A seroprevalence study in Tamil Nadu showed a decrease in antibodies after infection and vaccination with SARS-CoV-2

In India’s sixth most populous state, Tamil Nadu, three rounds of serological testing were conducted at the district level through two waves of COVID.

Round 1 of the survey was conducted from October 19 to November 30, 2020, round 2 from 7 to 30 April 2021 and round 3 from June 28 to July 7, 2021. The sample includes more women, several individuals aged 18-29 years and more older people than the general population.

The results of the study published on medRxiv* The preprint server helps to understand the epidemiology of severe acute coronavirus 2 respiratory syndrome (SARS-CoV-2) and to take effective measures, especially for the distribution of scarce vaccines.

Study: Sulfur prevalence in Tamil Nadu through the two waves of COVID in India: Evidence for antibody reduction after infection and vaccination.  Image Credit: Anyaivanova / Shutterstock

Seroprevalence results assess the extent to which reported cases underestimate population immunity

Seroprevalence data also help to understand the extent to which antibodies decrease after infection and vaccination by studying changes in seroprevalence data at the area level across the circles.

In rounds 1 and 2, the study obtained laboratory results for 26,135 people in 882 clusters and 21,992 people in 746 clusters, respectively. In round 3, results were obtained for 26,592 people and all groups were sampled.

Both seroprevalence and seropositivity follow a similar pattern, as shown by their respective results. In rounds 1, 2 and 3, the seroprevalence at the state level is 31.6%, 22.9% and 67.1%, respectively. Seropositivity varies in different areas in all three rounds. In rounds 1, 2 and 3, the variations varied from 12.1% to 49.3%, 11.1% to 49.1% and 36.8% to 84.9%, respectively.

In addition, seroprevalence is significantly higher in urban than in rural areas, but does not differ significantly between the sexes. However, it is relatively higher in the age group 40-49 years than in the adult population aged 70 years or more in circles 1 and 3 (31.7% vs. 26.5% in circle 1; 66.7% vs. 59, 6%, in round 3).

The seroprevalence among the older working age population is significantly higher than among the younger populations in round 2 (50-59 vs. 18-29: 25.6% vs. 19.5%). Also, seroprevalence is higher among vaccinated than unvaccinated populations (25.7% vs. 20.9% in round 2; 80.0% vs. 62.3% in round 3), and data from round 3 suggest that increases with the number of vaccine doses received (0 doses vs. 1 dose, 62.3% vs. 77.5%; 1 dose vs. 2 doses, 77.5% vs. 85.9%).

The decrease in antibodies after infection affects how well seroprevalence measures the previous degree of infection. Accordingly, seropositivity was found to have decreased between 31.6% and 62.6% in the six months between rounds 1 and 2.

No decrease in antibodies was observed in humans immediately after vaccination, but after three months of immunization, disintegration of antibodies cannot be ruled out.

In addition, humans were observed for about three months during this study. So infections in vaccinated individuals during the second wave in India may also have led to an increase in antibodies, and this is not just the result of vaccination.

Serological values ​​in circles 1 and 3.

Serological values ​​in circles 1 and 3.

Sulfur prevalence below 100% indicates ineffectiveness of the vaccine

The percentage of seroprevalence below 100% among vaccinated people, combined with the lack of evidence of a reduction in antibodies after vaccination, suggests that some doses may have been ineffective in triggering a detectable antibody response. However, it is uncertain whether this is due to the fact that some doses were not produced or maintained well or because the vaccines were not administered effectively.

The combination of vaccination and infection is associated with higher quantitative serological results. The distribution of the quantitative results has a similar form and manner among those not vaccinated in rounds 1 and 3.

Unvaccinated individuals show an unimodal distribution of low-dose test results. Individuals who received one or two doses showed a bimodal distribution with a low regimen, the same as in unvaccinated individuals.

Therefore, two doses of vaccine are unlikely to be responsible for the higher regimen among vaccinated individuals. This is established by the higher regimen observed in vaccinated individuals who received only one dose of vaccine.

Also, the high regimen is associated with the risk of infection, measured by seropositivity levels at the district level.

Although the Delta option is mainly responsible for the second pandemic wave in India among seropositive people, it is unlikely that this option is responsible for above average results.


According to the website www.covid19india.org, the actual number of COVID-19 infected is approximately 35, 25 and 21 times higher than the number of confirmed cases in rounds 1, 2 and 3. However, the overall seroprevalence results show that Nearly 23 and 48 million people were infected with COVID-19 between November 2020 and July 2021 in Tamil Nadu.

To summarize, the seroprevalence at the state level in rounds 1, 2 and 3 is 31.5%, 22.9% and 67.1%, respectively.

Seroprevalence ranged from 11.1 to 49.8% in round 1, 7.9 to 50.3% in round 2 and 37.8 to 84% in round 3.

The decrease in seroprevalence from rounds 1 to 2 suggests a decrease in antibodies after natural infection. A slight decrease in antibodies was observed in populations that received at least one dose of the vaccine between rounds 2 and 3.

*Important message

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered convincing, guiding clinical practice / health-related behavior, or treated as established information.

Reference in the magazine:

  • Source: TS Selvavinayagam, A. Somasundaram, Jerard Maria Selvam, Sabareesh Ramachandran, P. Sampath, V. Vijayalakshmi, C. Ajith Brabhu Kumar, Sudharshini Subramaniam, S Raju, R Avudaiselvi, V. Prakash, N. Gurunathanhni , DN Dhiliban, Sofia Imad, Vaidehi Tandel, Rajeshwari Parasa, Stuti Sachdeva, Anup Malani. Sulfur prevalence in Tamil Nadu through the two waves of COVID in India: evidence of antibody reduction after infection and vaccination. 2021 https://doi.org/10.1101/2021.11.14.21265758, https://www.medrxiv.org/content/10.1101/2021.11.14.21265758v1

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