The third phase of serosurveillance, which began on Monday, will cover 888 clusters in the State. Around 25,000 samples will be lifted for detection of SARS-CoV-2 IgG antibodies.
The first serosurvey, conducted by the Directorate of Public Health and Preventive Medicine in October-November 2020, found an overall seropositivity of 31%. This was followed by a second one this April (except in Chennai) which recorded an overall seroprevalence of 23%. During the second survey, the highest seropositivity was reported in Tiruvallur (49%) and lowest in Nagapattinam (9%).
In serosurveys, a combination of factors are at play, T.S. Selvavinayagam, Director of Public Health, said. “This includes the level of infection, vaccination and the individual’s immunity. At that point of time, it can indicate the level of susceptibility to infection in the community. It can increase or decrease,” he said.
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When seroprevalence crosses 70% — through infection and vaccination — it is established that there will be protection, he said, adding: “The challenge of newer variants remains. So serosurveys can only tell you what level of immunity there is within a population, through a combination of infection and vaccination.” The third phase will cover 888 clusters, spread across 46 health-unit districts.