Development and validation of the Vaccine Barriers Assessment Tool for identifying drivers of under-vaccination in children under five years in Australia

Figures & data

Figure 1. Overview of methods and outputs of three phases of VBAT development and validation.

Table 1. Participant characteristics at each phase.

	PHASE 1: Cognitive testing		PHASE 2a: Validation of draft 1 item set (n = 532)	PHASE 2b: Re-test^ (n = 263)	PHASE 3: Validation of draft 2 item set
	Australia (n = 18)	New Zealand (n = 10)			Participants wit AIR data^† (n = 156)	Participants without AIR data^† (n = 149)
Age of youngest child
0–5 months	0	0	43 (8%)	26 (10%)	56 (36%)	21 (14%)
6–11 months	4 (22%)	4 (40%)	41 (8%)	21 (8%)	47 (30%)	50 (34%)
12–17 months	5 (28%)	3 (30%)	47 (9%)	33 (13%)	40 (26%)	54 (36%)
18–41 months	9 (50%)	3 (30%)	207 (39%)	114 (43%)	0%)	0%)
42–47 months	0	0	47 (9%)	19 (7%)	13 (8%)	24 (16%)
48–72 months	0	0	147 (28%)	50 (19%)	0%)	0%)
Socio-economic status/income‡
High (least disadvantaged/$150,000+)	2 (11%)	1 (10%)	280 (53%)	138 (52%)	46 (30%)	58 (39%)
Medium ($75,000–$149,000)	3 (17%)	2 (20%)	124 (23%)	59 (22%)	63 (40%)	58 (39%)
Low (most disadvantaged/less than $74,000)	7 (39%)	7 (70%)	128 (24%)	66 (25%)	47 (30%)	33 (22%)
Missing	6 (33%)	0	0	0	0	0
Vaccine hesitancy level by VCF
High (major concerns/delay or refuse vaccines)	6 (33%)	4 (40%)	76 (14%)	32 (12%)	11 (7%)	14 (9%)
Medium (minor concerns)	5 (28%)	3 (30%)	143 (27%)	73 (28%)	34 (22%)	35 (24%)
Low (no concerns)	7 (39%)	3 (30%)	313 (59%)	158 (60%)	109 (70%)	99 (66%)
Missing	0	0	0	0	2 (1%)	1 (1%)
Vaccine coverage level of residential area
High	0	0	204 (38%)	81 (31%)	72 (46%)	48 (32%)
Medium	0	0	123 (23%)	76 (29%)	76 (49%)	83 (56%)
Low	0	0	205 (39%)	106 (40%)	6 (4%)	18 (12%)
Missing/not collected	18 (100%)	10 (100%)	0	0	2 (1%)	0 (0%)
Highest educational qualification
High school certificate	2 (11%)	5 (50%)	63 (12%)	23 (9%)	24 (15%)	24 (16%)
Trade certificate or diploma	5 (28%)	3 (30%)	123 (23%)	67 (26%)	59 (38%)	31 (21%)
Undergraduate degree‡‡	11 (61%)	2 (20%)	191 (36%)	102 (39%)	40 (26%)	53 (36%)
Postgraduate degree	n/a	n/a	148 (28%)	67 (26%)	30 (19%)	36 (24%)
None of the above	0 (0%)	0 (0%)	6 (1%)	3 (1%)	3 (2%)	4 (3%)
Missing	0 (0%)	0 (0%)	1 (0%)	1 (0%)	0 (0%)	1 (0%)

^{^}Subset of Phase 2a participants. ^†Reasons for unavailable AIR data described in Results. ^‡For phase 1, family income level was self-reported by participants. For phases 2 and 3, SEIFA level was calculated based on postcode.^{27 ‡‡}The top two education level options were combined for Phase 1 participants as “Degree or higher.”

Table 2. Characteristics of the VBAT draft 1 items reviewed for inclusion.

