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210 Silberg et al. | Journal of Clinical and Translational Research 2024; 10(3): 209-211
Table 1. Additive genetic, shared environmental, and non-shared environmental components of variance for vaccine hesitancy
Model Genes Shared environment Non‑shared environment −2 ln (L) Df χ diff P
2
ACE 0.0 0.77 0.23 489.77 3 - -
E 0.0 0.0 1.00 548.65 5 58.88 <0.0001
CE* 0.0 0.77 0.23 489.77 4 0 ns
AE 0.35 0.0 0.65 495.88 4 6.11 <0.001
Note: *Best-fitting model.
2
Abbreviations: −2 ln (L): Goodness of fit; A: Genes; C: Shared environment; E: Non-shared environment; Df: Degrees of freedom; ns: Non-significant; χ diff: Chi-square difference.
2.3. Data analysis (−2 ln[L]), and the Chi-square (χ ) difference between them. The
2
standardized genetic and environmental components of variance
The comparison in similarity of MZ versus DZ twins is
the foundation for estimating the contribution of genetic, are reported for each model. The full model comprised additive
genetic factors (A), shared environmental factors (C), and non-
shared environmental, and non-shared environmental factors
to vaccine hesitancy [6]. Additive genetic effects reflect the shared environmental factors (E) and was tested against three
average effect of individual alleles and genetic loci of a trait. alternative models: (i) A model with unique environmental factors
Because MZ twins (on average) share 100% of their genes alone (E); (ii) a model without a genetic influence (only C and E);
and DZ twins 50% of their genes in common, a higher MZ and (iii) a model without the shared environment (only A and E).
correlation to DZ correlation suggests that genetic factors are The full three-parameter model provided a good fit to the data (−2
influencing the trait. Common environmental effects describe ln[L] = 489.77). Eliminating the genetic parameter (A) did not affect the
influences which make family members more alike compared fit of the model (−2 ln[L] = 489.77; 1 degree of freedom [Df]), whereas
to random pairs of individuals, such as peers, family, and the eliminating the shared environment (C) resulted in a significantly worse
2
wider community.This shared environment is reflected in a DZ fit (−2 ln[L] = 495.88; χ difference = 6.11; 1 Df; P < 0.001). For the
correlation greater than one-half the MZ correlation. Unique best-fitting CE model, 77% of the variation in vaccine hesitancy is
environmental factors (including error of measurement) are accounted for by environmental factors shared by the twins.
those variables that affect only one MZ twin of the pair and 4. Conclusion
create differences in MZ twins despite their identical genotypes.
Tetrachoric correlations for vaccine acceptance in the MZ and This study provides strong empirical support for the role of
DZ twins were estimated using SAS software [7]. Genetic and the environment in vaccine acceptance. In contrast to studies
environmental models, controlling for age and sex, were fitted of other social–political attitudes, genetic factors do not play
to the twin data using the statistical program OpenMx [8]. a role. The overwhelming information from the media and
government agencies about getting vaccinated is the most likely
3. Results and Discussion explanation for these findings. A lack of trust was by far the most
Female twins, particularly MZ twins, were more highly important reason for vaccine hesitancy. The results underscore
represented than male twins. The age range of the twins was the need for bold new strategies to expedite the acceptance of
18.2 – 72.4 with a median age of 35.1 for the MZ twins and the COVID-19 vaccine and other vaccines that offer protection
34.8 for the DZ twins. from viral outbreaks in the future.
Nearly half of the twins (47%) indicated that they had acute
COVID-19 symptoms since 2020 (n = 540). By October 2021, Acknowledgments
90% of the sample indicated that they were vaccinated (n = 1035). The project was facilitated by the Mid-Atlantic Twin Registry
From a potential sample of 3586 twins, 1793 were successfully administered by the Cohort the Registry Administration (CARA)
contacted via email or phone. Out of these, 1150 individual twins, Core, a Virginia Commonwealth University shared resource.
comprising 325 MZ and 115 DZ twin pairs, provided their data. We would like to extend our gratitude to all of the MATR twins
Of the 115 twins that were not vaccinated: (i) 60% indicated “a who took part in this study.
lack of trust” was the reason for not getting the vaccine; (ii) 10 – 20%
said: “It will not help,” “Vaccination is worse than being ill,” “It is Funding
just a virus/not fatal/not necessary,” “It depends on the risks/adverse This study was funded by NIH UL1TR002649.
events,” “I am not in a risk group with underlying conditions,” and/
or “I need more information first;” and less than 10% said: “I will Conflict of Interest
not get/am never sick,” “I never get vaccinated,” “I do not want to
pay for it,” and/or “My region is not a high-risk area.” The authors declare no conflicting interest.
The tetrachoric correlations for vaccine acceptance indicate a Ethics Approval and Consent to Participate
high degree of similarity in the MZ and DZ twins (0.78 vs. 0.81,
respectively), suggesting that genes have little effect on one’s Informed consent was obtained before the subjects
willingness to be vaccinated. Table 1 displays the results of the participated in the study through the virtual data platform. The
model fitting, inclusive of alternative models, their goodness of fit study was approved by the VCU IRB (HM200021382).

DOI: http://doi.org/10.36922/jctr.24.00002