Baseline secondary attack rate in a household for a novel respiratory virus is estimated at 20%. Two independent interventions are available: Intervention A (source masking) reduces transmission probability per contact by 40%; Intervention B (hand hygiene + surface disinfection) reduces fomite-mediated transmission by 50%. Assuming these reductions apply independently to distinct fractions of transmission and that the combined effect on overall per-contact transmission probability can be approximated by multiplicative reduction, what is the new overall secondary attack rate if both interventions are implemented and affect independent portions of exposure?

• A. Approximately 10% because you average the two percent reductions (40% and 50%) to get 45% reduction and then apply it to 20% yielding 11%, rounded to 10% as a simple field estimate for planning purposes rather than precise calculation.
• B. No change (20%) because interventions targeting source masking and fomite control are redundant and cancel each other out in real-world settings, making any mathematical combination irrelevant for policy decisions.
• C. Approximately 12% because you subtract 40% then 50% sequentially from 20% (20% − 40% = negative so assume floor at 0%), and therefore the realistic remaining attack rate is around 12% after adjusting for implementation loss, which is a conservative practical estimate a policy maker might choose.
• D. Approximately 6% (a reduction from 20% to ~6% when the independent effects are combined multiplicatively).

Answer: (D)