The Hib vaccine became part of the routine immunisation programme 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 immunised – 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 immunised in the programme 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 programme 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 programme 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 amongst children, some adults found themselves less equipped than before to fight the infection.
Monitoring the Hib immunisation strategy quickly exposed trends and problems with the programme, say the authors, prompting swift corrective action – in this case resulting in the implementation of a national booster vaccination programme 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 immunisable diseases.