This targeted vaccination strategy, called cocooning, has been recommended for more than a decade, but its uptake has been limited, absent evidence of its field effectiveness, Helen E. Quinn, PhD, from the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, and the Discipline of Paediatrics and Child Health, University of Sydney, Children’s Hospital at Westmead, Australia, and colleagues write in an article published online September 15 in Pediatrics.
To assess the effectiveness of parental vaccination in preventing pertussis among infants younger than 4 months, the investigators conducted a case-control study focusing specifically on the effect of a government-funded cocoon program during a prolonged pertussis epidemic in New South Wales, Australia, that began in 2008.
The study population included 217 infants with confirmed pertussis who were younger than 4 months of age at onset and were diagnosed between April 1, 2009, and March 31, 2011, and 585 infants matched to cases by date of birth and demographics. Parents were considered immunized for the purposes of this investigation if they received the diphtheria-tetanus-acellular pertussis vaccine (Tdap) at least 4 weeks before the onset of symptoms in their infant.
The proportion of mothers reporting receipt of the Tdap vaccine at any time was similar between cases and matched control patients; however, a significantly lower proportion of cases’ mothers relative to control mothers reported receiving the vaccine at least 4 weeks before infant disease onset (22% vs 32%) or before pregnancy (12% vs 20%), the investigators report. Case fathers were also significantly less likely to be vaccinated at least 4 weeks before disease onset, they note.
Most of the vaccinated case and control parents were immunized in a primary care setting vs the delivery hospital.
A “high and similar proportion” of coresident children in case and control households (92% and 93%, respectively) were confirmed by the Australian Childhood Immunisation Register to have had 3 or more doses of pertussis-containing vaccine, the authors note. “Similarly, 83% and 86% of eligible coresiding child contacts among both case and control households were recorded on the [Australian Childhood Immunisation Register] as having received the booster dose scheduled at 4 to 5 years.”
After adjusting for potential confounding variables, the authors estimate that the protective effect of immunizing both parents was 51%, compared with vaccinating neither parent. The estimated independent protective effective of immunizing the mother alone, adjusting for the father’s status, was 48%, whereas the estimated protective effect of immunizing the father alone, adjusting for the mother’s status, was 5%.
The investigators also estimated the protective effect of vaccination any time before pregnancy in mothers and determined that after adjustment, the estimated vaccine effectiveness was 42% for the mother alone. If the father reported receiving the vaccine at least 4 weeks before the index date, the combined protective effect was 51%, the authors report.
Although the investigators were not able to assess the source of infection among cases in this study, “the observed reduction in risk among infants of vaccinated parents indirectly supports the importance of parents, especially mothers, as a source of infection among young infants,” they write. Because immunization status of fathers was highly correlated with that of mothers in this study, “we did not find any evidence of an independent protective effect among immunized fathers.”
The study was not sufficiently powered to address such specific questions as the minimum latency period for the protective effect of postnatal Tdap or the duration of protection for subsequent pregnancies, the authors note. “The presence and duration of any such protection is an important gap in knowledge needed to inform recommendations for programs for preventing infant pertussis.”