Background <p>While parents were traditionally the main reservoir for infant pertussis, transmission sources may have shifted with China’s multi-child policy. Our study aimed to investigate the household transmission patterns and clinical characteristics of pertussis in infants.</p> Methods <p>We investigated infant pertussis patients (aged ≤ 6 months) diagnosed at a hospital (Children’s Hospital of Zhejiang University School of Medicine) from 1 January to 1 May 2024. The laboratory diagnosis of pertussis was confirmed through bacterial culture. We collected basic and clinical information and conducted a telephone survey to investigate the transmission patterns within their households.</p> Results <p>A total of 275 infant pertussis cases were identified, including 162 (58.9%) infants aged ≤ 3 months. A total of 188 households completed the transmission pattern survey, of which 121 households had two or more children. The clinical characteristic analysis indicated that infants aged ≤ 3 months had more severe disease, with higher proportion of paroxysmal cyanosis (12.4% vs. 3.5%), elevated leukocyte count (13.3 vs. 9.8 × 10⁹/L), and prolonged hospital stay (10.0 vs. 5.5 days) (all <i>P</i> &lt; 0.05). The household survey revealed significant differences in the distributions of first and source cases between single-child and multi-child households (<i>P</i> &lt; 0.001). In multi-child households, the primary sources of pertussis transmission to infants were siblings aged 3–12 years, accounting for 46.4% of cases, while adults accounted for 32.1%.</p> Conclusions <p>This study highlights the growing involvement of older children in the spread of pertussis, with siblings emerging as the main sources of pertussis transmission to infants in multi-child households. Optimization of pertussis vaccination tactics is advised to protect infants better.</p>