More
evidence is needed to determine whether people who recover from the novel
coronavirus are vulnerable to reinfection.
Kate Baggaley July 16, 2020
Earlier this week,
Washington, DC-based physician D. Clay Ackerly described a worrying experience
with a patient of his who tested positive for the coronavirus that causes
COVID-19 three months after catching the disease in Vox. After dealing with a mild cough
and sore throat during his initial infection, the patient was now experiencing
more severe symptoms, including a high fever, shortness of breath, and
dangerously low oxygen levels.
“Despite scientific hopes
for either antibody-mediated or cellular immunity, the severity of my patient’s
second bout with COVID-19 suggests that such responses may not be as robust as
we hope,” Ackerly wrote, adding, “If my patient is not, in fact, an exception
but instead proves the rule, then many people could catch COVID-19 more than
once, and with unpredictable severity.”
There have also been a
handful of other cases in the past couple months, described by physicians,
where people have fully recovered from COVID-19 only to feel ill and then test
positive again for the novel coronavirus—officially known as SARS-CoV-2—months
later, Ackerly notes.
But how much evidence is
there that you can catch COVID-19 twice? In one recent experiment done on
monkeys, macaques that previously tested positive for SARS-CoV-2 did
not fall ill when exposed to the virus for a second time and had
“near-complete protection” from reinfection. Still, more research is needed
before physicians and other health officials can make any extrapolations to
humans. It’s simply too early, experts say, for us to know whether reinfection
is possible or how much of a danger it could pose. Here are a few issues they
are considering.
Diagnostic tests
So far, reports of people
becoming reinfected with COVID-19 are anecdotal and haven’t been confirmed,
says Luis Ostrosky, a professor of infectious diseases at McGovern Medical
School at the University of Texas Health Science Center at Houston. Diagnostic
tests for COVID-19 check for the presence of RNA from SARS-CoV-2. People can
“shed” this genetic material for a long period of time even if they are no
longer infectious.
This means true
reinfections, if they happen, won’t be easy to spot, according to a team of
researchers at the La Jolla Institute for Immunology in California who have
been studying the body’s immune response to COVID-19. “It is difficult to say
how many clear cases of reinfection have occurred, but, at this point, they
appear to be few in number,” several of the researchers—Alba Grifoni, Sydney
Ramirez, Shane Crotty, and Alessandro Sette—wrote in an email to Popular
Science.
In April, authorities
became concerned about the possibility of reinfection after a group of
recovered COVID-19 patients in South Korea began testing positive again.
However, researchers eventually concluded that the tests were probably detecting lingering bits of RNA. These survivors appear not to have transmitted the disease to
others, suggesting that they didn’t have an active COVID-19 infection the
second time around.
Some of Ostrosky’s patients
have tested positive for the virus as long as three months after catching it.
“It’s just kind of residual genetic material that we’re happening to find,” he
says. In cases where people recover from COVID-19, only to later feel ill again
and test positive for the virus, Ostrosky says, it’s not yet clear whether the
explanation for their symptoms is a relapse of COVID-19, a second COVID-19 infection, or
a completely different illness.
If it is possible to catch
COVID-19 shortly after recovering, it’s unclear how common an occurrence this
would be. However, the fact that the world has seen more than 13
million known cases of COVID-19 since late 2019 and there aren’t yet
any confirmed, clearly documented cases of reinfection in peer-reviewed medical
journals may be telling, Ostrosky says. “This would lead us to the conclusion
that this would be a rare event if it happens at all.”
Similar diseases
Generally speaking, it’s
unusual for a virus to infect people again after a brief period of time, both
Ostrosky and the La Jolla Institute researchers agree.
A number of viral
diseases—including measles, mumps, and chickenpox—typically lead to lifelong
immunity. But in other cases, the immune response our bodies mount to a virus
isn’t strong enough to ward against reinfection; respiratory syncytial virus is one such
pathogen.
