PHOTO: DWAYNE SENIOR/BLOOMBERG/GETTY IMAGES1304 10 DECEMBER 2021 • VOL 374 ISSUE 6573 science.org SCIENCET
esting stations and hospital wards in
Gauteng, South Africa’s most popu-
lous province. A company’s Christmas
party in Oslo, Norway, that became a
superspreading event. Infection pat-
terns in the United Kingdom. Sci-
entists are scouring patchy evidence from
around the world to better understand
Omicron, the new SARS-CoV-2 variant, and
what it might mean for the next phase of
the pandemic. Three weeks after Omicron
was discovered, there are still mostly ques-
tions, but a few hints have emerged—some
worrisome, others more encouraging.
Researchers are focusing on three key
questions: Can Omicron evade immunity
from vaccines or previous infections? How
transmissible is it? And how much severe
disease will it cause?
The most solid clues so far pertain to the
first question—and they are not reassuring.
The genome alone—with more than 30 mu-
tations in the all-important spike protein—
suggested the variant might well be the best
yet at dodging our immune defenses. Early
data from South Africa seem to confirm that
worry. That country has seen three massiveCOVID-19 waves so far, one with the origi-
nal SARS-CoV-2, one with the Beta variant
(which never made much headway outside
the country and has now disappeared), and
one with Delta. Past studies found a previ-
ous infection offered imperfect, but signifi-
cant protection against Beta and Delta, and
many had hoped South Africa had built up
enough population immunity to dampen
further spread of SARS-CoV-2.
But a new study, published as a preprint
on 2 December, suggests that won’t work
with Omicron. Juliet Pulliam, an infec-
tious disease epidemiologist at the South
African Centre of Excellence in Epidemio-
logical Modelling and Analysis, and her col-
leagues analyzed 35,670 reinfections among
nearly 2.8 million positive tests carried out
through late November. The proportion of
reinfections rose significantly as Omicron
started to spread, suggesting an earlier in-
fection only offers half as much protection
against the new variant as it does against
Delta. That’s a sign Omicron is able to es-
cape at least some of the immune system’s
defenses. “This does not bode well for
vaccine-induced immunity,” says virologist
Florian Krammer of the Icahn School of
Medicine at Mount Sinai.“The exact numbers are fraught with is-
sues,” says William Hanage, an epidemio-
logist at Harvard University. But, he says, “It
is a first pass that provides a good enough
comparison to show us that, as we might
have expected, reinfections are a big deal
with Omicron.” How big a problem that
will become depends on whether vaccina-
tions and previous infections still protect
against severe disease, says Justin Lessler,
an epidemiologist at the University of North
Carolina, Chapel Hill. “We knew there were
going to be variants and there was going
to be immune escape. The hope is that all
these reinfections are mild and it’s not such
a big deal,” he says. “That’s been my index
of when the pandemic is ‘over’—when we
have big waves of infection but not of seri-
ous disease.”
Whether Omicron is more transmissible
than its predecessors—as both Alpha and
Delta were—is harder to judge. Omicron
cases in South Africa have risen steeply in
the past few weeks, but that could be ex-
plained in part by chance or the variant’s
ability to infect those who are vaccinated or
had a previous infection.
But Jeremy Farrar, head of the Wellcome
Trust, sees cause for concern. “The evidenceIN DEPTH
The departure terminal at Cape Town International Airport on 3 December. Many countries have halted travel from southern Africa in a bid to slow Omicron’s spread.By Kai Kupferschmidt and Gretchen VogelHow bad is Omicron? Some clues are emerging
New variant appears to evade immunity and shows signs of spreading more rapidly
COVID-