What is the efficacy of vaccination for the prevention of Haemophilus influenzae type b (Hib) meningitis?

Updated: Jul 09, 2018
  • Author: Prateek Lohia, MD, MHA; Chief Editor: Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM  more...
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Answer

Several studies have demonstrated a significant reduction in the rate of carriage after vaccination. Carriage of the organism increases the risk of infection in the colonized individual. Reduction in rates of carriage also reduces the exposure to other children who may be at risk. The achievement of reduced nasopharyngeal carriage in older children, who received conjugated vaccines before their approval for use in infants, may account for the fact that many studies showed a decline in incidence of Hib meningitis in infants who were not as yet eligible for vaccination.

Several studies in the United States and abroad have demonstrated a significant reduction in the incidence of invasive Hib infection soon after the introduction of the vaccine. Within the United States, the incidence of invasive Hib diseases has fallen from 85% to 90%. These results have been reproducible in both regional and multistate studies and are not accounted for by interannual variations. The population that received the greatest benefit is that consisting of infants younger than 14 months, a group with the highest incidence of Hib meningitis.

North American immunization recommendations now include as many as 24 vaccines to be administered in an injectable form by 18 months of age. Because of pain as well as compliance, combination vaccines have been recommended where such combinations have been found to be effective.

Among these vaccines, a particularly important combination is that which immunizes against diphtheria, tetanus, pertussis, polio, and Haemophilus influenzae type b (DTaP-IPV/Hib). When instituted in a proper and complete schedule, this combination vaccine has been shown to be safe and effective for primary infant immunization and toddler booster immunization. [30]

Canada, which has had an immunization program since 1992, has discerned a shift in population prevalence for Hib meningitis, with cases occurring more frequently in infants younger than 6 months. Two thirds of cases occur in individuals with no or incomplete vaccination (due to age, parental refusal, or other delaying circumstances). However, some cases occur in individuals who have completed the primary series of immunizations.

It has also been demonstrated in Canada that the conjugate vaccine efficacy is not affected by coadministration of other typical age-indicated vaccinations. Higher case-fatality rates are observed in the postimmunization epoch in Canada and in older individuals, and two thirds of these cases occur in males. [31]

Studies in the Netherlands have detected a disturbing trend toward an increase in the rate of invasive Hib disease in children younger than 5 years. The increased annual incidence is from 0.66 cases per 100,000 in 1998 to 2.96 cases per 100,000 in 2001. The investigators are concerned that this increase is due to the change from the use of whole-cell pertussis vaccine to the conjugate DTaP-Hib vaccine. This newer vaccine has been associated with the achievement of lower levels of anti-Hib antibodies, although in the Netherlands that effect has not been observed. [32]

Unfortunately, even vaccination producing “adequate” Hib antibody levels may in rare instances not prevent the development of severe Hib infection, as has been observed recently in a case of fatal Hib septic purpura fulminans. [28]

Despite effective reduction in the incidence of disease, the case-fatality rate has remained about the same in the United States in the era of effective vaccination as it was prior to the availability of an effective vaccine. However, fewer deaths related to Hib meningitis in vaccinated populations have occurred annually since the number of cases has been so greatly reduced.

On the other hand, in developing nations, the effect of vaccination on case-fatality and case-morbidity rates may be expected to be much higher, since these outcome measures are so much worse in nations where diagnosis and treatment may be delayed due to the inadequacies of transportation and medical infrastructure.

Moreover, in developing nations the rates of antibiotic resistance (which increase morbidity and mortality) is high and steadily increasing. In Pakistan, where 35% of childhood meningitis is Hib, occurring mostly in the first year of life, the rates of Hib resistance to antibiotics is approximately 33% for ampicillin, 22% for chloramphenicol, and 49% for cotrimoxazole. [19]

The increasing role of nontypeable strains of H influenzae, for which no effective immunization is available, has been noted. So has recognition of such typeable strains as H influenzae type f (Hif), suggesting that the place of Hib as the overwhelmingly most common cause of invasive disease due to H influenzae may be taken to some degree by other capsular types. It is troubling that there has been possible clonal expansion of several strains of Haemophilus that are the same in the United States and Denmark. [33]


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