Sero surveillance survey yields ‘unexpected’ results

Survey in all districts using rapid antibody kits from June 8

Even as the government is preparing to do more sero surveillance studies in the containment zones in Malappuram district, it has been tight-lipped about the results of the first round of community surveillance studies across the State using rapid antibody kits.

The State launched the studies using the rapid antibody kits it had procured from HLL on June 8 and in the first round handed over 10,000 kits to all districts. The survey was to find out if there is community transmission and to determine the proportion of the population which has had prior silent exposure to SARS-CoV-2 virus.

‘Unexpected’ results?

An expert committee which analysed the data from the first sero surveillance study in the community has already submitted its report. Despite the huge public interest in knowing whether the virus has been silently spreading the disease in the community, the report remains under wraps. However, information trickling in from the districts put the positivity (both IgM and IgG) rates “at much higher rates than had been anticipated.” As a senior adviser to the government put it, the results thrown up by this preliminary screening tests have certainly been “unexpected.”

The authorities have now pointed to the unreliability of antibody tests. “More than the high positivity it was the population groups in which this was reported that surprised us. We expected a high positivity among those who returned from abroad but that was not so in this study. We are not convinced about the results because of the inherent issues with the test kits,” a senior health official said.

Screening tests

However, epidemiologists point out that the study should not be trashed because antibody tests are just screening tests and the concept of screening tests is different. Epidemiological studies use a two-step technique to study diseases of low prevalence, of which, the screening test is just the first step. It has to be followed up with detailed epidemiological investigations in the community.

Strenghts, weaknesses

“We chose to do the study with the antibody kits knowing fully well its strengths and weaknesses. In low prevalence disease settings, such a kit will throw up a lot of false positives. But it is possible to interpret the results epidemiologically and estimate false positivity and identify areas where we have to do focussed interventions,” a public health expert said.

One could test among the family members of those who were IgG positive to see how far an asymptomatic person can spread the disease. Or the IgG positive samples could be re-tested with ICMR-approved ELISA kits. It might also be possible to get a realistic estimation of R0 (R nought) in Kerala using the sero survey results and factoring in the sensitivity and specificity of the test kit. The government should give out the survey results to Community Medicine departments in medical colleges to interpret the data from an epidemiological perspective.