- Design
- Multicentre retrospective cohort, target trial emulation with propensity weighting
- Population
- 196 infants aged 12 months or under hospitalised with pertussis, 11 Italian centres
- Primary outcome
- Severe disease (Pertussis Severity Score above 5)
- Effect
- Azithromycin 60% vs clarithromycin 33%; OR 1.80 (95% CI 1.01-3.21)
This Italian study used records from 11 centres to emulate a trial comparing clarithromycin and azithromycin in infants aged 12 months or younger hospitalised with pertussis between November 2023 and December 2024. The choice of drug was the clinician's, and propensity weighting was used to balance the groups.
Of 196 infants (median age 86 days), 146 received clarithromycin and 50 azithromycin. Severe disease occurred in 60% with azithromycin and 33% with clarithromycin (weighted OR 1.80, 95% CI 1.01 to 3.21). Oxygen use, complications and intensive care admissions were more frequent with azithromycin. Both deaths were in the azithromycin group.
This is observational and small, with a confidence interval that only just excludes no effect. Azithromycin remains the usual choice in young infants because clarithromycin is not recommended under one month of age. The finding raises a question for a trial rather than a reason to switch.
- Start a macrolide promptly in any infant with suspected pertussis; delay is the bigger risk.
- Continue to follow current guidance on macrolide choice by age, particularly under one month.
- Monitor infants with pertussis closely for apnoea, hypoxia and hyperleucocytosis.
- Offer pertussis vaccination in pregnancy, which protects infants before their own doses.
Why it matters
It questions whether the two first-line macrolides are interchangeable in the sickest infants.
Don't overread it
The drug was chosen by clinicians, not randomised, so sicker infants may have received azithromycin.
The statistics, in plain English
An odds ratio of 1.80 with an interval from 1.01 to 3.21 is only just significant. Propensity weighting balances measured differences between groups, but sicker infants may still have been chosen for one drug for reasons not recorded.
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