Respiratory viral testing for young febrile infants presenting to emergency care: a planned secondary analysis of the Febrile Infants Diagnostic assessment and Outcome (FIDO) prospective observational cohort study.

Objective: To describe the association of respiratory viral test results and the risk of invasive bacterial infection (IBI) for febrile young infants presenting to emergency care.

Design: A planned secondary analysis within the Febrile Infants Diagnostic assessment and Outcome (FIDO) study, a prospective multicentre observational cohort study conducted across the UK and Ireland.

Setting: 35 paediatric emergency departments and assessment units across the UK and Ireland between 6 July 2022 and 31 August 2023.

Patients: Febrile infants aged 90 days and under presenting to emergency care.

Main outcome measures: IBI (meningitis or bacteraemia) among febrile infants, undergoing respiratory viral testing for respiratory syncytial virus (RSV), influenza and SARS-CoV-2.

Results: 1395 out of 1821 participants underwent respiratory viral testing, of those tested 339 (24.5%) tested positive for at least one of, SARS-CoV-2, RSV or influenza. A total of 45 infants (3.2%) were diagnosed with IBI. Of these, IBI occurred in 40 out of 1056 (3.8%) participants with a negative viral test and 5 out of 339 (1.5%) occurred in participants with a positive viral respiratory test (p=0.034). Infants aged 29 days and older with a positive respiratory viral test had a significantly lower rate of IBI (0.7%) compared with those with a negative test (3.2%) (p=0.015).

Conclusions: Young febrile infants with a positive respiratory viral test for SARS-CoV-2, RSV or influenza are at lower risk of IBI. Infants over 28 days of age with a positive viral test represent the lowest risk cohort.

Trial registration number: NCT05259683.