Needle and syringe program (NSP) coverage is often calculated at the individual level. This method relates sterile needle and syringe acquisition to injecting frequency, resulting in a percentage of injecting episodes that utilise a sterile syringe. Most previous research using this method was restricted by their cross-sectional design, calling for longitudinal exploration of coverage.

We used the data of 518 participants from an ongoing cohort of people who inject drugs in Melbourne, Australia. We calculated individual-level syringe coverage for the two weeks prior to each interview, then dichotomised the outcome as either "sufficient" (≥100% of injecting episodes covered by at least one reported sterile syringe) or "insufficient" (<100%). Time-variant predictors of change in recent coverage (from sufficient to insufficient coverage) were estimated longitudinally using logistic regression with fixed effects for each participant.

Transitioning to methamphetamine injection (AOR:2.16, p=0.004) and a newly positive HCV RNA test result (AOR:4.93, p=0.001) were both associated with increased odds of change to insufficient coverage, whilst change to utilising NSPs as the primary source of syringe acquisition (AOR: 0.41, p=0.003) and opioid substitution therapy (OST) enrolment (AOR:0.51, p=0.013) were protective against a change to insufficient coverage.

We statistically tested the transitions between time-variant exposure sub-groups and transitions in individual-level syringe coverage. Our results give important insights into means of improving coverage at the individual level, suggesting that methamphetamine injectors should be targeted, whilst both OST prescription and NSP should be expanded.