Abstract
The proportion of infant pertussis cases due to transmission from casual contact in the community has not been estimated since before the introduction of pertussis vaccines in the 1950s. This study aimed to estimate the proportion of pertussis transmission due to casual contact using demographic and clinical data from a study of 95 infant pertussis cases and their close contacts enrolled at 14 hospitals in France, Germany, Canada, and the U.S. between February 2003 and September 2004. A complete case analysis was conducted as well as multiple imputation (MI) to account for missing data for participants and close contacts who did not participate. By considering all possible close contacts, the MI analysis estimated 66% of source cases were close contacts, implying the minimum attributable proportion of infant cases due to transmission from casual contact with community members was 34% (95% CI = 24%, 44%). Estimates from the complete case analysis were comparable but less precise. Results were sensitive to changes in the operational definition of a source case, which broadened the range of MI point estimates of transmission from casual community contact to 20%–47%. We conclude that casual contact appears to be responsible for a substantial proportion of pertussis transmission to young infants.Medical subject headings (MeSH): multiple imputation, pertussis, transmission, casual contact, sensitivity analysis, missing data, community.
Highlights
Pertussis disease is poorly controlled among infants, adolescents, and adults in developed countries despite high immunization coverage rates [1,2,3] of ≥ 93 percent for both the primary infant series [4,5,6] and the booster at school entry [7]
We estimated the proportion of pertussis transmission to young infants due to casual contact with community members
Assuming that data were missing at random and that there was no misclassification of the outcome status when using polymerase chain reaction (PCR) and serological diagnostic methods, we inferred that in the absence of identifying a source case among close contacts of the index infant cases, pertussis transmission to these young infants was due to casual contact in the community
Summary
Pertussis disease is poorly controlled among infants, adolescents, and adults in developed countries despite high immunization coverage rates [1,2,3] of ≥ 93 percent for both the primary infant series [4,5,6] and the booster at school entry [7]. Emerging Themes in Epidemiology 2007, 4:15 http://www.ete-online.com/content/4/1/15 ing, where possible, one person as the most probable source of infection (where the difficulty in identifying the source case usually lies in identifying even one symptomatic source case as opposed to choosing from multiple potential cases). These studies identified close/household contacts as a source of infection for 40–53 percent of young infants with pertussis [12,13,14,15,16]. None of these studies investigated whether the remaining 47–60 percent of transmission was due to casual contact in the community or caused by transmission from unidentified close contacts as no attempt was made to rule out transmission from all identifiable household and other close contacts
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