Background: Viral infections remain the cause of key complications following haematopoietic stem cell transplantation (HSCT). The impact of multiple, concurrent viral reactivations/infections remains to be delineated.

Methods: The clinical correlates of single or multiple viremic infections following HSCT and especially the occurrence of respiratory viruses in the bloodstream were investigated. We retrospectively searched for 23 viruses in a total of 184 sera from 53 paediatric patients. The time-points of interest were pre-HSCT, one, two and three months post-HSCT, and at discharge or death. The viruses were analyzed by quantitative or qualitative PCR.

Results: Of the 53 patients, 13 (25%) had viraemias by multiple viruses and 27 (51%) by a single virus. Thirteen patients (25%) had no viruses detected by PCR during the study period. In the children with viremic co-infections, polyomaviruses predominated over herpes viruses. Nearly half the patients, 24/53 (45%) had a polyomavirus in their serum at one or more time-points. At 12/15 time-points and in 11/13 patients with co-infections polyomaviruses were involved, compared with 6/15 time-points and 6/13 patients for cytomegalovirus. Acute graft-versus-host disease (GvHD) and steroid use were significant risk factors for the viraemias caused by more than one virus.

Conclusions: Viral co-detection is a common finding in children undergoing HSCT. With large-scale viral screening also viruses other than CMV could be found as potential pathogens. In this study, BKPyV predominated over CMV as a contributor in viraemias caused by multiple viruses in children receiving HSCT.