FM November2020 Digital P

new infections continue to occur in a
constant or increasing trend, the population
prevalence of antibodies is likely to decline
with time after large spikes of infections.
Such “antibody decay” is likely the cause for
the declining prevalence of antibodies in the
population, according to Dr Pinto.
Others, however, beg to disagree. Some
of them refuse to accept that a drop in
antibody levels can be behind the survey
results. Antibodies against the virus may
drop to extremely low levels over a period
of time, but it is highly improbable for them
to vanish so quickly, they argue.
“The falling levels of seroprevalence is
a technical error and not due to waning
immunity,” says Dr V Anil Kumar, Clinical
Professor & Head, Microbiology, Amrita
Institute of Medical Sciences, Kochi, Kerala.
The microbiologist is convinced that the
immune response to SARS-CoV-2 is similar
to what was seen with its close cousin,
SARS-CoV. In case of the SARS epidemic,
antibodies remained in the system at
detectable levels for at least 12 to 24
months.
Antibody levels in mild or asymptomatic
cases can be low or even undetectable.
Antibody titres are higher in critical patients
when compared to non-critical patients
due to different rates of maximal antibody
response. Patients with severe disease
become antibody positive earlier than those
with mild disease. Longitudinal studies
on 98 SARS-CoV patients have shown
detectable levels of IgG for two years;
however, the titres may have declined over
this period. A similar study on 176 SARS CoV
infected people found that antibody titres
reduced by 33% within one year and 46%
by two years. Antibodies to SARS CoV have
been detected even 12 years after infection.
On the discrepancy in the numbers
of different surveys, Prof Anil Kumar
said it is impossible to accurately gauge
true prevalence by targeting selected
populations in a country like India which is
so heterogeneous with respect to culture,
lifestyle, beliefs, socioeconomic status,
the density of population and family size.
Further, various state governments follow
different approaches.
Besides, seroprevalence is bound to be
low when children and young. college-going
As time passes,
antibodies to SARS-CoV-2
are known to decline
rapidly.
Dr Lancelot Pinto
Consultant Respirologist
P D Hinduja National Hospital
and Medical Research Centre
Mumbai.
adults — the most active members of the
society — are confined to their homes. The
virus may have reached every nook and
corner of the country, he adds, but it will
still take long to hit every household.
Epidemiologists say that the sampling
methods adopted for these surveys
could also influence their outcome. To
ensure reliable results, surveys should
make sure the cohort to be tested is
representative of the larger population.
This means the individuals to be surveyed
are representative of the society at large
by age, social and economic status,
educational levels, areas of residence and
other parameters. Likewise, the size of the
sample should also be sufficiently large
enough to proportionately represent all
subgroups. Otherwise, such limitations
will skew the intended distribution of the
subjects.
The harmonisation of sampling rules
32 / FUTURE MEDICINE / November 2020