Twins with at least one with CHD and their immunisation status in direct comparison-are both twins complying with the German immunisation recommendations?

Background: Patients with congenital heart defects (CHDs) are at higher risk for infectious diseases. This may partly be due to frequent hospital stays and the associated exposure to pathogens. This study aims to provide a comprehensive overview of immunisation coverage among twins in which at least one twin has CHD. Confounding factors from shared environments and genetic components can be controlled through co-twin control analysis, thus minimising confounding effects. Methods: In the framework of the cross-sectional twin study "Same Same, but different?" twins, with at least one of them having CHD aged 3 to 99 years, were recruited nationwide in Germany between August 2019 and December 2022. Their primary immunisation status based on the German Standing Committee on Vaccination (STIKO) and immunisation against respiratory diseases, including influenza, respiratory syncytial virus (RSV), pneumococci, and coronavirus disease 2019 (COVID-19), were assessed and compared between the twins. Results: In total, 64 twins (128 individuals) were included for direct twin comparison. Overall, 56.3% of the twins reached complete primary immunisation status, negatively influenced by hospitalisation duration [odds ratio (OR): 0.98; 95% confidence interval (CI): 0.96-0.99; P=0.01]. Compared to their healthy twin, twins with CHD received their rotavirus vaccine significantly later (P=0.04). Only 3.1% of the twins with CHD received the pneumococcal vaccine recommended for high-risk patients. A higher number of catheter interventions can lead to a higher number of patients receiving the pneumococcal vaccine (OR: 1.79; 95% CI: 1.16-2.76; P=0.009). The direct twin comparison showed a significant difference between the twins in vaccination against influenza (P=0.007), although it is recommended for CHD patients and their household contacts-including their twin. A higher number of surgeries (OR: 1.51; 95% CI: 1.12-2.05; P=0.007) and catheter interventions (OR: 1.49; 95% CI: 1.00-2.21; P=0.049) increase the probability of influenza vaccination in CHD patients. Conclusions: In the direct twin comparison, twins are similarly vaccinated except for RSV and influenza. Immunisation against influenza in twins should be improved. With new upcoming RSV vaccines, existing recommendations must be reconsidered and adapted. Another disturbing fact is that only 30% of infants are vaccinated against pertussis and pneumococcus within the primary recommended timeframe, even though they are exposed at high risk during infancy. Further education of parents, patients, and medical staff might lead to higher vaccination coverage, especially in pneumococcal vaccines recommended for high-risk patients. We must provide sufficient information on the importance of vaccinations and their side effects for parents' and patients' decision-making.