Tamil Nadu has logged over 26.7 lakh coronavirus cases since the start of the pandemic.
Seventy per cent of people in Tamil Nadu have COVID-19 antibodies, the latest sero survey - the third since last year - revealed.
In April, the second sero survey had shown that only 29 per cent of the state's population had the antibodies.
Chennai and Coimbatore, which have been the hotspots, have reported the seropositivity at 82 per cent and 71 per cent in the third sero survey conducted between July and August. The Virudhunagar district is at the top spot in the list with 88 per cent of the population.
In April, a 3 per cent drop - as compared to the first survey (32 per cent) - had raised concern that the population turning more vulnerable to coronavirus.
Health Secretary Dr J Radhakrishnan told NDTV: "The current significant rise is due to the natural immunity among recovered patients after the second wave peaked and a spike in vaccination."
Seropositivity is a measure of antibodies present in the body that can protect against COVID-19.
Four districts - including Karur, Nilgiris, Ariyalur and Perambalur - have less than 60 per cent seropositivity rate.
Authorities are intensifying the vaccination drive in these areas. Health Minister Ma. Subramanian said, "We have sent more doses to these districts for the mega vaccination drive on Sunday."
Explaining the process, a statement from the state's health department says 24,586 samples were collected from 827 clusters and 17,090 persons had IgG antibodies against SARS -Cov-2 Virus.
While the seropositivity results certainly puts Tamil Nadu in what health officials call "a safe zone"; experts say there is no protection against any possible new variants.
With 5 crore doses, only 22 per cent of the population has been fully vaccinated. The state is seeing around 1,500 cases every day and it can't let its guard down yet.
Tamil Nadu has logged over 26.7 lakh coronavirus cases so far, and over 35,000 deaths since the start of the pandemic.