Trends in Haemophilus influenzae type b infections in adults in England and Wales: surveillance study. McVernon J, Trotter CL, Slack MPE, and Ramsay ME. British Medical Journal, 2004;329:655-658.
Explanatory note: Haemophilus influenzae type b (Hib) is a bacterium that can cause many serious and life threatening infections, especially in infants and young children. Hib is transmitted from person to person through mucus droplets that are spread by coughing or sneezing. The introduction of Hib conjugate vaccines led to a rapid decline in Hib disease. Hib vaccine not only protects the individual but also provides community immunity, preventing disease in the unimmunized, including adults, presumably by reducing the number of people carrying—and therefore potentially spreading—Hib.
A specific Hib vaccine to immunize some children in England and Wales proved to be less immunogenic (protective) than expected when given at 2,3 and 4 months with DTaP vaccine. The use of this vaccine coincided with an increase in the number of cases of children with Hib disease since 1998.
What was the number of invasive Hib infections in individuals aged 15 years of age or older in England and Wales between 1991 and 2003?
Researchers from the UK’s Health Protection Agency identified Hib cases in England and Wales—reported between 1991 and 2003—through data from the Haemophilus Reference Unit and the Communicable Disease Surveillance Centre.
The researchers also measured population immunity to Hib in adults by measuring Hib antibody concentrations from 100 blood samples taken from 30-39 year old men and women for each of the years 1991, 1994, 1997, 2000, and 2002.
This study identified 3,743 reports of invasive H influenzae infections in adults over the period 1991-2003, of which 656 (18%) were type b (Hib).
Adult Hib infections decreased after the Hib vaccine was introduced in October 1992, reaching a low in 1996. The number of new cases in people 15 years of age and older increased in 2002 and 2003.
The study also found that Hib antibody concentrations in adults fell between 1991 and 1994 and have remained low since then.
Hib immunization greatly reduced the number of cases of Hib in England and Wales as it has done in the US.
Although infant Hib immunization also resulted in a decline in Hib infections in adults, there has been resurgence of adult cases in 2002 and 2003 in England and Wales, coincident with the increase in childhood cases.
Hib immunization programs have dramatically reduced the number of cases of Hib disease in communities. For example, since Hib conjugate vaccines were introduced, the incidence of invasive Hib disease in infants and children in the U.S. has decreased by 99%.
It seems likely that the increase of Hib disease in adults seen in England and Wales reflects their increased susceptibility to infection (lower antibody levels than earlier) and increased exposure to children carrying Hib as a result of the use of a vaccine of reduced effectiveness. If this is the case, then the recent Hib immunization campaign in England and Wales, employing more potent vaccines, should lead to a rapid decline in both childhood and adult cases of Hib.