Infants who contract pertussis from a known source are now more likely to have gotten it from a sibling than from their mother. That represents a shift in the pattern of known sources of infection, although in about half of cases the source is unknown.
"[M]others and fathers still play an important role in transmitting disease to unprotected infants, but the transition to siblings and other school-age children as the main source of infection is not unexpected in this era of waning [acellular pertussis vaccine] immunity and an increasing burden of pertussis in these age groups," Tami Skoff, MS, from the Centers for Disease Control and Prevention, Atlanta, Georgia, and colleagues write in an article published online September 7 in Pediatrics.
Of 1306 cases of pertussis in infants, only 569 had a known source: 35.5% from siblings, 20.6% from mothers, and 10.0% from fathers. (Older studies had found that mothers accounted for 32% to 37% of known sources.) In total, immediate and extended family members accounted for 85.2% of known sources.
The Enhanced Pertussis Surveillance sites in seven states documented the infections in this study from 2006 to 2013. At these sites, public health personnel interviewed families of patients with laboratory-confirmed pertussis infections, collecting information on demographics, presentation, vaccination history, and epidemiologic information. The source of infection was defined as a person who had symptoms of pertussis and was in contact with the infant during the 7 to 20 days before the infant's onset of symptoms. If more than one person fit the description, the person with earliest onset of symptoms was considered to be the source.
Of infants (younger than 1 year) who contracted pertussis, 24.2% were younger than 2 months. Infants younger than 1 month were the most likely to have a known source of infection (P = .001).
At the start of the study period, mothers were the most common source of known infection, but they were overtaken by siblings around 2008. Infections from mothers decreased (P = .0014), and those by siblings increased (P = .0333); other sources did not show any significant trends.
Limitations of the study include uncertainty in the source of infection: the first contact to develop symptoms was considered the source, and the investigators did not consider how much contact each symptomatic person had with the infant. In some cases, the true source may have been asymptomatic, and thus could not be identified. Although infant infections were laboratory-confirmed, sources of infection were assumed on the basis of parents' recollection of cough symptoms.
Immunity from the acellular pertussis vaccine wanes over time, so children who received this vaccine after its adoption in the 1990s became susceptible to the disease later in childhood. School-age children now have higher rates of pertussis than they did in the past, and the authors write that this is likely what is driving the increase in siblings as a source of infection.
The Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices recommends two strategies for preventing infant pertussis infections: "cocooning" by vaccinating family members and caregivers, and vaccinating women in their third trimester of each pregnancy. Skoff and colleagues write that few adults get the recommended booster vaccines, and that immunity from these boosters also wanes over time. Studies in baboons also suggest that individuals vaccinated with the acellular vaccine can transmit pertussis asymptomatically. The authors conclude: "For these reasons, the cocooning strategy is less than ideal, and strong support of vaccination during pregnancy is needed to maximize the protection of infants in the first critical months of life."
The authors have disclosed no relevant financial relationships.
Pediatrics. Published online September 7, 2015. Full text
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