From Portside <[email protected]>
Subject It’s Not Whether You Were Exposed to the Virus. It’s How Much.
Date June 1, 2020 5:15 AM
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[The pathogen is proving a familiar adage: The dose makes the
poison.] [[link removed]]

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Apoorva Mandavilli
May 29, 2020
New York Times
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_ The pathogen is proving a familiar adage: The dose makes the
poison. _

Harry Henri, a research assistant, working with blood samples from
coronavirus patients at SUNY Downstate’s BioBank in Brooklyn., Misha
Friedman for The New York Times


When experts recommend wearing masks, staying at least six feet away
from others, washing your hands frequently and avoiding crowded
spaces, what they’re really saying is: Try to minimize the amount of
virus you encounter.

A few viral particles cannot make you sick — the immune system would
vanquish the intruders before they could. But how much virus is needed
for an infection to take root? What is the minimum effective dose?

A precise answer is impossible, because it’s difficult to capture
the moment of infection. Scientists are studying ferrets, hamsters and
mice for clues but, of course, it wouldn’t be ethical for scientists
to expose people to different doses of the coronavirus, as they do
with milder cold viruses.

“The truth is, we really just don’t know,” said Angela
Rasmussen, a virologist at Columbia University in New York. “I
don’t think we can make anything better than an educated guess.”

Common respiratory viruses, like influenza and other coronaviruses,
should offer some insight. But researchers have found little

For SARS, also a coronavirus, the estimated infective dose is just a
few hundred particles. For MERS, the infective dose is much higher, 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, Dr.
Rasmussen said.

But the virus has a habit of defying predictions.

Generally, people who harbor high levels of pathogens — whether
from influenza [[link removed]], H.I.V.
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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
[[link removed]] as those who are
seriously ill, according to some studies.

And coronavirus patients are most infectious two to three days before
symptoms [[link removed]] begin,
less so after the illness really hits.

Some people are generous transmitters of the coronavirus; others are
stingy. So-called super-spreaders seem to be particularly gifted in
transmitting it, although it’s unclear whether that’s 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

A higher dose is clearly worse, though, and that may explain why some
young health care workers have fallen victim even though the virus
usually targets older people.

The crucial dose may also vary depending on whether it’s ingested or

People may take in virus by touching a contaminated surface and then
putting their hands on their nose or mouth. But “this isn’t
thought to be the main way the virus spreads,” according to the
Centers for Disease Control and Prevention.

That form of transmission may require millions more copies of the
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cause an infection, compared to inhalation.

Coughing, sneezing, singing
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even heavy breathing can result in the expulsion of thousands of large
and small respiratory droplets carrying the virus.

“It’s clear that one doesn’t have to be sick and coughing and
sneezing for transmission to occur,” said Dr. Dan Barouch, a viral
immunologist at Beth Israel Deaconess Medical Center in Boston.

Larger droplets are heavy and float down quickly — unless there’s
a breeze or an air-conditioning blast — and can’t penetrate
surgical masks. But droplets less than 5 microns in diameter, called
aerosols, can linger in the air for hours.

“They travel further, last longer and have the potential of more
spread than the large droplets,” Dr. Barouch said.

Three factors seem to be particularly important for 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’s rarely used. A short
outdoor conversation with a masked neighbor is much 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 a window can
banish aerosols
[[link removed](20)30245-9/fulltext].

“Even the smallest breeze will do something,” said Daniel Bonn, a
physicist at the University of Amsterdam who led the study.

Observations from two hospitals in Wuhan, China, published in April in
the journal Nature, determined much the same thing: more aerosolized
particles were found in unventilated toilet areas
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patient rooms or crowded public areas.

This makes intuitive sense, experts said. But they noted that
aerosols, because they are smaller than 5 microns, would also contain
much less, perhaps millions-fold less, virus than droplets of 500

“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.

Apart from avoiding crowded indoor spaces, the most effective thing
people can do is wear masks, all of the experts said. Even if masks
don’t fully shield you from droplets loaded with virus, they can cut
down the amount you receive, and perhaps bring it below the infectious

“This is not a virus for which hand washing seems like it will be
enough,” Dr. Rabinowitz said. “We have to limit crowds, we have to
wear masks.”

_Apoorva Mandavilli is a reporter for The Times, focusing on science
and global health. She is the 2019 winner of the Victor Cohn Prize for
Excellence in Medical Science Reporting._

_She is the founding editor in chief of Spectrum, an award-winning
news site on autism science that grew an audience of millions. She led
the team there for 13 years. She joined The Times in May 2020, after
two years as a regular contributor._

_Apoorva has won numerous awards for her writing. Her work has been
published in The Atlantic, Slate and The New Yorker online, and in the
anthology ”Best American Science and Nature Writing.”_

_She co-founded Culture Dish, an organization dedicated to enhancing
diversity in science journalism, and was the founding chair of the
Diversity Committee for the National Association of Science Writers.
Apoorva has a Master of Arts degree in journalism from New York
University and a Master of Science degree in biochemistry from the
University of Wisconsin-Madison. She is fluent in English, Hindi,
Tamil, Telugu and Kannada._

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