	Criteria for considering exclusion
Reviewed items	Ceiling effect (>70% strongly agree)	High rate of “Don’t know/can’t say” (>5%)	Poor correla-tion with VCF (<0.2)	Poor test/retest (ICC <0.3)	Kept or cut	Rationale
I believe children’s immune systems respond well to vaccines even when they are very young	52.20%	9.20%	0.353	0.501	Cut	High ‘missing’ data
I believe my child is unlikely to experience a serious side effect from a vaccine	39.60%	9.40%	0.251	0.372	Cut	High ‘missing’ data
I believe people who do not vaccinate their children should be penalised	31.00%	9.40%	0.282	0.546	Cut	High ‘missing’ data
I believe vaccines can cause allergic conditions, for example asthma or eczema	20.00%	13.40%	0.248	0.482	Cut	High ‘missing’ data
I believe vaccines do not cause autism	51.80%	14.20%	0.32	0.398	Cut	High ‘missing’ data
I can find care for my other children when I need to vaccinate my child	48.00%	17.60%	0.134	0.353	Cut	High ‘missing’ data
I get information about vaccines from the radio, TV or newspaper	23.10%	2.70%	0.09	0.571	Cut	Cut because it does not provide meaningful actionable information for policymakers
I have a good relationship with my child’s doctor or nurse	59.80%	4.20%	0.153	0.384	Cut	Cut due to similarity to other items (e.g. “comfortable discussing vaccination”)
I prefer my child to get protection against many diseases through one combined vaccination rather than through separate vaccinations	40.80%	12.60%	0.168	0.428	Cut	High ‘missing’ data
My child has a severe fear of needles	21.80%	6.40%	0.199	0.594	Cut	High ‘missing’ data
I actively look for information about vaccines using social media	24.10%	2.90%	0.099	0.523	Kept	Not expected to be correlated to hesitancy so poor VCF correlation not relevant
I am comfortable discussing vaccination with my child’s doctor or nurse	71.40%	1.80%	0.276	0.278	Kept	Very close to threshold, retained
I believe children get too many vaccines in the first 2 y of life	22.00%	5.50%	0.326	0.543	Kept	Retained because previous research shows high association with vaccine hesitancy/refusal
I can afford any costs associated with vaccinating my child	52.40%	4.00%	0.184	0.538	Kept	Not expected to be correlated to hesitancy so poor VCF correlation not relevant
I can discuss vaccination in my preferred language with my child’s doctor or nurse	66.70%	6.20%	0.25	0.311	Kept	Important to retain for equity considerations (ie not relevant to majority, but important for minority groups)
I have enough knowledge to make a decision about vaccinating my child	58.20%	2.90%	0.224	0.288	Kept	Very close to threshold, retained
I know when my child’s vaccinations are due	56.90%	1.80%	0.062	0.442	Kept	Not expected to be correlated to hesitancy so poor VCF correlation not relevant
I know where to go to get my child vaccinated	71.20%	2.70%	0.231	0.368	Kept	Considered important to retain for equity considerations
I would feel guilty if I did not vaccinate my child and they got a vaccine	70.20%	2.00%	0.259	0.434	Kept	Very close to threshold, retained
I would reschedule my child’s vaccination appointment if they had a cold	57.80%	4.00%	0.051	0.373	Kept	Not expected to be correlated to hesitancy so poor VCF correlation not relevant
It is easy to remember when my child’s vaccinations are due	42.00%	2.50%	0.047	0.422	Kept	Not expected to be correlated to hesitancy so poor VCF correlation not relevant
It is my responsibility to make sure my child is vaccinated on time	71.80%	2.00%	0.281	0.387	Kept	Very close to threshold, retained
My child’s doctor or nurse recommends vaccination	68.00%	4.80%	0.224	0.218	Kept	Retained because previous research shows high association with vaccine uptake
My religious or cultural beliefs influence my vaccination decisions	19.80%	8.00%	0.171	0.508	Kept	Important to retain for equity considerations (ie not relevant to majority, but important for minority groups)

^Bold = results fall outside stated parameters for review.

Figure 2. Example of model performance and refinement (long form).

Figure 3. Short and long forms of the Vaccine Barriers Assessment Tool.

Figure 4. Confirmatory composite analysis of the Vaccine Barriers Assessment Tool (long form).

Australian Bureau of Statistics. Socio-economic indexes for areas. 2022. https://www.abs.gov.au/websitedbs/censushome.nsf/home/seifa#:~:text=Socio%2DEconomic%20Indexes%20for%20Areas%20(SEIFA)%20is%20a%20product,from%20the%20five%2Dyearly%20Census.

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Data availability statement

The data that support the findings of this study are available from the corresponding author, JK, upon reasonable request.