Tdap vaccine effectiveness in adolescents during the 2012 Washington State pertussis epidemic

Abstract

Background: Acellular pertussis vaccines replaced whole-cell vaccines for the 5-dose childhood vaccination series in 1997. A sixth dose of pertussis-containing vaccine, tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis, adsorbed (Tdap), was recommended in 2005 for adolescents and adults. Studies examining Tdap vaccine effectiveness (VE) among adolescents who have received all acellular vaccines are limited.

Methods: To assess Tdap VE and duration of protection, we conducted a matched case-control study during the 2012 pertussis epidemic in Washington among adolescents born during 1993-2000. All pertussis cases reported from January 1 through June 30, 2012, in 7 counties were included; 3 controls were matched by primary provider clinic and birth year to each case. Vaccination histories were obtained through medical records, the state immunization registry, and parent interviews. Participants were classified by type of pertussis vaccine received on the basis of birth year: a mix of whole-cell and acellular vaccines (1993-1997) or all acellular vaccines (1998-2000). We used conditional logistic regression to calculate odds ratios comparing Tdap receipt between cases and controls.

Results: Among adolescents who received all acellular vaccines (450 cases, 1246 controls), overall Tdap VE was 63.9% (95% confidence interval [CI]: 50% to 74%). VE within 1 year of vaccination was 73% (95% CI: 60% to 82%). At 2 to 4 years postvaccination, VE declined to 34% (95% CI: -0.03% to 58%).

Conclusions: Tdap protection wanes within 2 to 4 years. Lack of long-term protection after vaccination is likely contributing to increases in pertussis among adolescents.

Keywords: adolescent; diphtheria-tetanus-acellular pertussis vaccine; diphtheria-tetanus-pertussis vaccine; whooping cough.

FIGURE 1

Estimated Tdap duration of protection against pertussis among adolescents who received all acellular vaccines (birth years 1998–2000), restricted to confirmed cases or participants with complete and on-schedule childhood series. Acellular vaccine group: adolescents born from 1998 to 2000, assumed to have received all acellular pertussis vaccines for the childhood series (cases = 450, controls = 1256). Acellular vaccine group, confirmed case status: adolescents born from 1998 to 2000, assumed to have received all acellular pertussis vaccines for the childhood series and restricted to confirmed cases and their associated controls (cases = 355, controls = 984). Acellular vaccine group, complete and on-schedule childhood series: adolescents born from 1998 to 2000, assumed to have received all acellular pertussis vaccines for the childhood series and restricted to cases and controls with 5 childhood doses on schedule (cases = 288, controls = 728). A complete and on-schedule primary series is considered the following: doses 1 through 3 before the first birthday, dose 4 on or after the first birthday and before the second birthday, and dose 5 on or after the fourth birthday and before the seventh birthday. Refer to Table 3 and Supplemental Tables 5 and 6 for CIs for each of these time points.

FIGURE 2

Time since Tdap vaccination, by vaccine group. Acellular vaccine group: adolescents born from 1998 to 2000, assumed to have received all acellular pertussis vaccines for the childhood series (cases = 450, controls = 1256). Mixed-vaccine group: adolescents born from 1993 to 1997, assumed to have received a mix of whole-cell and acellular pertussis vaccines (cases = 386, controls = 1076).