Essentially, the number of new cases in a susceptible population
likely to arise from a single infection. With an R0 below 1.0, a virus
(as an outbreak) begins to sputter and dies out. Above 1.0, and
an outbreak can have `legs’.
where some people may be immune and control measures are in place
#19,334
Four months ago the big infectious disease story was an unusual outbreak of Andes Hantavirus (ANDV) aboard a cruise ship out of Argentina, which resulted in 13 identified cases (12 laboratory-confirmed and 1 probable) and 3 fatalities.
The incubation period for ANDV can run up to 6 weeks, with a 42-day monitoring period generally recommended. Some cases can be mild, or even asymptomatic, but the CFR (case fatality rate) can reach 30%-60%.
Although outbreaks of ANDV are fairly rare, and generally limited in size, we've followed several outliers over the years.
The WHO DON report stated:Including, most famously, a 2019 outbreak in Argentina - where, over a period of 3 months - a total of 34 laboratory-confirmed cases of Hanta Pulmonary Syndrome (HPS) - including 11 deaths - were reported in Epuyén, Chubut Province.
The index case had environmental exposure prior to symptom onset on 2 November, and subsequently attended a party on 3 November. Six cases who also attended the party experienced the onset of symptoms between 20-27 November 2018.
An additional 17 cases, all of whom were epidemiologically-linked to previously confirmed cases, experienced symptom onset between 7 December 2018 and 3 January 2019 (Figure 1). Potential human-to-human transmission is currently under investigation.
The idea that 20% of those infected may be responsible for 80% of the forward transmission of a virus has been around for decades, but gained momentum after the 2003 SARS outbreak.
But a small percentage of patients proved unusually efficient at spreading the virus, with some responsible for 10 or more secondary infections (see MMWR Severe Acute Respiratory Syndrome --- Singapore, 2003).
Super-spreading events are shaped by host, pathogen, and environmental factors. Often, more than one factor may be implicated in the same outbreak.
Superspreading events aren’t limited to coronaviruses, as they have been documented with measles, HIV, TB, S. aureus, Ebola, and various STDs . . .among others (cite)
In January of 2013, in Influenza Transmission, PPEs & `Super Emitters’ we looked at research that found five patients (19 percent) in their study were "super-emitters" who emitted up to 32 times more flu virus than did the rest. Patients who emitted a higher concentration of influenza virus also reported greater severity of illness.
All of which brings us to an EID Journal Dispatch which finds that the 20/80 rule also applies across a larger sampling of ANDV clusters reported over the years, and while the pooled average Rt remains at a reassuring .74, in some outbreaks (including the NEJM report) that number approached or exceeded 1.0.While the host and the pathogen are important parts to the equation, environment and opportunity also play a pivotal role in exacerbating these superspreader events.
Dispatch
Characteristics and Superspreading Potential of Andes Virus Person-to-Person Transmission
Zihao Guo1, Kailun Pan1, Yu Zhao, Sheikh Taslim Ali, Kai Wang, Lirong Cao, Zhuang Cui, Shengqiang Liu, Ka Chun Chong, Daihai He, Shi Zhao
Abstract
By using historical contact tracing data, we estimated that 23.4% of case-patients caused 80% of Andes virus (ANDV) person-to-person transmission. We demonstrated a low but nonnegligible probability of observing a large-scale ANDV infection outbreak in a rodent-free setting consisting of close contacts, despite the historically self-limited person-to-person transmission of ANDV.
(SNIP)
On May 2, 2026, an ANDV outbreak aboard the MV Hondius cruise ship was reported to the World Health Organization (WHO), with the first case illness onset on April 3 (10). Subsequent epidemiologic investigations proposed a working hypothesis that person-to-person transmission originated from a seed case in a person who probably acquired infection through environmental exposure during travel in Argentina before boarding the ship on April 1 (10). By July 2, a total of 13 ANDV cases had been identified, including 12 laboratory-confirmed and 1 probable case; 3 of those cases were fatal (10).According to the initial WHO notification for the outbreak on May 2, public health and control measures, including contact tracing, case isolation, clinical care, and medical evacuation of symptomatic passengers, had been implemented onboard the ship (11). By using historical contact tracing data, we aimed to estimate key epidemiologic characteristics of person-to-person transmission of ANDV, including the serial interval (SI) distribution and transmission heterogeneity, given that those traits strongly shape outbreak size.
(SNIP)
We identified a total of 88 case-clusters from the included studies, among which 50 (56.8%) were not associated with any secondary person-to-person transmission events. After accounting for variations across lineages or strains, the pooled estimate of Rt was 0.74 (95% credible interval [CrI] 0.28–1.29) and the estimate of k was 0.64 (95% CrI 0.36–1.16). Apart from inherent differences among ANDV lineages or strains, the substantial variation in Rt across lineages or strains could also be attributed to differences in study settings and individual-level heterogeneity in reproduction numbers (5).
We estimated that 23.4% (95% CrI 15.1%–30.3%) of the cases generated 80% of the transmission events, suggesting the person-to-person spread of ANDV exhibited relatively high heterogeneity, although it appeared lower than that observed for other pathogens known for superspreading events, including SARS-CoV, SARS-CoV-2, Middle East respiratory syndrome coronavirus, and Ebola virus (13,14).
(SNIP)
Conclusions
In summary, our results provide insights into the person-to-person transmission potential of ANDV, which exhibited substantial heterogeneity. We found a low but significant chance of observing a relatively large-scale ANDV outbreak in a rodent-free setting consisting of close contacts, which might be eradicated across a few generations of person-to-person transmission. Because no vaccine or antiviral treatment for ANDV infection is currently available, continuous surveillance of person-to-person transmission risk of ANDV is essential for preparedness against future outbreaks.
Dr. Guo is an associate professor in the School of Public Health, Tianjin Medical University, Tianjin, China. His primary research interests include epidemiology and transmission dynamics of emerging infectious diseases.
Simply put, while most ANDV outbreaks are likely to sputter out relatively quickly, there is enough variability in the transmission of the virus that occasional larger outbreaks are possible.
For more on superspreader events you may wish to revisit:
MMWR Early Release: COVID-19 Superspreading Event In A Church Choir
CDC Update: Rapid Assessment Of COVID Outbreak At 2023 EIS ConferenceZiad Memish: Two MERS-CoV Hospital Super Spreading Studies
The Lancet: Mapping The Korean MERS-CoV Superspreading Event
