Co-infection in unvaccinated infants with acute pertussis in Western China (2018-2019): pathogen distribution and impact on disease severity.

Background: Co-infections in pertussis patients are common, but there has been limited research on the distribution of co-infecting pathogens and their impact on disease severity in infant patients remaining unvaccinated against pertussis. This study aims to investigate the pathogen distribution in unvaccinated infants with acute pertussis and explore how the number and type of co-infecting pathogens influence disease severity.

Method: This cross-sectional study analyzed clinical data from 302 unvaccinated infants diagnosed with acute pertussis in western China. We compared clinical variables across different co-infection groups (bacteria, viruses, bacterial-viral combinations) and by the number of co-infecting pathogens (0, 1, ≥ 2).

Results: Of the 302 patients, 121 (40.1%) were infected solely with Bordetella pertussis, while 181 (59.9%) had co-infections with other pathogens. The most common co-infections were bacterial (93 of 139 cases), particularly Gram-negative bacteria, followed by viral co-infections, mainly parainfluenza virus type-3 (PIV-3), in 71.3% of viral cases. The number of co-infecting pathogens was positively associated with longer hospital stays, more severe pneumonia, and higher incidence of respiratory failure (P < 0.05). Notably, bacterial co-infections were associated with more severe clinical outcomes than viral co-infections, with significant differences in hospitalization duration, as well as in peak white blood cell and lymphocyte counts (P < 0.05). No significant differences were observed in co-infection types or pathogen numbers across different age groups.

Conclusion: Co-infections are prevalent among unvaccinated infants with acute pertussis in western China. Bacterial and viral pathogens are the most common co-infecting agents, and disease severity increases with the number of co-infecting pathogens. Bacterial co-infections may lead to more severe outcomes compared to viral co-infections, underscoring the need for targeted diagnostic and therapeutic strategies.