die of SIDS tend to be in poorer health than their peers in the week or two prior to death. Stanton and colleagues (1978) found that parents reported symptoms considered severe enough to warrant medical attention or close supervision in the 48 hours before death or interview for 69 of 145 (48 percent) children who died of SIDS and only 19 of 154 (12 percent) control children. Gilbert and colleagues (1990), in a similar study found that parents reported major or minor signs of illness in the previous week in 66 of 95 (69 percent) SIDS victims and only 71 of 190 (41 percent) control children matched with cases for age, area of residence, and time of year. In addition, Gilbert and colleagues (1990) found that 17 (18 percent) SIDS victims had been seen by their general practitioner during the week preceding death, whereas 11 (6 percent) control children had been seen by their general practitioner in the corresponding period. Less pronounced differences in the relative frequencies of reported symptoms before death or interview were found in the NICHD SIDS Cooperative Epidemiologic Study (Hoffman et al., 1988). Although parents of children who died from SIDS may be more likely to recall and thus report more symptoms in their children, reporting of doctor's visits over a short time period is likely to be complete for both cases and controls.
It is noteworthy that some of the factors associated with SIDS, such as low birth weight, young maternal age, and black race, are also associated with delaying early childhood immunization past the recommended age (Hoffman et al., 1987; Walker et al., 1987). The influence of such delays on the time of occurrence of SIDS in relation to the time of DPT immunization would depend on the specific ages over which such delays occurred. The effect could be to cause children to be immunized at ages associated with either higher or lower than expected rates of SIDS, and thus produce spurious direct or inverse associations, respectively, between SIDS and DPT immunization. Clearly, all factors associated with delaying immunization should be measured and controlled for as far as possible in studies of SIDS in relation to DPT vaccine administration. The ages of study subjects should be considered as precisely as possible as well. Although the Immunization Practices Advisory Committee advises the deferral of routine DPT immunization only for those with a febrile illness (Centers for Disease Control, 1985), in practice, some clinicians may postpone immunizations because of other minor illnesses (American Academy of Pediatrics, 1986). Since minor illnesses often precede SIDS, the effect of delaying immunization during such illnesses would be to produce a spuriously low rate of SIDS in the immediate postimmunization period. Thus, in addition to age and possible delaying factors, the potential role of minor illnesses in the timing of immunization is important to address in evaluating the studies of SIDS and DPT vaccine administration.