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On hantavirus #ANDV #hantavirus I think the Andes hantavirus data is being misread right now. Claims are circulating that the evidence doesn’t seem to support. I want to walk through them carefully.
The 40% fatality figure: an artefact of who gets counted. The 40% case fatality rate (CFR) figure comes specifically from hantavirus pulmonary syndrome (HPS), an outcome in a subset of hanta infections. This is the severe cardiopulmonary presentation. It excludes subclinical infections that resolved without anyone noticing more than “a touch of flu”. In Jujuy Province, Argentina, the seroprevalence (rate at which people have antibodies due to hanta infection) is 6.5%. Hospitalised HPS cases had a CFR of 13.3%, but most patients were described as having a mild clinical course. Disclaimer: although an Argentine outbreak, this has not been confirmed as ANDV. The 40% is the fatality rate among people sick enough to be diagnosed and hospitalised. It is not the infection fatality rate. These are not the same thing, and conflating them is causing significant confusion.
The true attack rate: barely any data. The Boat had approximately 180 exposed individuals, and around ten cases have been detected. Three - soon to be four, I predict - have died. Without antibody analysis of the full cohort, we don’t know how many mild or subclinical infections were missed entirely.
At Epuyén (The Birthday Party), around 100 people attended the event. Five secondary cases were traced to the index case. No systematic serology (antibody analysis) of all attendees was conducted. The true attack rate at that party is simply unknown. This matters enormously. If the true infected population is substantially larger than the diagnosed one, both the attack rate and the fatality rate look very different.
Household transmission: hard to explain by airborne spread Ferres et al. (2007) prospectively studied 476 household contacts of 76 index HPS cases in Chile. Overall secondary attack rate: 3.4%. Among sex partners of index cases: 17.6%. Among other household contacts: 1.2%. This is a statistically significant difference. People sharing the same house, the same air and the same bathroom have a 1.2% infection rate. Sex partners of the same index case have a 17.6% infection rate. Efficient airborne transmission does not produce this pattern.
Healthcare workers: a natural experiment. If ANDV spreads via respiratory or airborne transmission, healthcare workers are pretty much the highest-risk cohort. They are pumping lungs of fluid. Cleaning nasal passages and throats. They manage respiratory failure at close range, repeatedly, in the same room as very ill patients. If the virus were travelling efficiently through the air, they should be getting infected. Three prospective serological studies have looked at exactly this question.
Chaparro et al, Journal of Hospital Infection, 1998. 319 healthcare workers across an entire hospital in Coyhaique, Chile, tested during the 1997 HPS outbreak. Fewer than half always used gloves when touching patients or their fluids. Respiratory protection was not used. Antibody prevalence: 3.7%, indistinguishable from community background rates. Note: 3.7% of the local community had been, at some point, infected with hantavirus and lived to tell the tale. Crucially, there was no statistical association between antibody positivity and patient contact, nor with the type of hospital activities performed. The workers who had most contact with patients were no more likely to be seropositive than those who had least. Zero transmission to 319 healthcare workers.
Castillo et al, American Journal of Tropical Medicine and Hygiene, 2004. 109 healthcare worker contacts of 20 confirmed HPS index cases in Chile. Antibody seroprevalence: 0.0%. For comparison, family member contacts of the same index cases had a seroprevalence of 1.9%, itself lower than community background rates in endemic rural areas. Zero transmission to healthcare workers.
Martinez VP et al, New England Journal of Medicine, 2020. The Epuyén Birthday Party outbreak. More than 80 healthcare workers had direct contact with severely ill patients throughout the outbreak. PPE adherence was minimal by the authors’ own account. Zero infections documented among healthcare workers. The R0 of 2.12 reported in this paper is also worth reading carefully. It was derived entirely from transmission events within intimate social contexts in a small village: birthday parties, wakes, households. It is not a general population estimate and should not be treated as one.
An exception: no. Martinez-Valdebenito et al (2014) confirmed transmission to two healthcare workers in southern Chile using genomic sequencing of the virus. But it is a case linkage study, not a prospective cohort. There is no denominator - we don’t know how many other healthcare workers were exposed to the same index patient and remained uninfected. Both infected workers had direct body fluid contact during the early febrile phase, which appears to be the highest-risk window. The study confirms that transmission to healthcare workers can occur. It cannot tell us how often it does relative to total exposure events. And for that, we already have much wider cohort analyses.
Why are healthcare workers almost uniformly protected? It isn’t PPE. Across these studies, PPE use was inconsistent to poor. But I was thinking: healthcare workers (all medical, lab etc) have a pretty standard Golden Rule. Do not touch your face after patient contact. Do not touch your face with gloves. Wash your hands before picking your nose, eating or putting your hands anywhere near your mouth. We’ve all had this basic hygiene drilled into us from COVID.
It doesn’t look airborne. Even when you are pulling fluid from a severely ill patient without a mask, you don’t seem to catch it. If transmission requires transfer from a fluid source, healthcare workers don’t get it because they don’t put their hands in their mouths after touching a patient, a trolley or changing a bed.
There are a handful of patient-to-patient transmissions. Same room, shared bathroom?, same healthcare workers? The fella with the bladder infection in the same room as a HPS patient, touching the same taps and same trollies? Washing his face? Eating with sub-par hygiene? While healthcare workers are not touching their mouths?
Fomites: don’t put your hands in your mouth. For example, ANDV survives on fabric and bedding for up to several weeks in cool, dark, humid conditions. The Boat, with shared linen, communal dining, limited bathroom facilities and shared equipment is a fomite environment at least as much as it is an airborne one. And these two routes cannot be distinguished from the available data.
A note: I’m not downplaying a default to “airborne” as a precaution. I’d have put every passenger in a nice hotel in The Maldives for eight weeks. I’d certainly be seeking antibody testing in the entire cohort for that duration. It’s the kind of natural experiment that doesn’t come round too often.
Fomite transmission via hand-to-mucous membrane (mouths, noses, GI tract, genitals) contact is consistent with the scattered transmission pattern at The Birthday Party, which skipped people in closer proximity to the index case than those who were infected. It is consistent with patient-to-patient spread in hospital wards. It is consistent with healthcare worker immunity despite repeated respiratory exposure. And it is consistent with sex partner transmission versus other household members sharing the same airspace.
What the literature is missing. A systematic antibody survey of all exposed contacts, cross-referenced with symptom history. The Boat: approximately 180 exposed individuals, no cohort testing. The Birthday Party: approximately 100 attendees, no cohort testing. The 80+ Epuyén healthcare workers: none serotyped for a particular antibody type to establish whether any asymptomatic antibody prodction had occurred. Without this, we cannot establish the true attack rate, the true infection fatality rate or the transmission route with any confidence.
The evidence across multiple cohort studies points toward transmission requiring intimate contact or fomite-mediated hand-to-mucous membrane transfer, not efficient airborne spread. Look at the healthcare worker data. It’s almost the best we have to test the hypothesis that ANDV is airborne. And it doesn’t fit.
Papers: Chaparro et al, J Hosp Infect 1998;40:281-285 Castillo et al, Rev Med Chil 2000;128:735-9 Castillo et al, Am J Trop Med Hyg 2004;70:302-4 Ferres et al, J Infect Dis 2007;195:1563-71 Alonso et al, Emerg Infect Dis 2020;26:756-759 Martinez-Valdebenito et al, Emerg Infect Dis 2014;20:1629-36 Martinez VP et al, NEJM 2020;383:2230-2241
And a wry note: I'd tag Trisha Greenhalgh but she blocked me for saying men can't be women. Always follow the science ;)