrast with Canadian rates in the prevaccine era of 40 to 64/100,000 for this age group (18,19). Among PENTA-eligible children, the reported rate was 0.85 cases/100,000 child-years of observation.

Surveillance of invasive Hib infections is facilitated by the severity of disease caused in children. Because cases are rarely managed outside of hospital, hospital-based case ascertainment is appropriate. Moreover, cases almost always have a positive culture of blood or other normally sterile body fluid, providing a straightforward case definition and permitting laboratory-based surveillance as an efficient means of case ascertainment. Recovery of Hib organisms is occasionally thwarted by prior antibiotic therapy, but many such cases have detectable Hib capsular antigens in body fluids (20). Cases that were only Hib antigen-positive were reportable in this survey, but no instance was encountered. Although laboratory identification of H influenzae is straightforward, relatively few laboratories continue to stock antisera to identify type b organisms. In this survey, measures were in place to ensure that invasive Haemophilus isolates were fully identified at regional or provincial laboratories, or academic centres.

The surveillance networks in British Columbia and Alberta were designed to include all relevant hospitals. In Alberta, all hospital laboratories were included because regionalization made it feasible. In British Columbia, only the smallest community hospitals (less than four paediatric beds) were excluded, on the assumption that they would refer severely ill children to regional centres. One Vancouver area hospital did not participate but reported no cases to public health authorities during the survey period. Excellent cooperation was provided by the hospitals in responding to information requests. This was facilitated by the low frequency of cases and the limited information expected from laboratory personnel when cases were reported. Monitors faced some challenges in obtaining case details from attending physicians but were ultimately successful in every instance.

The surveillance system in Ontario, which relied on primary reporting of Hib cases from laboratories with back-up reliance on physician reporting, appeared to work as well as the systems in British Columbia and Alberta. The observed average annual incidence of infection differed no more than twofold between Ontario and the other provinces, and the incidence rate in 1997 was identical in Ontario and British Columbia. Physicians readily cooperated with case information requests from their local medical officer of health so that all desired information was available.

In none of the provinces was it feasible to confirm case ascertainment by independent means, apart from comparison with cases routinely reported to health authorities in the western provinces and annual audits at the five IMPACT hospitals.

The very low observed disease incidence rates indicate a high level of program and vaccine effectiveness. The latter could not be calculated directly because comparable pre-immunization era data are not available and intervening programs have modified disease risk. Few cases represented program failures (ie, resulted from nonvaccination or delayed vaccination), especially among children born during the survey period, but 25% of PENTA program failures had not been vaccinated because the parents refused. The number of cases declined progressively during the survey, largely because fewer failures attributable to Hib vaccines used before 1992 were encountered. In particular, PRP-D vaccine was less effective than current vaccines (8), but recipients were no longer in the age group at substantial risk of Hib infection. The number of PRP-T-related vaccination failures was remarkably low, only one to two per year and per province, amounting to less than one failure/100,000 infants immunized (Table 2). The observed PENTA failure rate was quite uniform among the three provinces (Table 2), suggesting consistent vaccine effectiveness. Among the failures detected, all occurred before booster immunization. Cases occurring after booster immunization are rare, but two instances have been reported to date by the full IMPACT network (21).

Recent publications also support a high level of effectiveness of PRP-T vaccine. A controlled, community intervention study in the Oxford region of the United Kingdom (22) demonstrated a vaccine efficacy of nearly 100% (95% CI 80% to 100%). Only one vaccine failure was detected during 16,484 child-years at risk. Nationwide immunization of infants in The Netherlands with PRP-T reduced the case total by 94% (23). Only two vaccine failures were detected during three years of surveillance. A randomized trial of PRP-T vaccine in Gambian infants demonstrated 95% protective efficacy for all types of invasive disease after three doses (24). A large scale, postlicensure study of Chilean infants also demonstrated 90.2% protection against invasive Hib disease (95% CI 74.5% to 100%) in an intention-to-vaccinate analysis (25). Additional evidence suggests that PRP-T induces immunological memory even when there is no measurable humoral response to the primary course or the response is not sustained afterward (26).

The pattern of illness caused by Hib was similar during the survey period to the preimmunization era (1,3), with most cases occurring in young children and with meningitis and epiglottitis as the most prevalent syndromes. Compared with PENTA program participants, those who refused vaccination had a relative risk of disease of 10.8 (P<0.005, 95% CI 2.9 to 39.8), based on an assumption that 3% refused vaccination. These observations suggest that Hib has lost none of its virulence despite the low frequency of infection.

CONCLUSIONS

PRP-T Hib vaccine, given as the PENTA combination product with DPT-IPV, has been highly effective. Hib disease incidence after three years of using this product reached an all time low in the provinces surveyed. Failures after completion of a three-dose primary series were rare. Follow-up in this survey was limited; the long term effectiveness of PRP-T will require extended disease surveillance. With the current low incidence of disease, targeted surveillance through systems such as IMPACT will be more practical than population-based surveillance.