 results were presented in [Supplementary Table S2](#SD1){ref-type="supplementary-material"}. The observed genotype frequencies of the rs1800818 and rs1800817 polymorphisms were in Hardy-Weinberg equilibrium in both patients and controls groups (all *P* \> 0.05, data not shown). By using multivariate logistic regression model to adjust for the effect from age, sex, and underlying disease, significant associations with SFTS were observed for the *PDGF-B* rs1800818. The association between rs1800818 and SFTS remained after multiple corrections ([Supplementary Table S2](#SD1){ref-type="supplementary-material"}). There was no association between rs1800817 polymorphism and SFTS ([Supplementary Table S2](#SD1){ref-type="supplementary-material"}). Therefore we focused on rs1800818 for further study. With DNA samples from more patients available over time, we genotyped rs1800818 from additional 770 SFTS patients and 1103 controls by PCR-direct sequencing, which results were combined with those obtained from initial small-scale association study for analysis. In all of the 1020 SFTS patients and 1353 controls, rs1800818 G allele was significantly overrepresented in SFTS patients than controls (9% vs. 5.6%, *P* \< 0.001). After adjustment for age, sex, and underlying medical conditions, the genotypes containing G allele (AG + GG genotypes) were significantly associated with increased susceptibility to SFTS (OR = 1.66, 95% CI = 1.28-2.16; *P* \< 0.001; Table [2](#T2){ref-type="table"}, Figure [1a](#F1){ref-type="fig"}).

###### Association of *PDGF-B* rs1800818 polymorphism with severe fever with thrombocytopenia syndrome

  Genotypes   Patients (n = 1020)   Controls (n = 1353)   Model          OR (95% CI)[^a^](#tfn_003){ref-type="table-fn"}   *P* value[^a^](#tfn_003){ref-type="table-fn"}
  ----------- --------------------- --------------------- -------------- ------------------------------------------------- -----------------------------------------------
  AA          799 (83.1)            1115 (82.4)                     