FM November2020 Digital P

second round of survey in August, which
covered 5,384 people in three wards, found
that 45% of the people residing in the city’s
slums were exposed to the coronavirus,
compared to the nearly 57% recorded in
the first survey in July. But for non-slum
areas, seroprevalence rose to 18% from the
16% earlier.
The surveys also showed that levels of
seroprevalence varies substantially from one
locality to another within the city.
Similar trends were also seen in the
third survey carried out in Delhi, which
found seroprevalence down by almost 4%.
Flaws in methodology;
erroneous sampling
Critics have pointed to several flaws in the
sampling design and methodology of these
surveys, while researchers explain that the
discrepancies may be technical in nature as
an entirely different set of people may have
been surveyed the second time.
Another, more worrying possibility
is that of a rapid drop in SARS-CoV-2
antibodies soon after recovery. “As time
passes, antibodies to SARS-CoV-2 are
known to decline rapidly,” says Dr Lancelot
Pinto, Consultant Respirologist, P D Hinduja
National Hospital and Medical Research
Centre, Mumbai.
He pointed to a recent study that
reported that concentrations of antibodies
in a recovered individual are halved roughly
every 36 days. This would imply that unless
MUMBAI:
FALL IN NUMBERS
Subsequent surveys
revealed a significant
drop in seroprevalence in
Mumbai slums
57% 45%
July
August
“Any definitive
conclusion
on SARS-CoV-2
immunity needs
more evidence”
Dr Chandrakant Lahariya is a medical
doctor and a leading public policy and
health systems expert. He has received
advanced training in epidemiology, vaccinology,
immunology and public health. In the last
15 years, he has worked closely with India’s
topmost health-policy makers to develop
and implement several health policies and
programmes at both the union and state
levels. He is the youngest fellow ever elected to
the Indian Public Health Association (IPHA).
In an interaction with FM, Dr Lahariya
points out that while serosurveys remain the
best tool to fathom the level of population
immunity due to past infection, it is important
that the results of these studies be interpreted
carefully, factoring in their context and
limitations.
Edited excerpts:
Data from metros like Mumbai and Delhi
show that seroprevalence is declining in
consecutive surveys across population
groups. Is the disparity a result of different
methodological approaches or is it indicative
of a deeper problem specific to SARS-CoV-2,
such as quickly vanishing antibodies?
We really cannot say that the trend
is declining till we see such a pattern in
comparable surveys and we have the
confidence intervals (CI) along with the point
prevalence, which are being reported. The
findings of the third serosurvey in Delhi are
not comparable with the previous two, and
definitely, the methods are much more robust.
As for serosurveys in Mumbai, we don’t really
know the CI. However, at least the difference
from 16% to 18% is unlikely to be statistically
significant.
It is too early to become concerned about
the level of antibodies. There is evidence that
even when there are no detectable antibodies,
the person could be protected. In addition,
the antibody response after vaccination is
30 / FUTURE MEDICINE / November 2020