E-Paper
TN serosurvey finds Covid antibodies in 23% population
The study was carried out by the Directorate of Public Health and Preventive Medicine across all districts, except Chennai where two serosurveys have already been done by the Greater Chennai Corporation.
A second serosurvey conducted in Tamil Nadu in April this year revealed that at least 23% of population has developed antibodies to SARS-CoV-2, a decline from 31% which was noted during the first survey conducted in October-November last year.
The study was carried out by the Directorate of Public Health and Preventive Medicine across all districts, except Chennai where two serosurveys have already been done by the Greater Chennai Corporation.
“We know that antibodies last only for a few months. So people who got infected in the early part of the first wave may not have detectable antibodies now in the second wave,” said Dr Prabhdeep Kaur, deputy director, National Institute of Epidemiology, explaining the decline. “This is anticipated and know from other serosurveys in India as well as around the world. Since we are vaccinating the population the only issue with serosurveys is that these tests do not distinguish whether you are infected or vaccinated.”
Of the 22,904 samples that were tested, 5,316 people were found with IgG (Immunoglobulin G-type) antibodies against coronavirus. The report also said that various coronavirus variants, including B.1.617.2 (Delta) of Indian origin, were detected in Tamil Nadu.
The highest seropositivity of 49% was found in Tiruvallur district and the lowest of 9% in Nagapattinam.
About 756 clusters were chosen and each cluster consisted of roughly 30 participants who were randomly picked for testing from a village, rural area, a street and an urban area for the survey.
During the first serosurvey conducted last year, 22,690 blood samples were collected of which 6,995 were found reactive to the antibodies.
The latest report attributed the ‘sharp decline’ to four factors — timing of serosurvey, waning of antibodies, vaccination coverage and emergence of SARS CoV-2 variants.
“The first survey was initiated around four weeks following the peak cases reported in August 2020 during the first wave,” the report said, adding that this time, the survey was conducted close to the beginning of the second wave.
“Waning of IgG antibodies during the time gap of five months between 1st and 2nd survey (November 2020 to March 2021) could be one of the valid reasons for low seroprevalence as reported by several studies published in international journals,” it said.
The government is planning to conduct a third serosurvey in July-August. The department said the period would be appropriate considering the current declining trend of Covid-19 cases combined with increased vaccinations for those above 18 years of age.
On Monday, the state reported 19,448 cases, taking the overall tally to 22,56,681. With 351 deaths in the same period, the toll rose to 27,356. As many as 97,35,420 people have been vaccinated so far.
