One-fifth of the population in the capital of Tamil Nadu, Chennai, have been in environments infested with coronavirus, a serosurvey (serological survey) for the transmission of the infection in Greater Chennai Corporation areas shows. “Overall, one-fifth of the population in Chennai was exposed to SARS-CoV-2 (the virus, severe acute respiratory syndrome coronavirus 2) infection,” the GCC said in its preliminary finding.
The sample size was around 12,000 to detect a seroprevalence of 2%, for a relative precision of 20% design effect of 2.5 and for a 95% confidence interval. Of the individuals tested, 2,673 individuals had evidence of past SARS-CoV-2.
“The reading (inference) is 21.5% of Chennai’s population reached the immunity levels by July-end,” GCC commissioner G Prakash told reporters. “Now, the (immunity) levels will be more and it would also mean a (comparatively) lower transmission rate,” he said adding the scientific study would be useful to alter strategies to tackle COVID here.
Around 80% of the population is still susceptible to SARS-CoV-2 here and “we need to understand the reasons for varying seroprevalence across zones to improve our control measures,” the civic body said. “There is huge variation in seroprevalence across zones ranging between 7.1% in Madhavaram and 44.2% in Tondiarpet.” The infection to case ratio is lower compared to some of the published studies (Santa Clara, National survey) -indicating high testing performance.
The field data/sample collection activities happened between 18 and 28 July and the testing of the samples was completed by August 6, the GCC said in an official release adding the mean age of individuals surveyed was 41.1. The serosurvey was conducted in the GCC areas to estimate the proportion of the population exposed to SARS-CoV-2.
The study design was a multi-stage cluster sample survey among individuals aged more than 10 years.
Serosurvey and seroprevalence
The ministry of health and family welfare had in the month of May decided to conduct a population-based serosurvey in select districts of the country with an aim to monitor the trend in the prevalence of SARS-CoV-2 virus which causes the COVID-19 disease.
A serosurvey involves testing of blood serum of a group of individuals and this is used to monitor trends in the prevalence of the novel coronavirus, or SARS-COV-2, infection at the district level. The Indian Council of Medical Research and the National Centre for Disease Control conduct the surveillance in collaboration with key stakeholders and state health departments.
The facility-oriented surveillance is an expansion of the testing of flu and serious respiratory cases in hospitals being carried out by the government.
Seroprevalence is the number of persons in a population who test positive for a specific disease based on serology (blood serum) specimens; often presented as a percentage of the total specimens tested or as a proportion per 1,00,000 persons tested. As positively identifying the occurrence of the disease is usually based upon the presence of antibodies for that disease (especially with viral infections such as herpes simplex, HIV, and SARS-CoV-2), this number is not significant if the specificity of the antibody is low.
Sero testing in a serosurvey
A more focussed population-based serosurvey of high and low-risk groups in select districts is in addition to routine testing.
The move is not only helping the government and its agencies monitor COVID-19 trends but also checking for community transmission in any part of the country.
Only some affected parts of the country have witnessed community transmission of the virus.
How is it conducted?
The enhanced exercise sees 10 health facilities — six public and four private — from each district being tested.
For the serosurvey, population groups comprise low-risk — as well as high-risk — populations. The low-risk group includes outpatient attendees (non-ILI patients) and pregnant women while healthcare workers are surveyed among the high-risk population.
The survey includes a total of 200 samples per week and 800 samples per month. This includes at least 100 samples per week and 400 per month from the selected districts from the high-risk population. For the low-risk population, 50 samples per week and 200 per month are collected from outpatient attendees (non-ILI patients).
Using a combination of RT-PCR and Elisa kits
The agencies use a combination of RT-PCR and Elisa antibody kits for these surveys. Throat and nasal swabs are collected for RT-PCR tests and samples tested in a one-time pool of 25.
However, the results of this sample pooling are only for surveillance purposes and not for the diagnosis of individual patients.
In addition to RT-PCR tests, blood samples are collected for detecting IgG antibodies for Elisa testing. In subsequent rounds, IgG Elisa-based testing of serum samples replaced RT-PCR based testing for surveillance purposes.
Elisa testing kit replaced rapid antibody test kits from China
The National Institute of Virology in Pune along with Zydus Cadila developed the Elisa testing kit. The kit is playing a critical role in surveillance of a proportion of the population exposed to the infection after rapid antibody test kits from China failed the ICMR quality tests. Elisa kits have a specificity of 97% and sensitivity of 92%.