When experts recommend wearing masks, staying at least 6 feet away from others, washing your hands frequently and avoiding crowded spaces, what they are saying is: Try to minimize the amount of virus you encounter.
A few viral particles cannot make you sick — the immune system would kick in first. But how much virus is needed for an infection to take root? A precise answer is impossible, because it is difficult to capture the moment of infection. “The truth is, we really just don’t know,” said Angela Rasmussen, a virologist at Columbia University.
For severe acute respiratory syndrome, or SARS, also a coronavirus, the estimated infective dose is just a few hundred particles. For Middle East respiratory syndrome, or MERS, the infective dose is on the order of thousands of particles. The new coronavirus, SARS-CoV-2, is more similar to the SARS virus and, therefore, the infectious dose may be hundreds of particles, Rasmussen said.
But the virus has a habit of defying predictions.
Generally, people who harbor high levels of pathogens — whether from influenza, HIV or SARS — tend to have more severe symptoms and are more likely to pass on the pathogens to others. But in the case of the new coronavirus, people who have no symptoms seem to have viral loads — that is, the amount of virus in their bodies — just as high as those who are seriously ill.
Some people are generous transmitters; others are stingy. So-called superspreaders seem to be particularly gifted in transmitting it, although it is unclear whether that is because of their biology or their behavior.
On the receiving end, the shape of a person’s nostrils and the amount of nose hair and mucus present — as well as the distribution of certain cellular receptors in the airway that the virus needs to latch on to — can all influence how much virus it takes to become infected.
A higher dose is clearly worse, though.
The crucial dose may also vary depending on whether it is ingested or inhaled. For example, touching a contaminated surface may require millions more copies of the virus to cause an infection, compared to inhalation.
Coughing, sneezing, singing, talking and even heavy breathing can result in the expulsion of thousands of large and small respiratory droplets carrying the virus.
Larger droplets are heavy and float down quickly — unless there is a breeze or an air-conditioning blast — and cannot penetrate surgical masks. But droplets less than 5 microns in diameter, called aerosols, can linger in the air for hours. “They travel farther, last longer and have the potential of more spread than the large droplets,” said Dr. Dan Barouch, a viral immunologist at Beth Israel Deaconess Medical Center in Boston.
However, aerosols also contain much less, perhaps millions-fold less, virus than droplets of 500 microns. “It really takes a lot of these single-digit size droplets to change the risk for you,” said Dr. Joshua Rabinowitz, a quantitative biologist at Princeton University.
Three factors seem to be key to aerosol transmission: proximity to the infected person, air flow and timing. A windowless public bathroom with high foot traffic is riskier than a bathroom with a window, or a bathroom that is rarely used. A short outdoor conversation with a masked neighbor is safer than either of those scenarios.
Recently, Dutch researchers used a special spray nozzle to simulate the expulsion of saliva droplets and then tracked their movement. The scientists found that just cracking open a door or window can banish aerosols.
“Even the smallest breeze will do something,” said Daniel Bonn, a physicist at the University of Amsterdam.
Apart from avoiding crowded indoor spaces, the most effective thing people can do is wear masks, experts said. Even if masks do not fully shield you from droplets loaded with virus, they can cut down the amount you receive, and perhaps bring it below the infectious dose.
“This is not a virus for which hand washing seems like it will be enough,” Rabinowitz said. “We have to limit crowds, we have to wear masks.”