Our study examined factors associated with protective antibody against vaccine preventable diseases in a large cohort of internationally adopted children, focusing on the five countries from which the most children emigrated during the years of this study. In the univariate analyses, country specific differences were detected and varied by vaccine antigen. Similar to our findings, other studies have reported lower levels of protection in Chinese children [11
]. However, the results of previous studies compared to our study varied with respect to age [10
], institutionalization [9
], and nutritional status [10
]. Given these differences, we felt it was important to adjust for these factors in multivariable analyses.
In our study, 93% had documentation of receiving at least one vaccine in their birth country compared to other studies where documentation varied greatly from 35 to 85% [5
]. Our high proportion of children with an immunization record may have been due to our emphasis on bringing vaccination records to the clinic appointment, improved vaccine coverage, and/or improvements in immunization documentation in the child’s birth country. Interestingly, we found very few errors in the vaccination records as discussed in other studies [9
]. Instead, the major reason for a lack of UTD-V status in our study was not receiving enough doses and not due to incorrect dates.
To our knowledge, this is the first study to examine different methods for assessing the immunization records of internationally adopted children with regard to protective antibody levels. The two methods yielded fairly similar results for diphtheria, tetanus, polio, and HBV. However, for measles, mumps, and rubella, children with ≥ 1 recorded dose (ND-R) were more likely to be protected than children who were UTD-V. This could be due to the fact that measles immunization if often given prior to 1 year of age in areas in which measles is endemic.
Our study adjusted for several factors in the multivariable analyses to determine which factors were associated with immunity. Compared to Guatemalan children, we found Chinese children had decreased protection for diphtheria, tetanus, mumps, and rubella, while Russian children had decreased immunity for mumps and Kazakhstani children were less likely to be immune to rubella. The only other factors besides vaccination status that were significantly associated with protection were increasing age for polio, measles, mumps, and rubella, and non-institutionalization for polio. For varicella, since so few children were vaccinated, increasing age and history of disease were the most important predictors. For all antigens except polio and varicella, the optimal model for predicting protection was the least restrictive model where vaccination status was defined as the ND-R.
Only two previous studies used multivariable analyses to evaluate protection using immunization records in internationally adopted children [15
]. In the Verla-Tebit
study, Chinese children were more likely to be unprotected against tetanus, polio serotype 1 and HBV compared to Russian children [16
used “record with updated number of doses for age,” similar to UTD-V (except that doses were not excluded for being administered too young or within an insufficient interval between doses), and found a significant association with protection against measles, mumps, rubella, and HBV, but not diphtheria, tetanus, or polio [15
]. In our study, UTD-V was significant for all antigens except for diphtheria, measles and mumps. Differences between our results and Cilleruelo
’s for tetanus and polio may exist because children (especially younger children) may still have had protection from prior vaccinations and age cutoffs for defining up-to-date were different. In our analysis, children with an adequate number of prior doses who had reached the minimum
recommended age for a subsequent vaccination were considered UTD-V until they exceeded
recommended age, whereas Cilleruelo
used a younger age cutoff [15
]. In Cilleruelo
’s study, it is possible that younger children defined as not up-to-date were still protected by prior doses, which would explain the lack of association.
While there are several strengths to our study including a large sample size and ability to use multivariable analyses to examine factors associated with protection, there are limitations to our study. We were only able to include the five countries from which most children immigrated to the US. Therefore, our conclusions may be different for other countries. In order to perform statistical comparisons, we wanted to use a country that had high levels of overall protection as our reference group. Both Guatemala and South Korea met this specification. Because the number of children from Guatemala was larger than the number from South Korea, we decided to use Guatemala as our reference group. Children from Guatemala do have some demographic differences – notably that they are often not institutionalized (as they live in private families/foster care) and are younger. In addition, the time period of the study may not be generalizable to the current time as the quality of vaccines and vaccine uptake may have changed over time. In addition, there have been changes in the demographics of international adoptees with an increase in the prevalence of Ethiopian adoptions. It will be important to examine whether similar results will be seen in children from Ethiopia. Similar to other studies, we were unable to determine if the presence of protective antibody was due to immunization, wild-type disease in the community (polio, measles, mumps, rubella, and varicella), vaccine-associated varicella or polio virus, or waning maternal antibody. However, we do not feel this is a major limitation given the strength of the association with protection seen with the increased number of immunization doses recorded.