Recommendations for UseThe IAPCOI recommends offering the Hib vaccine to all parents who can afford the vaccine (Category 2). The Government of India has also approved introduction of Hib vaccine in EPI in a phased manner. The vaccination schedule for Hib consists of three doses when initiated below 6 months, 2 doses between 6-12 months and 1 dose between 12-15 months, with a booster at 18 months. For children aged more than 15 months a single dose suffices. The interval between two doses should be at least 4 weeks. As Hib disease is essentially confined to infants and young children, catch up vaccination is not recommended for healthy children above 5 years. However the vaccine should be administered to all individuals with functional/anatomic hyposplenia irrespective of age. Hib vaccines are now used mostly as combination vaccines with DTwP/DTaP/Hep B/IPV details of which will be discussed separately. |
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Capsulated H. influenzae has six serotypes of which type b is most important. Hemophilus influenzae type b (Hib) is an important invasive pathogen causing diseases such as meningitis, bacteremia, pneumonia, cellulitis, osteomyelitis, septic arthritis and epiglottitis. Most of invasive Hib disease occurs in children in the first two years of life and natural protective immunity is acquired by the age of 3-4 years. Non capsulated Hib disease including, bronchitis, otitis media, sinusitis and some pneumonia is not amenable to prevention at present and can occur at all ages.
The burden of Hib disease is underestimated in India as cultures are often not sent, the organism is difficult to culture especially when antibiotics have been administered and as a large proportion of pneumonia may be non bacteremic. Data from the Invasive Bacterial Infections Surveillance (IBIS) group from six referral hospitals in India show that Hib is a common cause of meningitis in India.
All Hib vaccines are conjugated vaccines where the Hib capsular polysaccharide (polyribosyl ribitol phosphate or PRP) is conjugated with a protein carrier so as to provide protection in the early years of life when it is most needed.
Currently available vaccines include Hb OC (carrier CRM197 mutant C. diphtheria toxin protein), PRP–OMP (carrier N meningitidis protein outer membrane complex) and PRP–T (carrier tetanus toxoid). PRP–D has been withdrawn due to relatively poor efficacy. HbOC and PRP–T vaccines show only a marginal increase in antibody levels after the first dose with a marked increase after the second and even better response after the third dose. On the other hand, PRP–OMP shows an increase in antibody level after the first dose itself with only marginal increases after the second and third doses. The onset of protection with PRP–OMP is thus faster. Additionally while 3 doses of HbOC and PRP–T are recommended for primary vaccination, only 2 doses of PRP–OMP are recommended for this purpose. Only HbOC and PRP–T are currently available in India.
The vaccines should be stored at 2 to 8° C and the recommended dose is 0.5 ml intramuscularly. Efficacy trials have demonstrated 90-100% efficacy against culture proven invasive Hib disease for 1 year after vaccination. A trial in Gambian infants has shown 21% protection against episodes of severe pneumonia. The serologic correlate of protection at the time of exposure has been fixed at 0.15 ěg/ml and that for long term protection as 1ěg/ml. Side effects are mild and usually local. Developed countries where the vaccine was introduced for universal immunization have witnessed virtual elimination in Hib disease with no serotype replacement. The vaccine has also been shown to impart herd protection by reducing nasopharyngeal carriage.
A notable exception to the Hib success story was an increased incidence of Hib disease in vaccinated children between the years 1999-2003 in the UK occurring after a remarkable initial decline in Hib disease in the early 1990’s. Most of the cases of invasive Hib disease occurred in the late 2nd year of life. The major factor responsible for this phenomenon was omission of the 2nd year booster.
Vaccine induced immunity wanes over time and
reduced carriage of the organism in the environment
compounds the problem by lack of natural boosting. It is also
now recognized that immunological memory is insufficient
for protection against Hib disease. Hence a booster dose is
mandatory for sustained protection.