This register-based study was made possible by linkage of perinatal data and immunisation records of over 1.1 million children born in NSW and WA. It shows that despite an increase in vaccination coverage during the study period, the herd immunity level for the MMR vaccination (95%) was still not reached, especially in certain socio-demographic groups. Stratified analysis by maternal country of birth showed that, in both states, children of foreign-born mothers had persistently lower coverage of MMR1 than children of Australian-born mothers, with increasing inequalities since 2006. Stratified analysis of other socio-demographic indicators showed that over the years, the gaps in coverage have diminished, especially in NSW. In contrast, by the end of the study period certain inequalities remained among WA-born children, especially among the Aboriginal population and children of young mothers. For almost all birth years, coverage for most of the disadvantaged groups was lower for children born in WA when compared to NSW.
Children with foreign-born mothers constitute almost one third of our study population. Increasing the levels of vaccination in this group is therefore not only an issue of equity, but also an important opportunity to increase coverage at the population level. Our findings confirm the results of previous studies in Australia [16, 28, 29] and elsewhere [30]. The increasing differences in coverage within the study period may have multiple explanations. One possible factor is the change in the composition of the migrant population since 2006. There was increased migration from Southern Asian countries during the time period of analysis and a recent study of DTP3 coverage, using the same study dataset as our present study, showed declining on-time coverage since 2008 in children whose mothers migrated from South-East and Southern Asia for reasons that are unclear [31]. Another factor may be the healthcare system’s inability to adjust to the changing composition of the migrant population [32, 33].
Numerous studies have shown Aboriginal status to be another strong determinant of vaccination coverage [17, 34, 35]. However, in both states there was a considerable increase in vaccination coverage among Aboriginal children over the study period and a closing of the gap in coverage between non-Aboriginal and Aboriginal children. This was particularly evident in NSW where the implementation of local projects and state-level policy changes that focused on the Aboriginal population may have played an important role [36]. For example, since 2003 immunisation programs in NSW have sought to strategically integrate immunisation coordinators and Aboriginal health specialists [37], a process that did not begin in WA until 2015 [38]. The lower levels of coverage in Aboriginal children in WA might also be explained by the lack of a systematic state-level immunisation strategy in contrast to the two strategic plans developed in NSW [37, 39, 40]. Additionally, NSW implemented policy directives that required public health services to report to AIR and to follow up on children with overdue vaccinations. These directives may have played a role in increasing the vaccination coverage for the Aboriginal population to a greater extent in NSW and could serve as a model for tackling other social inequities.
Patterns of coverage by maternal age at childbirth mirror coverage disparities related to socioeconomic disadvantage. This is explained by the overrepresentation of disadvantaged backgrounds among the youngest mothers [41]. Furthermore, studies have shown Aboriginal mothers to be more likely to give birth at a younger age compared to mothers with a non-Aboriginal background [42, 43]. The lower vaccination coverage amongst the children with mothers in the oldest age-group compared to the preceding age group (Table 1) could be explained by older women having, on average, more children. A recent study by Gidding et al. [28] showed birth order to be one of the strongest predictors of delay in DTP vaccination with children with older siblings having the greatest delay.
An interesting finding was the decline in vaccination coverage in the least disadvantaged group in WA, observed during the last few years of the study period. Previous literature has shown that vaccine-hesitant parents often belong to advantaged socioeconomic groups, which might explain the decline observed in this study [44]. One reason for the absence of a decline in vaccination coverage in the least disadvantaged group in NSW could be due to how vaccine hesitancy is dealt with at an organisational level. For example, from the early 1990s, primary schools and day-care centres in NSW have required parents to provide documentation about the child’s immunisation status upon enrolment [45]. Additionally, educational programs within healthcare services [40] could also have helped to tackle vaccine hesitancy. Neither of these organizational changes were implemented in WA during this period.
Vaccination coverage by level of remoteness suggest that there is an equitable infrastructure for the provision of vaccines to remote areas in WA and NSW. One possible explanation for this could be the presence of local public health units and Aboriginal medical centres in remote regions. There are no obvious reasons that can explain the sharp decline in coverage for the last birth cohort living in outer regional areas of NSW. This observation needs further monitoring in more recent data to see if the pattern is continuing.
Our findings show that NSW has managed to reduce the social gap in coverage in relation to young mothers, Aboriginal background, and socio-economic disadvantage to a greater extent than WA. One reason for the disparity between the states might be because primary care doctors (GPs) have been the main vaccine providers in NSW since the mid-1990s, whereas in WA the delivery of vaccinations has been dispersed among multiple providers with the introduction of child and adolescent healthcare units in 2005 [14, 38]. Previous studies have shown that a unified and well-coordinated healthcare service can decrease social inequalities in vaccine coverage [46]. By minimising the need for parental knowledge and self-initiative, which are related to parents’ level of social disadvantage, such a healthcare service may make it easier for individuals who belong to a disadvantaged population to navigate the system and reach the services they need.
Strengths and limitations
To the best of our knowledge, this is the first study to report trends in MMR1 vaccination coverage within social groups over a 10-year period across two states in Australia. This was made possible by linking the Australian Immunisation Register to individual level health data in which numerous social indicators were available for analysis [21].
Unlike most research on social inequalities in vaccination coverage, which studies complete immunisation levels for all recommended childhood vaccinations, our paper focuses specifically on the MMR vaccination. By analysing social inequalities in MMR vaccination coverage, we may be able to develop a more refined understanding of why measles cases have increased in recent years. However, the observed trends in MMR1 coverage are likely to apply to other childhood vaccines.
The study has limitations. The measurement of the socioeconomic status variable is at the area level (i.e. census collection district), which is composed of approx. Two hundred fifty households in an urban setting [47]. However, previous studies in Australia have shown this measurement to be at a small enough scale to draw valid conclusions [23].
The AIR is linked to Medicare enrolments (Australia’s universal public health scheme) which covers 99% of Australian residents by 12 months of age. At the time of this study, AIR recorded details of vaccinations (type, brand and date of administration) given to children < 7 years of age. As of 2016 AIR included all ages [48]. Due to possible underreporting (~ 2–3% in 2001), the levels of vaccination can be seen as minimal estimates of the actual coverage [49]. However, since then, completeness has reportedly improved in both states [50, 51].
The children who were excluded from the study population due to missing a birth registration record overrepresented Aboriginal children, children with younger mothers, mothers living in remote regions or with a low Socio-Economic Indexes for Areas (SEIFA) score [21]. However, since this group constituted only 1.3% of the total cohort population, we believe the study cohort is representative of all registered births and includes a large cohort involving 97.5% of all live births.
The aim of our study was to provide a descriptive analysis of trends over time in MMR1 coverage by 2 years of age by selected socio-demographic factors in 2 states with differing vaccination policies, healthcare structures, and strategic immunisation plans. Therefore, no multivariable analyses were conducted. We were unable to study some other relevant individual-level socioeconomic factors that could explain the observed results and it is possible that relative differences in coverage changed after 2 years of age due to catch up vaccinations. It would have been useful to have information on access to primary care and maternal vaccination coverage to further investigate the increasing inequities with respect to maternal country of birth, but these data were unavailable.