Background/Aims: Of 100 US deaths from pertussis during 2000--2004, 90% occurred among infants aged <4 months at onset of illness. In 24% of cases, a cough illness in the mother, father, or grandparent has been found. Patients with pertussis are highly infectious; attack rates among exposed, nonimmune household contacts reach 80%--90%. Single dose Adacel (sanofi pasteur) containing tetanus and diphtheria toxoids and acellular pertussis antigen (Tdap), was licensed June 2005 only for persons 11-64 years. A second Tdap vaccine, Boostrix (GlaxoSmithKline) was also licensed for adults 19-64 December 2008. This study examined off-label use of Tdap among seniors in a Midwest HMO June 2006 to February 2009. Methods: The number and percent of members 65 years and over in a Midwest HMO who received Tdap between June 2006 and early February 2009 were determined using the HMO’s medical records. Results: Between June 2006 and early February 2009, among a total of 55,930 Tdap vaccinations given, 853 (1.5%) of them were given to persons 65 years and over, of which 61% were female, 49% aged 65-69, 22% aged 70-74, 10% aged 80-84 and 6% aged 85 and over. Their racial/ethnic distribution was: 88% Caucasian, 6%
African American, 4% Asian, and 1% Hispanic, which was similar to racial/ethnic distribution of HMO patients aged 65 and over. A total of 280, 232, 231 and 110 Tdap vaccinations were given to persons 65 years and over in years 2006, 2007, 2008 and January/February 2009 respectively. As of February 2009 Tdap cumulative coverage in this age group was 1.9%. Conclusions: Although Tdap is not currently licensed for use in people aged 65 and over because U.S. prelicensure trials did not include this age group, its off-label use has been consistently observed among older persons since it was licensed in 2005. Research needs to be conducted to further quantify this use, determine the reasons Tdap vaccination is given to patients 65 years and over and help inform future potential consideration for study of the safety and efficacy of Tdap in this age-group.