A model to assess the infection potential of jet injectors used in mass immunisation.

Jet injectors are needleless injectors that penetrate skin with high-pressure fluid. They have potential advantages over needles and syringes in mass immunisation programs, but concerns over their capacity to transfer blood-borne viruses have been a barrier to acceptance. Hepatitis B infection can transmit in 10 pl of blood; detection of such low volumes presents severe difficulties to such assessments. A model to assess jet injector safety was developed using injection of an inert buffer into calves and assaying the next injector discharge, representing the next dose of vaccine, for blood using a highly sensitive ELISA. Four injectors were tested: two with reusable heads and direct skin contact, one with single-use injector heads and one where the injector head discharged at a distance from the skin. All injectors tested transmitted significant (over 10 pl) volumes of blood; the volumes and frequency of contamination varied with injector. The source of the contamination was consistent with contamination by efflux of injected fluid and blood from the pressurised pocket in tissue that is formed during injection. This insight should inform the design of safe jet injectors.