What is COVID-19?
SARS-CoV-2, a new betacoronavirus of zoonotic origin, has
produced the worst pandemic (coronavirus disease 2019,
COVID-19) since the Spanish Flu. Apart from the lung, the virus can attack the cardiovascular
system (Liu et al., 2020), kidneys, the digestive tract, and the central
nervous system. Approximately 20% of the cases lead to severe complications (Chen et al., 2020),
with varying estimates of the case fatality rate (CFR) (Baud et al., 2020; Kim and Goel, 2020).
The reproductive number (Ro) in Wuhan, China was reported to be
between 2 and 3 (Li et al., 2020).
How
are the Lungs and the Airways Affected?
SARS-CoV-2, after gaining access to the respiratory system,
binds to the cell-surface receptor angiotensin-converting enzyme 2 (ACE2) to
enter the host cells, using the serine protease TMPRSS2 for S protein priming (Hoffmann et al., 2020).
ACE2 and TMPRSS2 were previously implicated in the disease progression of SARS
epidemic, also caused by a closely related coronavirus (Li et al., 2003; Matsuyama et al., 2010).
Coronaviruses, including SARS-CoV-2, use the class I fusion “spike” protein (S
protein) to attach themselves to cell surface receptors, before being taken up
into endosomes and releasing the viral genome into the cytoplasm (Fehr and Perlman, 2015). Structural features of the crucial
S protein for SARS-CoV-2 have been established (Lan et al., 2020; Wang et al., 2020; Wrapp et al., 2020),
and it’s a viable target for vaccines in development (Amanat and Krammer, 2020).
In the lungs, ACE2 is abundantly expressed on the surfaces of type
2 alveolar epithelial cells (Qi et al., 2020).
Destruction of these cells may reduce the production of lung surfactants, which
may in turn, negatively impact the gas-exchange function of the alveoli. Recent
single cell RNA sequencing by researchers at the Wellcome Sanger Institute
indicates that other epithelial cells in the airway, especially nasal
epithelial cells also express ACE2 receptors (Sungnak et al., 2020).
Did
Anyone See This Coming?
It’s well known that Chinese bats (especially horseshoe bats)
are a potent reservoir of coronaviruses (Menachery et al., 2015).
Although east Asia been through a pandemic caused by coronavirus before (SARS),
COVID-19 has surprised most countries outside east Asia through its high
transmissibility coupled with a large percentage of infected (and infectious)
population that’s asymptomatic.
What
are the Implications for India?
There are a few advantages for the Indian population: (a) a
very hot summer (droplet transmission may be less effective at high
temperatures), (b) a relatively young population, and (c) the promptness of the
central and state governments in imposing a fairly strict lock-down. However,
there are a few factors that work against disease containment: (a) the quality
of the public healthcare system, (b) a large migrant population, and (c) the
difficulty of contact-tracing and isolation in a country of 1.4 billion people.
I expect (this is a guess) that we would reach a peak in hospitalizations in
May, with a drop-off in the next four months. I’m not sure whether to expect a
re-emergence in October–November.
References:
1.
Amanat, F., and Krammer, F.
(2020). SARS-CoV-2 Vaccines: Status Report. Immunity 52, 583-589.
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Qiu, Y., Wang, J., Liu, Y., Wei, Y., et
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