A study on the first national sero surveillance to estimate the prevalence of the novel coronavirus, conducted in May, shows that as many as 8.56 lakh cases were present in the 233 districts deemed ‘zero-caseload’ at the time — confirming the under-detection of cases in the country.
The data also reveals that these districts may have actually accounted for a significant 13 per cent of the total Covid-19 caseload in May, two months after the first coronavirus case in the country.
The study, published in the Indian Journal of Medical Research on Friday, says that of the 64.68 lakh infections estimated in the country at the time, 8.56 lakh were in the zero districts; 18.17 lakh in low-prevalence districts; 15.18 lakh in medium-prevalence districts; and 22.76 lakh in high-prevalence ones.
The study covered 15 districts that were zero-case at the time. Out of 150 clusters covered in these 15 districts, only 11.3 per cent were in urban areas.
As pointed out by the researchers, this shows that first, there was poor surveillance and contact-tracing of infections in the districts with low caseloads, resulting in the infection spreading; and second, there was poor testing in the areas at the time the survey was conducted, resulted in cases going undetected.
The study looked at 6,014 adults covered by the sero survey in the zero districts (the survey had a total sample size of 28,000). It found that 19.7 per cent of them were employed in occupations with high exposure to the virus, 53 per cent had sought medical care for respiratory symptoms, 34.3 per cent had a history of hospitalisation, 1.5 per cent had a history of contact with a Covid-19 case, and that just 0.1 per cent had been tested for Covid-19 by the RT-PCR test.
“The present findings of seropositivity in the strata of districts with zero to low incidence of Covid-19 cases underscore the need to strengthen surveillance and augment the testing of suspected cases in these areas,” the researchers say.
Elaborating on the reasons for coronavirus cases not being detected in the 233 districts, the researchers say, “First, the stratification of districts was done based on the reported number of Covid-19 cases as on April 25, 2020. The sero survey in the 15 districts of (this) strata was conducted during May 11 to June 4, 2020 after a median interval of 23 days. During this period, as per the ICMR laboratory database, three districts had reported Covid-19 cases at least two weeks before the initiation of survey and thus were no longer reporting zero cases.”
The study adds, “Second, there could be under-detection of Covid-19 cases in the zero-stratum districts on account of low testing as well as poor access to the testing laboratories. In four of the 15 districts in this stratum, Covid-19 testing laboratory was not available at the district headquarters and the samples were transported to the state headquarter hospitals for diagnosis.”
The detailed findings also reveal that the infection fatality rate (IFR) per 10,000, which estimates the proportion of deaths among all infected individuals, on May 24 ranged between 0.18 in zero-stratum districts and 11.72 in high-stratum districts. “IFR per 10,000 infections as on June 1 ranged between 0.27 in zero-stratum and 15.04 in the high-stratum districts,” the study says.
The study also states that the unweighted prevalence of IgG antibodies against SARS-CoV-2 was 0.47 per cent in the stratum with zero reported cases; 0.48 per cent in the stratum with low incidence; 0.74 per cent in the stratum with medium incidence; and 0.59 per cent in the stratum with high incidence.
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