Tdap Vaccine Protects Against Pertussis During Outbreak

From Reuters Health Information CME

News Author: Karla Gale, MS
CME Author: Penny Murata, MD

July 9, 2010 — The Tdap vaccine — a tetanus, reduced-dose diphtheria, and acellular pertussis booster — effectively protected adolescents during a pertussis outbreak in the U.S. Virgin Islands, investigators reported online June 25th in Clinical Infectious Diseases.

Tdap's licensure in 2005 was based on serological surrogate end points rather than direct vaccine efficacy data. In this study, the first published evaluation in an outbreak setting, Tdap was 65.6% effective.

The outbreak occurred in the autumn of 2007 at a nursery through 12th grade school on St. Croix. With 51 confirmed or probable cases among 499 students, the attack rate was 10%. Coughs lasted up to four months or more, with a median duration of 38 days.

According to senior author Dr. Stacey W. Martin, from the U.S. Centers for Disease Control and Prevention, Atlanta, Georgia, and colleagues, all but three cases occurred in grades six through 12, with an overall attack rate in those classes of 17%. The highest incidence was in the 10th grade (38%).

Among students age 11 and older, overall Tdap coverage was 12%. There were two confirmed or probable cases among 33 vaccinated students (6.1%) and 41 among 233 not vaccinated (17.6%, relative risk 2.9). The vaccine's effectiveness was not statistically significant (p = 0.092) due to limited sample size.

Local authorities collected nasopharyngeal aspirates or swab samples for culture and for pertussis polymerase chain reaction (PCR) testing from students whose cough had started no more than 14 days earlier. They also collected blood samples from students with any duration of cough. During the convalescence phase in winter 2008, nasopharyngeal specimens were again collected from kids with cough, and all students age 11 and over were offered serological testing regardless of cough history.

In confirmed cases, Bordetella pertussis was isolated in culture or patients had positive PCR or serological test results. Clinical cases (cough for at least 14 days along with whoop, post-tussive vomiting and/or paroxysmal cough) that were not laboratory-confirmed were classified as probable.

Because the Advisory Committee on Immunization Practices recommends Tdap vaccination only for adolescents and adults up to age 65, the research team limited its vaccine efficacy analyses to the 287 students age 11 and older.

Among 162 students who provided clinical specimens, six had culture-confirmed cases. These six also had the only positive results on PCR and convalescent serology.

The authors report that geometric mean concentrations (GMC) of anti-pertussis IgG varied in the serum samples obtained in the convalescent period, from 107.2 ELISA Units/mL in 40 patients with confirmed or probable cases, to 20.2 EU/mL in 45 students with a history of cough not meeting the case definition, and 29.4 EU/mL in 72 students with no history of cough (p < 0.001 comparing cases with noncases).

They also note that only 26 of 40 case patients had positive convalescent serological test results, and one had indeterminate results.

On the other hand, 12 of 72 noncoughing students also had positive serological test results, indicating "evidence of asymptomatic infection and the potential for unrecognized transmission."

The investigators also point out that serology results identified 20 patients who didn't present for testing until they'd been coughing for more than two weeks. These students didn't provide nasopharyngeal specimens, but if they had, the tests would likely have been negative because of the timing. Given the usefulness of serology in these cases, the authors recommend that the Council of State and Territorial Epidemiologists include serological testing in its case definition.

"Higher Tdap coverage rates are needed to minimize the negative impacts of waning immunity, imperfect (vaccine effectiveness) and high secondary transmission rates," the research team concludes.

Clin Infect Dis. 2010;51:315-321.

Reuters Health Information 2010. © 2010 Reuters Ltd.
Clinical Context

In the March 30, 2007, issue of MMWR. Morbidity and Mortality Weekly Report, McNabb and colleagues reported that 53% of pertussis cases in 2006 occurred in children at least 15 years old. According to Kretsinger and colleagues in the December 15, 2006, issue of MMWR. Recommendations and Reports: Morbidity and Mortality Weekly Report, a combined vaccine, Tdap, was recommended for all adolescents and adults up to age 65 years. In the March 2006 issue of Pediatrics, it was noted that approval of Tdap use was based on serologic responses in adolescents and adults.

This study of a pertussis outbreak assesses the vaccine effectiveness of Tdap and whether serologic testing is useful to detect pertussis.