How to understand why COVID numbers in the US are dropping
Scientists at IST Austria show that little differences in behavior decide
between success and complete failure of epidemic control.
INSTITUTE OF SCIENCE AND TECHNOLOGY AUSTRIA
What
has fluid physics to do with the spreading of the Corona virus?
Whirlpools and
pandemics seem to be rather different things, certainly in terms of comfort.
Yet, newest findings about epidemic spreading come from Physics professor Björn
Hof and his research group at the Institute of Science and Technology Austria
(IST Austria), who specialize in fluids and turbulent flows.
When early last
year Björn Hof had to cancel his scheduled visit to Wuhan, his wife's hometown,
his focus abruptly shifted to epidemic spreading.
"My
group normally investigates turbulent flows in pipes and channels", he
explains, "Over the last 10 years we have shown that the onset of
turbulence is described by statistical models that are equally used to describe
forest fires and epidemics."
Given this experience, programming an
epidemic model was a straightforward exercise for Burak Budanur, the group's
theorist and computational expert.
The
epidemic curve does not flatten, it collapses
Standard
epidemic models suggest that the level of mitigation has a continuous effect on
the height of the epidemic peak.
"The expectation is that the curve
flattens in proportion to the level of social distancing", says Davide
Scarselli, main author of the paper. However, when he first simulated epidemics
taking limits in testing and contact tracing into account, the picture was a
very different one.
The maximum of infected people initially decreased as
expected but then suddenly collapsed to almost zero as the mitigation level
reached a certain threshold. In one limit, approximately half of the people got
infected during the epidemic.
In the other one only three percent caught the
disease. Surprisingly, it was impossible to obtain a result in between these
two outcomes: Either there is an outbreak of considerable size, or there is
almost none whatsoever.
Failure
yields faster than exponential growth
Testing
of known contacts (not testing per se) is one of the most powerful ways to slow
down an epidemic. However, the number of cases that can be traced every day is
limited and so is the number of tests that can be administered. As the
researchers found out that exceeding these limits at one point during the
epidemic has far-reaching consequences.
"If this happens", says
Timme, "the disease begins to spread faster in the unchecked areas and
this unavoidably causes a super-exponential increase in infections."
Already, exponential growth is immense. It means doubling infections every few
days. Super-exponential though signifies that even the rate of doubling becomes
faster and faster.
As
long as this acceleration can be avoided, epidemic curves collapse to a
comparably low case level. Interestingly, it matters relatively little whether
contact tracing is protected by a small or a large safety margin. The numbers
remain comparatively low. If on the other hand the limit is only surpassed by a
single case the super-exponential growth causes the total case numbers to jump
to a tenfold level.
Marginal
differences and disproportionate effects
"Like
most nations, Austria didn´t react early against the second wave. Once not all
contacts could be traced anymore during last September, it wasn't difficult to
predict that case numbers would soon surge at a faster than exponential
rate", says Scarselli.
While over the last year it has become apparent
that an early and decisive response is essential when facing exponential growth,
the team's study shows that test limits make timing even more crucial.
The
difference between success and failure of a lockdown is marginal, or as Budanur
puts it: "A policy that would have worked yesterday will not only take
much longer to take effect, but it may fail entirely if it is implemented a
single day too late."
Hof adds: "Most European countries only reacted
when health capacity limits became threatened. Actually, policy makers should
have paid attention to their contact tracing teams and locked down before this
protective shield fell apart."
More
recently the team has looked into optimal strategies, where lockdowns are used
as a preventive tool rather than an emergency brake. A manuscript that outlines
the optimal strategy, which minimizes both, the number of infected people and
the required lockdown time, is currently in progress.
What
has fluid physics to do with the spreading of the Corona virus?