Our advisor, Thomas Fekete, MD, FCPP, is the author of today’s post. Dr. Fekete, a frequent contributor to this site and an active Fellow here at the College, has many responsibilities at Temple University Hospital and School of Medicine: Section Chief, Infectious Diseases; Professor, Medicine; Associate Professor, Microbiology and Immunology; and Executive Vice Chair for Clinical Affairs, Department of Medicine.
Twenty-plus years ago, an improved (protein-conjugated) vaccine for Haemophilus influenzae type b (Hib) was approved and deployed extensively in young children in most developed countries. There was a rapid and sustained drop in serious infections caused by Hib. This reduction of meningitis, pneumonia, blood infections, and so on was dramatic and was accompanied by a major reduction in the formerly common and seemingly innocent carriage of Hib in the throats of children. As a result, even unvaccinated children had protection from Hib via herd immunity.
But that is not the whole story: Hib did not disappear entirely from the landscape. While the rate of Hib infection decreased >500-fold in children, there are still occasional episodes of Hib disease in adults. Modern adults, like adults in the pre-vaccine era, are not prone to much serious Hib disease, but they can occasionally develop pneumonia or other complications. The bad news is that adults who were never advised to get vaccination against Hib are still occasionally prone to develop significant infections. Ironically, this problem could be exacerbated because natural immunity in adults was sustained by occasional exposure to children with Hib. Adults who are getting sick with Hib in 2013 are those who are predisposed to infections (people with underlying medical conditions, for example) but have neither been vaccinated nor sufficiently exposed to Hib to keep up their immunity.
This opens up the possibility of finding out whether adult immunization against Hib might still be worthwhile. Haemophilus influenzae infections in adults are largely not due to Hib but rather to other strains of Haemophilus that are not affected by any of the Haemophilus influenzae type b vaccines. Starting a policy of adult vaccination for Hib would only make sense if we show that the vaccine offers protection to people receiving it later in life and perhaps at a time when they have underlying medical problems.
The final twist in these statistics is that even with the occasional serious illness caused by Hib in adults, the total burden of Hib in adults is at its lowest point ever. Even if we could show that vaccination of adults works to prevent infection, we would be chasing a very small number of people destined to get Hib. As we enter 2014, we can say that in Western countries, serious Haemophilus influenzae b infection has almost been eradicated and its major effect on health has been nearly eliminated. Occasional Hib cases in adults with underlying medical problems show that bacteria can be tenacious even when they have been nearly vanquished. They have an uncanny ability to pop up in the unfortunate person with diminished immunity, and they remind us not to rest on our laurels because even old foes can lurk out there ready to return if our vigilance falters.
For background on the topic of Hib disease in adults, see this article in Clinical Infectious Diseases: Invasive Haemophilus influenzae Type b Disease in England and Wales: Who Is at Risk After Decades of Routine Childhood Vaccination?