Can Adolescent Vaccination Reduce Infant Pertussis?

Can Adolescent Vaccination Reduce Infant Pertussis? CME/CE

News Author: Steven Fox
CME Author: Charles P. Vega, MD, FAAFP

CME/CE Released: 11/07/2013; Valid for credit through 11/07/2014

Clinical Context

Infants bear a disproportionate burden of morbidity and mortality from infection with pertussis, so a clear understanding of the primary sources of pertussis in this vulnerable age group is a public health priority. Bisgard and colleagues addressed this issue in a study published in the November 2004 issue of the Pediatric Infectious Disease Journal. They found that 75% of infant cases of pertussis were the result of contact with an infected family member, with the mother being the single most common source of familial transmission. Slightly more than half of primary source contacts with pertussis were adults, but 20% were between ages 10 and 19 years.

The recognition that adolescents are a significant source of pertussis led to recommendations in 2006 for universal vaccination of adolescents with the tetanus-diphtheria-acellular pertussis (Tdap) vaccine. By 2011, a total of 78% of adolescents had received Tdap. The current study by Auger and colleagues evaluates the effect of this effort on the rate of severe pertussis among infants.

Study Synopsis and Perspective

Widespread vaccination of adolescents against pertussis, spurred by 2006 recommendations from US health authorities, appears to have helped reduce pertussis-related hospitalizations among infants.

Katherine A. Auger, MD, from the Division of Hospital Medicine, Department of General Pediatrics, Cincinnati Children's Hospital Medical Center, University of Cincinnati, Ohio, and colleagues, report the results of their study in an article published online October 21 in Pediatrics.

In 2006, the Centers for Disease Control and Prevention's (CDC's) Advisory Committee on Immunization Practices recommended universal administration of the Tdap vaccine to adolescents, who are a known source of pertussis in infants.

The recommendations have significantly improved vaccination rates among adolescents. By 2011, 78% of adolescents in the United States had received Tdap immunizations. "We sought to understand if patterns of pertussis hospitalization for infants changed with adoption of Tdap vaccination among adolescents," Dr. Auger and colleagues write.

Therefore, the investigators mined data from the Nationwide Inpatient Sample to identify infants younger than 1 year diagnosed with pertussis. They used data from 2000-2005 to estimate the rate of infants' hospitalizations if Tdap vaccinations had not increased in teenagers and compared those with observed hospitalization rates for 2008-2011. They excluded the 2 years immediately after the Advisory Committee on Immunization Practices recommendations (2006 and 2007) because the recommendations had not yet been widely implemented.

During 2000, the incidence of pertussis-related infant hospitalizations was 5.82 (95% confidence interval [CI], 4.51 - 7.13) discharges per 10,000 infants in the US population. "The rate increased during pre-Tdap years by a mean of 0.64 pertussis discharges per 10 000 infants per year (P for trend = .004)," the researchers note.

In contrast, after implementation, the observed rate was lower than the expected rate in 3 of 4 years examined. In 2008, the observed rate was 3.85 hospitalizations per 10,000 infants (95% CI, 2.90 - 4.81) compared with an expected rate of 10.09 (95% CI, 7.41 - 12.76). In 2009, the observed rate was 5.84 (95% CI, 4.41 - 7.27) vs an expected rate of 10.72 (95% CI, 7.63 - 13.82), and in 2011, the observed rate was 3.27 (95% CI, 2.37 - 4.16) vs an expected rate of 12.00 (95% CI, 8.06 - 15.94).

The exception to the pattern was 2010, in which there was no significant difference between the observed rate (7.78; 95% CI, 5.10 - 10.46) and the expected rate (11.36; 95% CI, 7.84 - 14.88).

The findings suggest that adolescent vaccination may help reduce the burden of severe infant pertussis cases that require hospitalization yet are not consistent enough to have mitigated infantile hospitalizations for pertussis in 2010, the first peak year of pertussis incidence after Tdap recommendations were published.

"Our findings of an inconsistent impact on infant pertussis hospitalizations in the Tdap era through 2011 indicate that infants may need further protection by 1 of 2 immunization efforts: improving vaccination rates among adolescents and implementing expanded vaccination recommendations among adults," the authors write.

Dr. Auger has received salary support from the Robert Wood Johnson Foundation through the Clinical Scholars Program. The other authors have disclosed no relevant financial relationships.

Pediatrics. Published online October 21, 2013. Abstract

Study Highlights

• Researchers used the Nationwide Inpatient Sample, the largest all-payer database of hospitalizations across the United States, to evaluate their study question. The researchers focused on children admitted to the hospital at age younger than 1 year with a diagnosis code for pertussis.
• Hospitalization rates for infants were compared in 2000-2005, when Tdap was not recommended for adolescents, with 2008-2011. By 2008, more than 40% of adolescents had received Tdap. Admissions from 2006 and 2007 were excluded because of the nascent implementation of Tdap vaccination for adolescents during these years.
• Researchers performed analyses that attempted to adjust for changes in the propensity of clinicians to admit infants with pertussis with time as well as changes in the prevalence of pertussis from year to year.
• The rate of hospitalizations for pertussis increased from 2001-2005, from 4.98 hospitalizations per 10,000 infants to 12.89 hospitalizations per 10,000 infants.
• The expected rate of hospitalizations was lower than expected in 2008, 2009, and 2011, with an actual rate of 3.27 to 5.84 hospitalizations per 10,000 infants.
• In 2010, the expected rate of hospitalizations (11.36 per 10,000 infants) was similar to the actual rate of hospitalization (7.78 per 10,000 infants).
• Overall, the rates of hospitalization for pertussis were lower than expected in 3 of the 4 years evaluated after adoption of the adolescent Tdap vaccine.
• Among infants who were hospitalized for pertussis, the mean age at the time of hospital discharge was 87 days. The mean length of hospital stay was 5.4 days, and 5.7% of infants were judged to have life-threatening pertussis infection.
• There was no evidence that pertussis vaccination among adolescents affected the percentage of admitted infants with life-threatening illness or the mean age of admitted infants.
• The researchers compared their rates of pertussis hospitalizations with the number of pertussis cases reported by the CDC to validate their data set. However, there were clearly years in which the number of cases in the current study either exceeded or was less than the number of cases from the CDC. The 2 data sets generally agreed on general trends in pertussis activity from year to year.

Clinical Implications

• A previous study by Bisgard and colleagues found that 75% of infant cases of pertussis were caused by contact with an infected family member.
• The current study by Auger and colleagues suggests that Tdap vaccination among adolescents may have a modest effect in reducing the risk for hospitalization for pertussis among infants.