Objective To evaluate the epidemiology and population trends of early-onset sepsis in very preterm neonates admitted to neonatal intensive care units (NICU) in Australia and New Zealand.

Design Retrospective observational cohort study using a dual-nation registry database.

Setting 29 NICUs that have contributed to the Australian and New Zealand Neonatal Network.

Participants Neonates born at <32 weeks’ gestation born between 2007 and 2018 and then admitted to a NICU.

Main outcome measures Microorganism profiles, incidence, mortality and morbidity.

Results Over the 12-year period, 614 early-onset sepsis cases from 43 178 very preterm admissions (14.2/1000 admissions) were identified. The trends of early-onset sepsis incidence remained stable, varying between 9.8 and 19.4/1000 admissions (linear trend, p=0.56). The leading causative organisms were Escherichia coli (E. coli) (33.7%) followed by group B Streptococcus (GBS) (16.1%). The incidence of E. coli increased between 2007 (3.2/1000 admissions) and 2018 (8.3/1000 admissions; p=0.02). Neonates with E. coli had higher odds of mortality compared with those with GBS (OR=2.8, 95% CI 1.2 to 6.1). Mortality due to GBS decreased over the same period (2007: 0.6/1000 admissions, 2018: 0.0/1000 admissions; p=0.01). Early-onset sepsis tripled the odds of mortality (OR=3.0, 95% CI 2.4 to 3.7) and halved the odds of survival without morbidity (OR=0.5, 95% CI 0.4 to 0.6).

Conclusion Early-onset sepsis remains an important condition among very preterm populations. Furthermore, E. coli is a dominant microorganism of very preterm early-onset sepsis in Australia and New Zealand. Rates of E. coli have been increasing in recent years, while GBS-associated mortality has decreased.