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Despite isolated reports suggesting possible human-to-human transmission of the Andes virus, experts urged caution in drawing conclusions about its pandemic potential based on limited observations. The virus is not currently considered a highly contagious respiratory pathogen, and available evidence suggests that transmission occurs primarily during close and prolonged contact, particularly within households or between partners.

The cruise ship M/V Hondius, where an outbreak of the Andes virus infection occurred, arrived at Rotterdam, Netherlands, on May 18. Most passengers had already disembarked earlier in Tenerife on the Canary Islands and returned to their home countries under strict safety precautions.

As of May 17, the European Centre for Disease Prevention and Control (ECDC) had identified 12 cases, including nine confirmed cases, two probable cases, and one case of uncertain status. Three patients died due to the infection.

Passengers who left the ship were under medical monitoring in their home countries. The World Health Organization (WHO) and the ECDC later issued recommendations on quarantine procedures and the use of protective equipment to reduce the risk for further transmission.

According to a report published in The BMJ, human-to-human transmission of the Andes virus is not merely theoretical. Researchers have documented evidence suggesting this transmission route on several occasions over nearly three decades.

One widely cited example was the 2018 outbreak in Epuyén, Argentina, in which researchers reconstructed transmission chains involving 34 confirmed cases and 11 deaths. The outbreak was triggered by a single zoonotic event and appeared to intensify after symptomatic individuals attended large social gatherings, including birthday parties and funerals.

Birthday gatherings, in particular, provide important clues about possible transmission routes. More than 100 people attended the event, and the index patient remained there for approximately 90 minutes despite having symptoms. Secondary infections later developed even among guests seated up to approximately 2.5 m away. Epidemiologists have also reported a case involving only a brief encounter with the index patient with no direct physical contact.

The authors of the Epuyén study concluded that the infection may have been transmitted through the inhalation of droplets or aerosolized virions.

An opinion piece in The British Medical Journal (BMJ) calling on the WHO to shift its default response to emerging respiratory viruses. The authors argued that such findings support the early implementation of precautionary airborne measures when new cases appear rather than delaying intervention. They also noted that repeated associations with shared indoor spaces, prolonged encounters, large gatherings, and transportation settings were consistent with possible aerosol spread.

Transmission Mechanisms

According to Jonas Schmidt-Chanasit, PhD, professor and head of the Department of Arbovirology, Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany, human-to-human transmission of the Andes virus has been reported, particularly among household contacts, partners, caregivers, and in situations involving close and prolonged exposure. Based on earlier outbreaks, respiratory bodily fluids may have played a role. Whether individual cases involve droplets, short-range aerosols, or a combination of both is not always clearly distinguishable.

The WHO noted that transmission risk may be especially high during the prodromal phase and therefore recommends beginning contact tracing up to 2 days before symptom onset. The International Hantavirus Society has also cautioned against assuming that transmission occurs only after clearly recognizable symptoms appear.

How Should Doctors Protect Themselves?

Speaking with the Science Media Center, Schmidt-Chanasit said, “In the medical field, I do not consider protection against droplets alone — using a simple medical mask — to be sufficient when treating a confirmed or highly suspected case of the Andes virus. In my view, the combination of gloves, protective gowns, eye protection, and FFP2 respirators is appropriate because it provides significantly better protection not only against classic droplets but also against smaller respiratory particles at close range.”

For passengers and contacts, however, the situation is different. There, a medical mask primarily serves as a source control measure and a pragmatic additional precaution, for example, during transport, screening, or situations where social distancing cannot always be reliably maintained. However, it does not replace measures such as quarantine, contact tracing, symptom monitoring, ventilation, social distancing, or immediate isolation upon the onset of symptoms.

Isabella Eckerle

Speaking with the Science Media Center, Eckerle , MD, DTMH, clinical virologists and cohead of the Geneva Centre for Emerging Viral Disease, Department of Microbiology and Molecular Medicine at the Medical Faculty of the University of Geneva, Geneva, Switzerland questioned whether current evidence supports substantial airborne transmission.

According to Eckerle, the BMJ analysis did not emphasize that respiratory detection of the virus occurred in only a small proportion of patients “And from an even smaller proportion, only the virus could be isolated — the main viral load is found in the blood. It is not a respiratory virus, and the pulmonary symptoms are not caused by viral pneumonia but by fluid shifts,” she said.

She also cited studies arguing against efficient airborne spread. One report described a patient with an Andes virus infection who had contact with 53 passengers during a flight without transmitting the virus to any of them. Another household study found infections in only 3.4% of contacts, although the risk increased among sexual partners or individuals with very close physical contact, such as kissing or sharing a bed.

“I feel that airborne transmission is often equated with ‘highly contagious’ and ‘pandemic risk,’ which I think is misleading. We’re not dealing with a new virus here, but with a pathogen that has been known for decades. That doesn’t mean we should take it lightly, but it’s simply not a respiratory virus with a high transmission rate.”

Conclusion

The outbreak aboard MV Hondius has reignited the debate about possible human-to-human transmission of the Andes virus. Although epidemiologic observations and experimental findings suggest that transmission may occur under certain conditions, experts have emphasized that the virus has not shown high respiratory transmissibility.

Close and prolonged exposure appears to be the main risk factor. Experts, therefore, advocate targeted precautions and careful contact tracing rather than alarmist responses.

This story was translated from Medscape’s German edition.