Coronavirus is most contagious before
and during the first week of symptoms
People stop making infectious virus once the
body’s antibody response kicks in. The virus can
grow easily in people’s noses and throats and may be transmitted before people
are aware they are sick, a new study suggests.
NATIONAL
INSTITUTE OF ALLERGY AND INFECTIOUS DISEASES-ROCKY MOUNTAIN LABORATORIES/NIH
As sweeping efforts to contain the COVID-19 pandemic go into
effect around the globe, researchers are starting to get hints of just when
patients are most contagious.
People
infected with the SARS-CoV-2 virus, which causes the disease, may test positive
for the virus both before and after they have symptoms. But a new study of nine
people who contracted the virus in Germany suggests that people are mainly contagious before they have symptoms and in the first week of
the disease.
Infectious
viruses were isolated from about 17 percent of nose and throat swabs and more
than 83 percent of phlegm samples during that first week, researchers report
March 8 in a study posted at medRxiv.org.
Patients
produced thousands to millions of viruses in their noses and throats, about
1,000 times as much virus as produced in SARS patients, Clemens Wendtner,
director of infectious disease and tropical medicine at Munich Clinic
Schwabing, a teaching hospital, and his colleagues found. That heavy load of
viruses may help explain why the new coronavirus is so infectious.
Scientists identified these nine people some time after they had
been exposed to the coronavirus, so researchers don’t know for sure when
exactly people begin giving off the virus.
After
the eighth day of symptoms, the researchers could still detect the virus’s
genetic material, RNA, in patients’ swabs or samples, but could no longer find
infectious viruses. That’s an indication that antibodies that the body’s immune
system makes against SARS-CoV-2 are killing viruses that get out of cells,
Wendtner says.
The
study brings an important point to light; finding RNA or pieces of a virus in a
swab or sample is no guarantee that the virus is “live,” or infectious, says
Ali Khan, dean of the College of Public Health at the University of Nebraska
Medical Center in Omaha. “Some of it is discouraging news because when you are
mildly [ill] or just [getting] sick, you’re putting out a whole lot of virus,
which explains why we’re seeing so much transmission within our communities,”
says Khan, was not involved in the study.
But there’s encouraging news, too. Plummeting numbers of
infectious virus after antibody production turns on “means that after about 10
days or so, you’re not likely to be infecting other people,” Khan says. Other
studies also suggest that people with very mild or asymptomatic infections
don’t shed as much virus and aren’t as likely to infect other people as people
with more severe cases, he says.
Wendtner
and colleagues put the nine patients through tests every morning during their
hospital stay, collecting blood, urine, stool, nasal and throat swabs and
asking the people to cough up sputum, or phlegm. “We were learning with the
patients because we did not know when would be the best and safest time to
discharge them,” Wendtner says.
The
high levels of virus shedding from the nose and throat happened very early in
infection — by the time of testing, most patients’ upper airway virus
production had already peaked. As the infection progresses, the virus moves
deeper into the lungs, the findings suggest.
The
team never found evidence of the virus in blood or urine and has stopped
collecting those samples from a “second wave” of 23 COVID-19 patients now being
treated at the hospital. Researchers did detect viral RNA in feces, but no
infectious virus there. That suggests that the virus isn’t spread through
stool, an unknown until now.
All
nine patients are employees of Webasto, an auto supplier in Stockdorf. They
caught the virus from a male coworker, who became known as Patient 1. He
originally got the virus from a business colleague from Shanghai who came to Germany in
January for a series of meetings (SN: 1/31/20). Both
Patient 1 and his Shanghai colleague transmitted the virus before developing
symptoms, the first documented cases of asymptomatic spread.
As
health officials tested other employees of the company, they found the study
participants and placed them in isolation at the Munich clinic. In one case,
Patient 1 sneezed during a meeting with one person, Wendtner says. “That was
enough for infection.” In other cases, “they had simple business meetings,
sitting together for 60 minutes, 90 minutes [at a table or] in front of a
computer, with no physical contact — just one handshake, that’s all,” Wendtner
says. “The infectivity is quite high.”
Most
had coughs, but only two developed a fever, the most common symptom reported in
other studies. Most symptoms were mild and one person never developed any
at all. One patient developed severe pneumonia.
Two
of the nine had runny noses, previously reported as a rare symptom of COVID-19.
Another four had stuffy noses and reported that they couldn’t smell or taste
anything. “In all of our patients, it cleared up, but it was a little bit
annoying for two weeks or so,” Wendtner says. “They could order anything they
wanted [to eat], but [if] you can’t taste it, it doesn’t matter.”
A
temporary lack of smell or taste also affected some SARS patients in 2003, he
says. That symptom may indicate that in addition to causing swelling in the
nose, the virus may infect nerve cells responsible for identifying odors, he
says.
Patients
in the study started making antibodies against the virus about six to 12 days
after symptoms started. Once antibody production kicked in, researchers still
found high levels of viral RNA in phlegm and in nose and throat swabs, but
patients were no longer giving off infectious virus.
The
early and extreme contagiousness of the virus “tell us that gatherings of
people should be avoided,” Wendtner says. But the results also may suggest that
isolation periods could be shorter for people who have RNA but no virus.
Researchers thought that because tests could still detect RNA for up to weeks after symptoms had cleared (SN: 2/28/20),
patients were infectious for that long. Most patients are not released from the
hospital until two separate tests within 24 hours come back negative, Wendtner
says.
But
Wendtner isn’t suggesting letting people out of quarantine before their two
weeks are up. “Fourteen days is safe, and you have to keep it simple,” Wendtner
says. “Maybe it’s safe 10 days after symptoms start, but you have to prove they
have those neutralizing antibodies.” Tests for the antibodies, however, are not
widely available.
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