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Tdap cocooning against pertussis effective in Australian cohort
Recent data show a government-funded cocoon program was effective in protecting newborns from pertussis during an epidemic in New South Wales, Australia.
Helen E. Quinn, PhD, MAE, of the National Centre for Immunisation Research and Surveillance of Vaccine-Preventable Diseases in Westmead, Australia, and colleagues determined receipt and timing of Tdap vaccination among parents or close contact adults of 217 infants diagnosed with pertussis between April 2009 and March 2011. Researchers age-matched 585 children and their parents to serve as the control. Parents were interviewed to establish timing and receipt of Tdap vaccination. Parents were considered immunized if they received Tdap at least 4 weeks before pertussis onset.
There was a similar proportion of mothers in the case and control groups who received Tdap vaccine at any time (76% vs. 79%). However, 22% of case mothers received Tdap at least 4 weeks before onset compared with 32% of control mothers. Further, more mothers in the control group received Tdap vaccination before pregnancy than those in the case group (20% vs. 12%).
Fathers in the case group were also less likely to receive Tdap vaccine at least 4 weeks before pertussis onset compared with fathers in the control (20% vs. 31%).
Most siblings who lived in case (92%) and control (93%) households received at least three doses of a pertussis-containing vaccine.
Analysis indicated having both parents immunized had a protective effect against pertussis of 51% (95% CI, 10-73). Adjusting for the father’s status indicated an immunized mother alone had a protective effect of 48% (95% CI, –2 to 74). An immunized father alone had a protective effect of 5% (95% CI, –79 to 50) after adjusting for mother’s status.
A mother who was vaccinated before pregnancy had a vaccine effectiveness of 42% (95% CI, –24 to 73). If the father also was vaccinated 4 weeks before pertussis onset, the combined protective effect was 51% (95% CI, 0-76).
“Our study provides evidence of a moderate reduction in the risk of laboratory-confirmed pertussis in infants aged less than 4 months whose parents have been booster immunized with acellular pertussis vaccine at least 4 weeks earlier,” the researchers concluded.
Disclosure: Some of the researchers report financial ties with GlaxoSmithKline.
Perspective
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Quinn and colleagues are to be applauded for being the first group to demonstrate protection against pertussis for young infants through “cocooning.” Cocooning is a biologically plausible strategy based on household transmission studies and was recommended in the United States and other countries during the past decade to prevent young infant pertussis. Vast logistical barriers and lack of data about cost-effectiveness have precluded its widespread implementation in the United States. The few centers that implemented successful cocooning were unable to prove effectiveness, but unlike this study, they were inadequately powered to demonstrate moderate benefits.
It is clear that with current pertussis vaccines no single immunization strategy will control young infant pertussis. Studies performed in the United States and in the United Kingdom show that immunization of pregnant women in the third trimester is safe and reduces pertussis illness in this vulnerable population. Due to the rapid waning of pertussis antibodies, this protection is likely improved when Tdap is given in every pregnancy so that maternal antibodies are available to protect infants from birth. But as shown by Quinn and colleagues, Tdap immunization of mothers pre-pregnancy also offers some infant protection. This study demonstrates that efforts to implement cocooning should continue for three reasons: 1) to reduce the reservoir of infection in those who transmit pertussis to infants; 2) to protect infants whose mothers did not receive, did not adequately respond to Tdap vaccine during pregnancy either because the booster is given only a few days before delivery or because of a poor immune response; and 3) to protect preterm infants in whom passively-acquired maternal antibody levels are insufficient or older infants where maternal antibody levels have waned but who have not yet completed their primary immunization series. Quinn and colleagues also remind us of deficiencies in our own health care infrastructure to limit the impact of cocooning.
Perspective
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This study provides timely evidence of the benefit of maternal Tdap immunization in prevention of pertussis. Although the effect is modest, this is a very important benefit, as newborns are at greatest risk for severe illness due to pertussis. There are some limitations to the study, particularly that all vaccinations were by report, but clearly a great deal of effort went into the study and the authors should be commended.
Here in California, we experienced another large pertussis outbreak in 2014. Pertussis appears to be on the rise, with larger epidemics and a widening cohort of children who have been vaccinated with only acellular pertussis vaccines. Studies have revealed that acellular vaccines are less effective than earlier whole-cell vaccines, and wane rapidly, leaving recipients and their families vulnerable to infection, and most likely contributing to the rising incidence of pertussis. Revaccination with each pregnancy makes sense, since antibodies are carried transplacentally and, if given near term, should last sufficiently long enough to protect the newborn. This study provides welcome evidence of the utility of this strategy.
