When experts recommend wearing masks, staying at least a meter away from others, washing your hands frequently, and avoiding crowded spaces, they really say: Try to keep the amount of viruses you come across as low as possible.
You can’t make a few virus particles sick – the immune system would defeat the invaders before they could. But how much virus does it take for an infection to take root? What is the minimum effective dose?
“The truth is, we really don’t know,” said Angela Rasmussen, virologist at Columbia University in New York. “I don’t think we can do anything better than an educated guess.”
Common respiratory viruses such as influenza and other corona viruses should provide an insight. However, researchers have found little consistency.
For SARS, also a coronavirus, the estimated infection dose is only a few hundred particles. For MERS, the infectious dose is much higher and is on the order of thousands of particles.
The new SARS-CoV-2 coronavirus is more similar to the SARS virus, and therefore the infection dose can be hundreds of particles, said Dr. Rasmussen.
But the virus has a habit of defying predictions.
Generally people with a high proportion of pathogens – whether from Influenza, H.I.V. or SARS – tend to have more severe symptoms and are more likely to pass on the pathogen to others.
But in the case of the new corona virus, people who have no symptoms seem to have a viral load – that is, the amount of viruses in their body – According to some studies, as high as those who are seriously ill.
Some people are generous transmitters of the corona virus; others are stingy. So-called super spreaders seem to be particularly gifted when it comes to transmission, although it is unclear whether this is due to their biology or their behavior.
On the receiving side, the shape of a person’s nostrils and the amount of nasal hair and mucus present, as well as the distribution of certain cellular receptors in the airways to which the virus has to attach, can affect how much virus it infects.
However, a higher dose is significantly worse, and this could explain why some young healthcare workers have become victims, although the virus usually affects the elderly.
The critical dose may also vary depending on whether you are taking it or inhaling it.
Humans can ingest viruses by touching a contaminated surface and then placing their hands on their nose or mouth. “However, this is not the main cause of the spread of the virus,” said the centers for disease control and prevention.
“It’s clear that you don’t have to be sick and cough and sneeze to get a transmission,” said Dr. Dan Barouch, a viral immunologist at the Beth Israel Deaconess Medical Center in Boston.
Larger droplets are heavy and quickly float down – unless there is a breeze or air conditioning – and cannot penetrate surgical masks. However, droplets with a diameter of less than 5 micrometers, known as aerosols, can remain in the air for hours.
“They travel on, last longer, and have the potential to spread further than the big droplets,” said Dr. Barouch.
Three factors appear to be particularly important for aerosol transmission: proximity to the infected person, air flow and time.
A windowless public bathroom with high pedestrian traffic is riskier than a bathroom with a window or a bathroom that is rarely used. A quick outdoor conversation with a masked neighbor is much safer than either of these scenarios.
Recently, Dutch researchers used a special spray nozzle to simulate dropping droplets of saliva, and then tracked their movement. The scientists found that only one door or window was torn open can banish aerosols.
“Even the smallest breeze will make a difference,” said Daniel Bonn, a physicist at the University of Amsterdam who led the study.
Observations from two hospitals in Wuhan, China, published in Nature magazine in April, showed almost the same thing: more aerosolized particles were found in unventilated toilet areas than in airier patient rooms or crowded public areas.
This makes sense intuitively, experts said. However, they found that aerosols, being smaller than 5 microns, would also contain much fewer, perhaps millions, fewer viruses than droplets of 500 microns.
“It really takes a lot of these single-digit droplets to change your risk,” said Dr. Joshua Rabinowitz, a quantitative biologist at Princeton University.
Aside from avoiding crowded interiors, people can most effectively wear masks, all experts said. Even if masks do not fully protect you from droplets loaded with viruses, they can reduce the amount you receive and possibly lower them to the infectious dose.
“This is not a virus for which hand washing appears to be sufficient,” said Dr. Rabinowitz. “We have to limit the amount, we have to wear masks.”