Purpose: This study determined the concentration of sodium hypochlorite and contact times necessary to ensure significant reduction of representative human norovirus GII.4 Sydney from stainless steel surfaces.
Methods: Clarified 20% suspensions of human feces confirmed positive for GII.4 Sydney were used with or without the addition of a tripartite soil load. Aliquots were dried on stainless steel coupons and exposed to sodium hypochlorite (concentrations from 100 PPM – 5,000 PPM) for contact times ranging from 1-30 min. Following neutralization and elution, the suspension was subjected to RNAse pre-treatment and RT-qPCR. Log10 reduction was calculated based on genomic copies. Triplicate samples were tested per data point.
Results: For treatment in the absence of soil load, a 750 PPM sodium hypochlorite solution produced a 5.3 ± 0.5 log10 genome copy number reduction after a 1 min contact time and a solution as low as 400 PPM showed 5.1 ± 0.5 log10 reduction after 5 min. Addition of a supplemental soil load (5%) to clarified virus significantly reduced disinfectant activity, such that a 5000 PPM sodium hypochlorite solution resulted in only a 3.4 ± 0.2 log10 reduction in genome copy number after a 1 min exposure.
Significance: For clean surfaces, a 400 PPM sodium hypochlorite solution is the minimum concentration necessary to reliably cause a 4 log10 reduction in genome copy number with a contact time less than 5 min. The presence of excess soil almost completely abolished disinfectant efficacy. It may be possible to use lower concentrations of free chlorine, if thorough surface cleaning is done prior to disinfection.