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The first serosurvey of Ahmedabad district shows an overall seropositivity of nearly 20 per cent among the 2300 samples covered

The study sampled three groups across the nine talukas of the district - 299 RTPCR-confirmed Covid-19 patients from the months of June, August and September, 1,430 of their close contacts and 1,143 RT-PCR-negative patients, taken as the control arm.

Written by Sohini Ghosh | Ahmedabad |
March 26, 2021 5:27:21 am
Ahmedabad newsThe first serosurvey of Ahmedabad district, excluding the areas under the Ahmedabad Municipal Corporation (AMC) conducted in December last year, results of which were published recently, shows an overall seropositivity of nearly 20 per cent (File Photo)

The first serosurvey of Ahmedabad district, excluding the areas under the Ahmedabad Municipal Corporation (AMC) conducted in December last year, results of which were published recently, shows an overall seropositivity of nearly 20 per cent among the 2300 samples covered.

The study sampled three groups across the nine talukas of the district – 299 RTPCR-confirmed Covid-19 patients from the months of June, August and September, 1,430 of their close contacts and 1,143 RT-PCR-negative patients, taken as the control arm. Of these, 2,301 valid samples were taken.

Of all the nine blocks, Sanand saw maximum seropositivity, with 70 per cent of the confirmed RTPCR samples testing positive for Covid-19 antibodies. The study however notes that “the difference in the positivity rate as per the talukas in rural Ahmedabad is statistically not significant but it was found that the taluka in the vicinity of the Ahmedabad city showed greater antibody positivity…”

Across all three groups, those aged 30-59 years had a higher antibody positivity as compared to the age group of 0-29 years and those above 60 years, as against the AMC serosurvey that had found greater seopositivity in the elderly population.

The serosurvey study, authored by Ahmedabad district development officer Arun Mahesh Babu and then chief district health officer Dr Shilpi Yadav (who is now the additional Ahmedabad district health officer), published earlier this month in the International Journal of Science and Research is however careful to note that the “selection of study participants was not completely random, and this could have led to an overestimation of the seroprevalence estimates.”

Interpreting the results, the study estimates that nearly 3.95 lakh cases were missed among the general population if it is assumed that the entire population of 16 lakh in the jurisdiction was infected and surmises that vaccination is the only way to develop immunity against the infection.

Of the 299 confirmed cases, antibodies were found in only 46 per cent of them (that is in 107 cases of the 232 valid samples), indicating absence of evidence of long-lasting antibody after Covid-19 infection. In the Ahmedabad Municipal Corporation (AMC) second serosurvey conducted in the second half of August 2020, 60 per cent of the 1,800-odd samples had reflected antibodies against the infection. In the third serosurvey by the AMC conducted in the second half of October 2020, 54 per cent among the confirmed Covid-19 samples taken in the survey had antibodies.

The first serosurvey conducted by the AMC between second half of June and first week of July had found antibodies in 17 per cent of the total 29,891 samples that it tested. The survey had concluded that even in the areas highly affected by SARS CoV-2 back then, very low level of seropositivity was seen.

Speaking to this paper Babu added that a smaller trial was first conducted on Covid-19 positive cases detected in the Ahmedabad rural jurisdiction in between March to June and those detected in July had to be excluded from the December serosurvey owing to “logistical issues”

Among the 1,430 close contacts of the confirmed RT-PCR-positive cases, 21 per cent (that is in 236 samples of the 1,104 valid samples) were found with antibodies against the novel coronavirus.

In the second serosurvey of AMC of August 2020, close contacts of cases, including family members and others, an overall seropositivity of nearly 32 per cent was reported.

“With 19.8% seropositivity of our total sample…we are consistent with other studies showing that even in the areas highly affected by SARS CoV-2 during this pandemic have shown very low level of seropositivity…. It can be probably due to the association with the time duration from their diagnosis of Covid-19 as they might have lost their antibodies over the long time-gap or it may be on account of the severity of their clinical illness,” Ahmedabad rural serosurvey notes.

Assuming a best-case scenario where overall seropositivity in the population is considered at 25 per cent, the survey estimates that for the 16 lakh populace of Ahmedabad rural, four lakh may develop antibodies, leaving the remaining 12 lakh vulnerable. Taking into account that Ahmedabad rural as of January 31 had reported a total of 4,053 Covid-19 positive cases, the survey notes that “we might have missed 3, 95,947 cases in general population.” In light of this, the survey notes that “the population of Ahmedabad is still largely susceptible,” and the immunity provided by the antibodies cannot be relied upon.

“What we are telling is, since antibody development is less, vaccination is the key, we cannot wait for more people to get infected and wait for antibodies to develop,” emphasised Babu.

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