Seroprevalence high in Kerala, but herd immunity elusive

Antibodies found in 82% people in seroprevalence survey, but largely through vaccination |, Expert says natural infection in 65% could have led to herd immunity

Published: 13th October 2021 06:31 AM  |   Last Updated: 13th October 2021 06:31 AM   |  A+A-

Covid testing, coronavirus

A medic collects a swab sample (File photo | Ashishkrishna HP, EPS)

Express News Service

THIRUVANANTHAPURAM: Even as the state has over 82% of its adult population developing antibodies against Covid, the ultimate goal of herd immunity remains elusive. While most states in the country have achieved herd immunity even with lesser seroprevalence, Kerala continues to contribute a major chunk of the new positive cases. 

The test positivity rate that hovers around 10% shows that the situation is unlikely to change immediately.
Achieving herd immunity was considered a major goal for checking future pandemic waves. Health experts were hoping that the state, which had low seroprevalence in the previous surveys conducted by the Indian Council of Medical Research (ICMR), could reach the herd immunity level with the help of improvement in vaccination coverage. 

The antibodies develop by way of infection and through vaccines. However, the presence of the Delta variant of Covid has altered the calculations. Now, the health experts are of the view that the seroconversion achieved through vaccination has limitations.

“It is important to know how the seroprevalence is built. If it is through natural infection, then 65% seroprevalence is enough to get herd immunity. But it would require a seroprevalence of over 95% for seroconversion achieved through vaccination,” said Dr Anish T S, a member of the state’s Covid management committee and assistant professor at the Department of Community Medicine, Government Medical College Hospital, Thiruvananthapuram. 

He pointed out that the people who were infected with Alpha variant have been reinfected with Delta variant.  As many as 15,71,183 persons were infected with the former variant before the presence of Delta came to be known in May. These people along with those who got antibodies only through vaccination are susceptible for reinfection or breakthrough infection along with the others who are yet to get antibodies.
Health experts have already said vaccines could not prevent infection but effectively reduce morbidity and mortality due to infection.

As a result, the proportion of breakthrough infection increases as the vaccination coverage goes up, contributing majorly to the daily new positive cases.  The state has given first dose of vaccines to 2.5 crore (93.64%) of the adult population as on Tuesday.

Antibodies identified

13,198  IgG SARS CoV-2 S1 RBD (Antispike Antibody)

13,339  IgG SARS CoV-2  Nucleocapsid Antibody (develops due to natural infection or after administering vaccines other than Covishield)


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