Why Kerala's topping COVID-19 charts? ICMR serosurvey provides missing link


The curious case of Kerala's rising COVID-19 tally is the talk of the town now. It is perplexing when a state, which gained international recognition for its COVID-19 containment strategy, becomes one of India's worst-affected in the second wave.

As on Thursday, Kerala accounted for almost 50 per cent of the daily COVID count in the country.

The reason is simple. The state is experiencing a surge in COVID cases since it has a greater number of population unexposed to the infection in the first wave.

The findings of the fourth round of national serosurvey done across 70 districts of India confirms this. Kerala has the lowest seroprevalance rate (44.4%) among the states surveyed by the ICMR between June 14 and July 6 compared to the 67.6% for India.

Onmanorama speaks to health economist and ​adjunct professor at ​Rajagiri College of Social Sciences Rijo M John to find out what this means.

Can you explain seroprevalence and seropositivity in simple terms?

A seroprevalence survey tests for the COVID-19 antibodies in the general population. These antibodies may have been developed either through a natural infection from the virus or through COVID-19 vaccination. However, at the time of conclusion of this fourth serosurvey only about 3.3% of India’s population was fully vaccinated and 12.6% had received at least a single dose of vaccine.

The fourth round of the ICMR serosurvey has shown that Kerala has the lowest seroprevalence in the country. What does this imply?

The seroprevalence in Kerala is the lowest among all the 21 states surveyed. In simple terms, it means Kerala has the largest proportion that is still vulnerable and not yet exposed to the virus. The state has a long way to go before it reaches any level of threshold immunity for the virus to stop spreading. It also means, Kerala has been the most successful in containing the spread of this virus compared to all the other states in this survey.

Kerala had an effective COVID-19 containment strategy since the beginning of the pandemic. But why is the state showing a surge in COVID cases now?

There are several factors that are contributing to an increasing number of cases in Kerala. One most important factor is that Kerala has a substantial share of the population that is not yet infected and the virus keeps spreading until the population reaches a certain threshold of immunity. Epidemiologists refer to it as herd immunity. For highly transmissible variants of viruses like the current COVID-19 variants, this herd immunity threshold is expected to be beyond 85% and Kerala is quite far from that threshold.

Along with this factor, Kerala has also eased the lockdown restrictions in recent times and there is increased mobility among the population even during the lockdown days. This is also contributing to a rise in the cases.

Kerala has higher testing rates as opposed to other states. Are the high COVID-19 numbers associated with higher testing and reporting?

Yes, the daily testing has significantly gone up in recent days reaching an average of 1.5 lakh a day in the past 7 days and that is sure to increase the detected cases too. So far, Kerala has tested more than double the number of people per million population compared to the national average.

Apart from the number of tests itself, the way of testing is also important. For example, targeted testing after careful tracking and tracing can lead to better detection and more cases per each 100 tests done. So, one cannot strictly compare the testing numbers between states as the testing strategy could be widely different between each state depending on the efforts put in for tracking and tracing before each test.

Rijo M John
Rijo M John, Health economist and ​adjunct professor at ​Rajagiri College of Social Sciences

Is the sample size of serosurvey enough to draw significant conclusions on the COVID-19 situation?

Yes, the sample size is large enough to find an average prevalence at the national or state level. However, only select districts are included in the survey and to that extent, the representativeness of the sample itself may be questioned.

For example, if the virus had an uneven spread across different districts, the percentage prevalence estimated for Kerala may not hold good for the whole of Kerala as the survey samples were taken from just three districts —Ernakulam, Palakkad and Thrissur — in Kerala.

It is important that the state government takes necessary steps to conduct its own serosurveys from all the districts of Kerala to get a reasonable picture of the seroprevalence in all the districts and that can help us design our policies with respect to lockdown, testing and other pandemic management efforts in the immediate term. 

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