Fall of antibodies does not necessarily mean decrease in level of immunity, say experts
The antibody prevalence in the slums of Mumbai’s three wards has gone down, according to the second serosurvey results released by the Brihanmumbai Municipal Corporation (BMC) October 2, 2020. The rate has, on the other hand, gone up in the non-slum population.
A serosurvey is conducted to understand the estimates of infection spread in a sample population. The second survey in the R-North, M-West and F-North wards of Mumbai was conducted in the second half of August, about a month after the first round. This does not include the famous slum of Dharavi.
The seroprevalence was found to be 45 per cent in the slum and 18 per cent in the non-slum areas. The first round had estimated 57 per cent seroprevalence in slums and 16 per cent in non-slum.
The second-round exercise included 5,384 participants, about 1,552 lesser than the first round. The participants also included 728 healthcare workers (HCWs).
How should one interpret the decline in antibodies? Tata Institute of Fundamental Research (TIFR)’s Ullas S Kolthur told Down To Earth (DTE) that antibodies did wane over a period of time in all infections. But this survey’s results should not be interpreted as a decline in immunity, he added.
TIFR, along with other organisations, conducted the survey for the BMC this time and earlier.
Though the geographical areas and the sample population in the second survey were the same as the first, the individuals were not, Kolthur said.
“So, to say that immunity has declined and by what rate, the individuals ought to be the same in both rounds. However, in repeat serosurveys, the same individuals are not recruited. We also didn’t do so,” Kolthur said.
Only 1-2 per cent of the samples belonged to people who had participated in both exercises.
Jayaprakash Muliyil, a leading epidemiologist and former principal of Christian Medical College, Vellore, too cautioned against reading too much into the decline of seropositivity in slums.
“The new infections have declined there substantially. Hence, only the waning effect of antibodies in the overall population would be in the survey and not the additional or new formation of antibodies in it,” he said.
In areas where there is high seroprevalence, which was already the case with slums as revealed in the first serosurvey, herd immunity demands no new cases occur, Muliyil said. And, this is precisely reflected in the serosurvey.
Any serosurvey does show antibody prevalence. However, it does not reveal the proportion of neutralising antibodies out of the total found in an individual, that actually fight the virus. For that, a full-fledged antibody characterisation is required.
Kolthur said that had not been done in the second round. In fact, the antibody characterisation of the first round was itself ongoing.
Kolthur also spoke about why the second survey had a smaller sample in comparison to the first. “It was decided keeping in mind several factors, including statistical and logistical,” he said. He disagreed when asked whether the decline in size would have had any impact on the survey results.
Also, the same kit — Chemiluminescence Assay by Abbott — was used in both rounds to avoid any discrepancy.
So, what can be made of this survey? “Not anything more significant than the fact that virus is still very much around as a large proportion of the population does not still have antibodies. So, wearing mask and adopting other COVID-appropriate behaviour is the key,” Kolthur said.
Sampling did not distinguish between symptomatic and asymptomatic people. Another significant fact is that sampling was done in active containment zones, where the rate of infection is quite high as compared to the others.
As against the first serosurvey, which estimated no difference in the rate of infection among various age groups, the second one revealed slightly higher rates in the age group above 40 years.
The rate of infection in HCWs remained 27 per cent, as it was in the first round. This was higher than other sample populations or civilians because HCWs are more exposed to the virus.
The full seroresults and the technical report has not yet been released in the public domain. “We are waiting for the BMC to give us a go-ahead,” Kolthur said.
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