The race to understand how rat virus spreads

 One of the most important questions scientists are racing to answer about the hantavirus outbreak is exactly how it spreads from person to person.

It sounds like a simple question, but it is not. There are dozens of variables that dictate the “transmission dynamics” of a virus and scientists are only just starting to grapple with a few of them.

Here are some of the crucial variables that scientists are investigating:

Large droplets

Following an outbreak of hantavirus in an Argentinian village in 2018 in which 34 people were infected and 11 died, US Army researchers concluded that “the inhalation of droplets or aerosolised virions” almost certainly explained the transmission of the virus.

Three of those infected victims were found to have been “super-spreaders” – people who infected multiple others at two key social events: a birthday party and then a wake for one of the victims.

Droplet transmission occurs when an infected person coughs, sneezes, talks or breathes heavily, releasing respiratory particles into the air. But not all droplets are the same.

Larger droplets cannot travel very far. Their weight means they fall quickly, landing on nearby surfaces and presenting a possible “touching” risk for others.

A droplet expelled at a party might come to rest on a plate, a buffet or a napkin. From there, it could be transferred directly into someone’s mouth or hands.

If the latter happens, then light switches, doorknobs and handshakes all become possible secondary routes for transmission.

Environmental factors can hugely influence their spread.

In a hot, dry environment, droplets may not survive for more than a few minutes on the surfaces they land on, while in a humid room, they may survive for hours or perhaps days.

A gust of wind or a powerful air conditioning system could make the difference between a good-sized droplet falling close to the person who expelled it or travelling the distance of a room.

Aerosols

Large droplets are one thing, small droplets are another entirely. Small droplets are light enough to float in the air and can be described as airborne or “aerosolised”.

The risk here is less about touch and more about absorption through the permeable membranes of eyes, nose and mouth.

But here again, environmental factors may make all the difference.

Humidity can have a big impact on the spread of airborne viruses. The less moisture there is in the air, the longer aerosol particles are likely to float about.

That may make a cruise ship sailing across the Atlantic in winter especially vulnerable.

Not only would the air be dry, but it would be cold outside, necessitating the heating to be on and doors to be closed.

Shared cabins, group dining and other confined indoor spaces would all add the chances of onward transmission.

“Cruise ships are floating Petri dishes for infectious disease: hundreds of people in close quarters, shared ventilation, communal dining, and constant person-to-person contact for weeks,” Dr Tom Frieden, President and CEO of Resolve to Save Lives and former CDC Director, said in a recent social media post.

“Generally, spread on a cruise ship doesn’t mean that a pathogen will necessarily be highly transmissible,” Dr Frieden said.

Infectious dose

The theoretical dynamics or vectors of transmission are one thing. But on their own, they are not enough to know if someone will be infected.

Many other variables determine how infectious a virus is, including the size of an infectious dose.

A healthy body may be able to quickly fight off a few virions inhaled here or there, meaning that a much larger dose is required to overcome our body’s initial defences.

The Andes strain of the hantavirus is known to be capable of passing from person to person, but it is also evident that it does not spread as efficiently as many other respiratory bugs.

This strongly suggests that the risk of contracting the virus via very small airborne droplet transmission is low.

Perhaps such particles are simply not large enough to carry an infectious dose.

The early evidence suggests that close and perhaps sustained contact is needed for the virus to spread: face-to-face conversation, kissing, sharing a drink or perhaps a toothbrush.

“It doesn’t spread the same way as coronaviruses do. It’s very different. It’s that close, intimate contact that we’ve seen [in this outbreak]”, said Maria Van Kerkhove, Head of the Emerging Diseases and Zoonoses at the World Health Organisation (WHO) last week.