Some viruses, like the ones
that cause the seasonal flu, also mutate so quickly that you might recover from
one strain only to encounter another that is different enough that your body’s
immune system doesn’t recognize it. So far, the coronavirus that causes
COVID-19 doesn’t
appear to be mutating especially rapidly or in ways that would hinder vaccine development.
“One way to make guesses
about immunity to SARS-CoV-2 is to compare it to related viruses,” said Grifoni
and her colleagues. Little is known, though, about whether people can be
sickened twice by the coronaviruses that cause SARS and Middle East Respiratory
Syndrome (MERS). One well-known
study from the 1990s demonstrated that people could be reinfected by
one of the coronaviruses that cause common colds after a year, but none of the
participants developed cold symptoms.
It’s possible that when
people recover from COVID-19, their immune response won’t prevent reinfection
but can protect them from becoming noticeably sick. “But, depending on the
levels of the virus present, they may be able to transmit it to other people
without themselves feeling sick,” noted Ramirez and her team.
During their research, they
have also observed that immune cells from blood samples collected years before
the pandemic reacted to proteins from SARS-CoV-2. One explanation is that the
people who donated those blood samples had been exposed to common cold
coronaviruses. “You may be accumulating antibodies against previous coronavirus
infections and those may offer a certain degree of protection,” says Ostrosky,
who was not involved with the research.
A few diseases—most
notoriously dengue fever—can make people more severely ill when they catch them
a second time. But there are also cases where people who vanquish one virus
retain some protection against its relatives; this could explain why the swine
flu pandemic of 2009 hit young people, who hadn’t had time to encounter similar
strains, particularly hard.
Research on antibodies and T cells
People who recover from
SARS—which is caused by the virus most closely related to SARS-CoV-2—tend to lose
their antibodies within a few years. Two recent reports have indicated that antibodies to SARS-CoV-2 may wane within two to
three months, particularly in those who had milder cases of
COVID-19.
It’s actually common for
antibody levels to drop over a period of six months or so after a viral
infection or immunization, the La Jolla Institute researchers said. For
COVID-19, though, it’s still too soon to tell how sharply antibodies will
decline over time and what impact that will have. Scientists may learn that low
levels of antibodies can still prevent people from becoming reinfected, or that
a different arm of the immune system could offer protection. This could mean
that even if you no longer have antibodies to SARS-CoV-2, your immune system
could potentially still effectively fight the virus off.
Grifoni, Ramirez, and their
colleagues have previously
found that in addition to antibodies, people who have recovered from
COVID-19 produce immune cells called T cells that recognize the virus, even in
cases where people are gravely ill. T cells play an
important role in fighting disease; the researchers observed both “helper” T
cells, which assist the immune system in responding to a pathogen, and “killer”
T cells that destroy infected cells.
However, as with
antibodies, further studies are needed to pin down how long-lasting and useful
the T cells produced in response to COVID-19 will be. In the case of the SARS
virus, the researchers said, T cells that recognize the pathogen have been
observed over a range of one to 17 years after infection. With COVID-19, it
appears that T cells stick around for at least two to three months. They may
even help the immune system react swiftly and pump out more antibodies if it’s
faced with SARS-CoV-2 a second time.
But again, it’s too early
to determine what these findings all add up to. “We’ve only been dealing with
this virus for seven months, so we’re just getting to know it,” Ostrosky says.
We just don’t have enough evidence to know yet if or when COVID-19 can strike
the same person twice and what that would mean for our chances of developing an
effective, long lasting vaccine.
For now, though, there are
two things we can do to deal with all the uncertainty.
“One is to follow the
‘Hitchhiker’s Guide to the Galaxy’ mantra, which is don’t panic,” Ostrosky
says. “We need to get more information to see whether this really happens or
not and if it’s a common or rare event.” Further, researchers still need to
better understand whether an individual who has some immunity can still spread
it to others. So if you have recovered from COVID-19, you should still be
social distancing and wearing a face mask when you plan to come into contact
with other people.
“People that had the
infection already should not think they’re immune automatically,” Ostrosky
says. “They still need to be careful and they still need to follow
precautions.”
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