Secondary attack rate

The secondary attack rate (SAR) measures how often the disease spreads from a primary case to their close contacts. It's calculated as the number of new cases among contacts divided by the total number of contacts, within a defined incubation period after the initial case's symptom onset. SAR is most often measured within households (household attack rate) where contact intensity is high and exposure is well-defined.

Reported secondary attack rates vary widely by pathogen and setting. Measles household SAR exceeds 90% in unvaccinated populations. SARS-CoV-2 household SAR ranged from 15% to 40% depending on variant and intervention level. Ebola household SAR is typically 12% to 18% — high enough to drive outbreak spread but well below the explosive household transmission of respiratory pathogens. Hantavirus SAR is essentially zero in households because of the rodent-to-human transmission route, with Andes virus the documented exception.

SAR is the workhorse measure of person-to-person transmissibility because it can be measured empirically during an outbreak, while R0 requires modeling. It informs containment strategy: high household SAR means quarantine and case-isolation are essential; low SAR means broader vector or environmental measures may dominate the response.