Cases of the Hib infection (haemophilus influenzae type b) among children and adults have risen in recent years in England and Wales, despite a vaccination program which initially proved successful, say researchers in this week's British Medical Journal.
The Hib vaccine became part of the routine immunization program for babies in 1992, cutting deaths from the disease amongst children significantly and reducing infection rates. In 1992 for instance 21.91 cases per 100,000 children were reported, dropping to 0.65 by 1998.
Adults, most of whom were not immunized, also benefited, with Hib infections in the population at large also dropping from 0.17 cases per 100,000 to 0.03 in the same period. Although infection in adults is rare, infection rates are higher among the age groups in regular contact with children, suggesting that adults usually catch infection from these young children, say the authors.
From 1998 however, Hib cases in children started to rise significantly, mostly among those who were immunized in the program as babies, though infection rates remain well below those seen before vaccination was introduced. Among adults, rates have now reached levels higher than before the vaccination program was introduced (0.27 cases per 100,000 in 2003, compared with 0.17 in 1992.)
The apparent fall in the effectiveness of the initial vaccination program for children may be due to several factors, say the researchers, including issues with the vaccine used, which has now been changed. For adults the situation was more complex. They had initially benefited from herd immunity say the authors, where the drop in infection rates among children resulted in reduced exposure to the disease for adults, and consequently fewer infections. But adults' reduced exposure to the disease also meant their antibody levels, or natural immunity, was no longer being boosted. When the disease began to rise once again among children, some adults found themselves less equipped than before to fight the infection.
Monitoring the Hib immunization strategy quickly exposed trends and problems with the program, say the authors, prompting swift corrective action -- in this case resulting in the implementation of a national booster vaccination program for children younger than four years, which should once again benefit adults. Such high quality surveillance systems are vital, say the authors, in the fight against immunizable diseases.
Source: British Medical Journal
