Abstract
BACKGROUND: While the live attenuated measles virus vaccine is generally safe and highly effective, it has been associated with a significant number of vaccine-associated rash illnesses (VARIs). Some VARIs had been identified through the measles case-based surveillance system in Shandong province. METHODS: In our study, we comprehensively examined the epidemiological characteristics of VARIs using a cross-sectional study design with comparative subgroup analysis. Additionally, we conducted a detailed analysis of the genetic properties of VARIs. RESULTS: Among the 189 cases of VARIs, 90.47% (171 cases) were children aged 8 to 18 months who had received a single dose of measles-containing vaccine (MCV). Additionally, 16.40% (31/189) of these VARIs exhibited at least one symptom such as cough, coryza, or conjunctivitis. A significant linear correlation was observed between measles morbidity and the incidence of VARIs (R = 0.76, p = 0.03). Of the 189 VARIs, 109 cases were sampled throat swabs and 47.09% cases (89/189) were sampled within 5 days after the onset of rash, and only 30.28% (33/109) of these were successfully genotyped. Among the successfully genotyped sequences, 90.91% (30 sequences) were 100% consistent with the Shanghai-191 strain, with nucleotide identities ranging from 99.7% to 100%. Selective pressure analysis revealed no evidence of positive selection within the genotype A sequences from Shandong. CONCLUSION: Our data demonstrate that VARI cases predominantly occur in children after single-dose MCV vaccination. Genotyping remains essential to prevent misdiagnosis and ensure accurate surveillance. This highlights the urgent requirement for either improved genotyping efficiency or vaccine-strain-specific rapid tests. CLINICAL TRIAL: Not applicable.