CHENNAI: The three rounds of the seroprevalence study in Tamil Nadu, conducted amid the two waves of the Covid-19 outbreak, suggest a decline in antibodies after a natural infection, but no significant decline among people who took at least one dose of the vaccine between rounds two and three.
The study, approved by the Directorate of Public Health and Preventive Medicine and the Institutional Ethics Committee of Madras Medical College, was funded by the State government and National Health Mission, Tamil Nadu.
The pre-print study (not yet peer reviewed), which was posted in medRxiv, a server for health science, showed that the State-level seroprevalence in rounds 1, 2, and 3 was 31.5 per cent, 22.9 per cent, and 67.1 per cent respectively.
The first round was conducted between October 19 and November 30, 2020; second round between April 7 and 30, 2021, and the third between June 28 and July 7, 2021. The study was conducted at the district level in Tamil Nadu, which has a population of about 72 million. In each round, blood test were conducted on 25,000 people.
The seroprevalence in urban areas is higher than in rural areas. “While the decline in seroprevalence from rounds 1 and 2 suggests a decline in antibodies after a natural infection, we don’t find a significant decline in antibodies among those receiving at least one dose of the vaccine between rounds 2 and 3,” the study noted.
The seroprevalence rate among the unvaccinated in round 3 is 62.3 per cent, compared to 77.5 per cent among those who took one dose of the vaccine, and 85.9 per cent among those who took both doses. The daily rate of antibody decay post vaccination among those who took just one dose by round 3 is insignificant, the study said.
The overall seroprevalence implies that at least 22.6 million people in Tamil Nadu were infected after the first wave by November 30, 2020, and 48.2 million after the second wave by July 7, 2021. The actual number of infections was roughly 35, 25, and 21 times more than the number of confirmed cases in rounds 1, 2, and 3.
The study also found that seropositivity declined between 31.6 and 62.6 per cent over six months (170 days) between rounds 1 and 2, and there was no decline in antibodies following vaccination. However, individuals were only observed for three months, and 95 per cent of those who took only one dose had been vaccinated in the previous 126 days, so a decline in antibodies in the subsequent months cannot be ruled out.
The study also noted that vaccinated individuals may have experienced breakthrough infections in the second wave, and those infections - not just vaccination - may have caused a spike in antibodies.
8.36 lakh get the jab in ninth mega vax camp
Chennai: The Health Department vaccinated 8.36 lakh people in the ninth mega Covid-19 vaccination camp on Thursday. According to official data, the State vaccinated a total of 8,36,796 people, of whom 3,36,468 received the first dose and 5,00,328 received the second. TN has stock of 1.31 crore vaccines, and people should make use of the vaccination camps, Health Minister Ma Subramanian said.