Transmission timing and containment risk in human Andes virus outbreaks

The 2026 multinational Andes virus (ANDV) outbreak linked to cruise-ship travel from Argentina triggered extensive contact tracing and quarantine. Whether intervention on this scale was necessary or sufficient depends on the risk of onward transmission, which can be difficult to estimate early in an outbreak. Using transmission-pair data from the 2018--2019 outbreak in Epuyen, Argentina, and a mechanistic model linking viral growth, symptom onset, and transmission, we estimated that 42.4% (95% CrI: 18.2-69.7\%) of transmission occurs before symptom onset and that the median incubation period is 18.5 days (95% CrI: 16.5-19.5). We simulated outbreaks under an Epuyen-calibrated estimate (R = 2.12, k = 0.316) and a pooled estimate from 15 prior ANDV investigations (R = 0.24, k = 0.39), with intervention beginning after two generations, as in the cruise-associated event. Under the higher-transmission scenario, missing 10% of exposed individuals or downstream contacts resulted in a 29.9% probability of continued spread and a 20.6% probability that at least 5,000 ultimately uninfected people would be quarantined. Under lower transmission, continued spread stayed below 8%. However, the risk of onward transmission could not be estimated precisely from the first several detected cases. These findings therefore support rapid, comprehensive contact tracing and quarantine after large international exposure events despite substantial costs.