[T]he epidemiology of pertussis in Australia and the United States of America has also changed in recent times, with an increasing proportion of disease occurring in children.
Household contacts are the most likely sources of infant pertussis, but there is variation in the proportion of sources reported to be parents as opposed to siblings.
A prolonged outbreak of pertussis occurred in Australia, including south metropolitan Perth, between 2008 and 2012. A cocooning strategy involving the vaccination of caregivers of newborns was implemented in Western Australia and ran for 2011 and 2012 in attempts to protect newborns during the outbreak.
[T]he South Metropolitan Population Health Unit (SMPHU) collected enhanced surveillance data for pertussis cases in children under 5 years of age.
Recent studies have shown an increasing incidence of pertussis in children but the implications of this for the source of infant pertussis have not been fully described.
Identifying the source of pertussis in infants 6 months of age and under is crucial for the development of effective preventive strategies in this age group.
[A] recent Australian study reported the vaccine effectiveness of acellular vaccine to be 83.5% in infants aged 6–11 months, falling to 70.7% in children aged 2 years, and 59.2% in children aged 3 years.
Cocooning programs are challenging to implement and there is no definitive evidence that they are successful in reducing the incidence of infant pertussis.
In the peak epidemic period, sibling sources of infection were most likely to be aged 2 or 3 years.
The only other recent Australian study of infant pertussis sources had similar findings, demonstrating that siblings aged 3 and 4 years were particularly important sources of infant pertussis…
Fully vaccinated siblings aged 2 and 3 years were the most important infant pertussis sources in the peak epidemic period of this study, suggesting that immunity may wane in this age group before the vaccine booster at 4 years.
Even if it were possible to fully cocoon infants through a combination of parental vaccination and ensuring siblings were fully vaccinated, the possibility of transmission via breakthrough disease in siblings would persist.