The impact of epidemic depends on three factors: the infectivity and virulence of the agent,
the suspecptibity of the host, and the environment favorability of the agent.
That means simply, if the infectivity is high, it’s going to attack more people, but if the
virulence is high, it will kill more people without being able to be transmitted.
If the host is not susceptible to infection then also epidemic will fade or may not produce
increased case fatalities. And if the environment is not conducive to viral survival then also
the epidemic will fade.
Let’s discuss these factors on Indian context.
- Agent : According to a Chinese study in Peking university on 103 patients, they identified
two strains of COVID 19, the original S type and L type, which is a mutated type [1] ; but
their study found more of L type. It suggests that L type is more aggressive mutated form.
We don’t know which type is predominant in India? But overall the infectivity of COVID 19 is
pretty high with a favorable mortality profile which is actually pro-epidemic. So the agent
remains same as for the rest of the world, till now. - Host: The host factor is very important in an epidemic. The infectivity and severity of the
disease, as well the mortality are determined by immune response of the host. The COVID
19 causes more suffering in elderly and more mortality. Though the young patients are
infected, they usually show mild or no symptoms. In 80% of cases symptoms are mild, 15%
cases symptoms are moderate and 5% cases having severe symptoms like ARDS and overall
mortality is around 2.5%. The most of the mortality is above 60 years of age with highest
above 80 ( 14 % in Italy vs 10% in Wuhan) . The mortality is almost insignificant among
individuals less than 20 years of age ( 0.3%) , and nil upto 9 years [2]. So age composition of
a country will determine it’s mortality. A country like Italy where 22% of population are
elderly (more than 65 years) the mortality is very high [3]. Whereas in India 6.4% are above
65, so naturally the mortality will be less here.
Apart from this age composition another factor is important which is immunity of the host.
We have two types of immunity. The first line is Innate immunity and 2nd line is Adaptive
immunity. The innate immunity does not have a memory and it is mediated by NK cells,
macrophages and neutrophils. The Adaptive immunity is mediated by T and B lymphocytes
and it needs a memory of a previous encounter with the pathogen. But here our Innate
immunity is important as the COVID 19 is a new virus. Do we have any proof that we
Indians have strong innate immunity?
An Indo- US team of researchers has found that Indians, compared to other world
populations carry more NK cells that can detect and terminate infections at early stage [4].
Indians acquired the activating KIR (killer cell immunoglobulin receptor) genes as a result of
natural selection to survive environmental challenges [5]. So there is some proof that we
have a stronger first line defense, may be as we are exposed to more infective agents and
that determine a specific microbiome inside our body. Apart from that, observation in the
current epidemic is, mortality is less still in malaria endemic areas. We still don’t know the
causal relationship. But studies have shown Plasmodium Falciparum requires Zinc for
parasitic growth [6], and zinc also inhibits the RNA dependent RNA polymerase of COVID 19
and Chloroquine is Zinc Inophore. So, there may be an interrelationship ! But being a
malaria endemic country we can breathe a sigh of relief !
There is another factor as well. A new study is recruiting health workers to evaluate effect
of BCG vaccination in prevention or manifestation of COVID 19 infection [6]. BCG vaccine is
given in children to modulate their immunity against tuberculosis. It actually enhances T
cell mediated immunity which is a form of Adaptive immunity, but at the same time with
the help of IL1 Beta it stimulates innate immunity, as well. BCG vaccination has been shown
to reduce 30% of viral infections, with the help of this Innate immunity. So BCG,
Tuberculosis and Innate immunity all seem to link together. We don’t know till now whether
we will have another advantage, in fight against this virus, of being a Tuberculosis endemic
country . - Environment: It is hot topic now that whether the hot climate of our country will be able
to kill the virus ? We still don’t know. But let’s see the temperature and latitude of the
affected countries! The ‘Global Virus Network ‘ has predicted that weather modeling can
explain spread of COVID 19 [7]. Their observation is that the spread of COVID 19 is along a
narrow corridor of 30-50″ N at consistently similar weather conditions of 5 to 11 degree
Celsius and 47% to 79% humidity. They also suggested that a temperature rise of 12
degrees Celsius or higher, the viral transmission may be difficult. A study in China also found
that that the virus transmission is best at a temperature 8.72 degree Celsius and with every
1 degree rise in minimum temperature , the total number of cases go down [8]. So what will
be the fate of the virus in boiling temperature of country is a valid speculation. We didn’t
have a single casualty in MERS epidemic in 2012, which was a deadly Coronavirus. We don’t
know whether it was due to a proper isolation or our climate !
Amidst of fearful theories, I tried to offer you a ray of hope. But remember all these
conjectures are without firm evidence. Still I believe we can win the disease with positivity.
So just hold tight till mid April, maintain a lock down properly, and leave the rest to
scorching sun rays.
A Compilation by my better half – Dr Rashi Upmanyu , Associate Prof EMNP, San Francisco
- Happy Reading.
References:
- https://academic.oup.com/nsr/advance-article/doi/10.1093/nsr/nwaa036/5775463
- https://www.cebm.net/covid-19/global-covid-19-case-fatality-rates/
3.https://www.indexmundi.com/italy/age_structure.html - https://www.natureasia.com/en/nindia/article/10.1038/nindia.2008.254
- Du, Z. et al. KIR2DL5 alleles mark certain combination of activating KIR genes. Genes
Immun. 9, 470-480 (2008) - https://www.clinicaltrialsarena.com/news/australia-bcg-vaccine-trial-covid-19/
7.https://gvn.org/enhanced-model-for-monitoring-zones-of-increased-risk-of-covid-19-
spread/
8.https://www.firstpost.com/health/coronavirus-myth-busted-covid-19-cases-may-notdecline-
in-warm-climate-8133671.html
American Precoat 