Sero survey: 70% in Kerala may have acquired natural immunity against COVID-19

When the State was organising mass vaccination drives to protect the community from the ill effects of COVID-19 infection, natural infection had already penetrated to 70% of the State’s population.

The difference in sero positivity amongst the unvaccinated (70%) and the vaccinated (90%) in population samples indicates that 20% of the State’s population have only vaccine-derived immunity. File.

The third round of sero survey conducted by Kerala across all districts to assess the levels of COVID-19 antibodies in the population has reported that post second wave, at least 70 % of the population may have achieved natural immunity against COVID-19.

While the overall sero prevalence in the adults in the community is estimated at 82.61% (either Anti -spike IgG antibody or Anti-nucleocapsid IgG antibody), the Health department’s report states that the proportion of sero positivity in those who were unvaccinated was 70%.

This essentially means that even when the State was organising mass vaccination drives to protect the community from the ill effects of COVID-19 infection, natural infection had already penetrated to 70% of the State’s population.

“The seropositivity of 70% observed among the unvaccinated population may be an indicator of the background infection rate in the population”, is how the Health department has chosen to put it.

The other side of the story is that since natural immunity has been found to be stronger and more long-lasting than the immune cover provided by any of the vaccines currently in use, Kerala is in a much better position now as far as population immunity against COVID-19 is concerned, than it had been in June this year.

In June, the fourth round of serial sero survey conducted by ICMR had estimated a sero prevalence of only 44.4% (42.7% for S1spike protein receptor binding domain (RBD) neutralising antibodies) in Kerala against a national average of 67.6%.

Sero positivity amongst those who were fully vaccinated has been pegged at nearly 90% (89.92).

The difference in sero positivity amongst the unvaccinated (70%) and the vaccinated (90%) in population samples indicates that 20% of the State’s population have only vaccine-derived immunity, among whom, breakthrough infections will continue to be reported for some time.

Given the heterogenous pattern of natural immunity in the State (as seen in the variation in sero prevalence across districts) and waning vaccine-immunity, in the near future, these breakthrough infections are likely to occur in clusters in regions where vaccination coverage has been high but natural infection has been low

“COVID-19 transmission had been active and sustained in the State since the last round of ICMR’s sero survey in May. At the same time, the State’s vaccination coverage with the first dose of the vaccine also went up from approximately 30% in May to over 80% by September, when the current sero survey was undertaken. Hence we would like to believe that a chunk of Kerala’s population now have hybrid immunity (combination of immunity from natural infection and vaccination), which is stronger and sustainable,” said T. S. Anish, public health expert.

Low sero positivity amongst children

The fact that only 40.2% of the State’s children (5-17 years) have natural immunity against the virus is quite concerning because this is one group which is totally vulnerable to COVID-19 as they do not have the protection of the vaccine. The low sero prevalence in this group indicates that when schools reopen next month, large-scale clustering of COVID-19 cases could be expected in schools and within families.

Children are the only group where the sero conversion has been due to natural infection alone. District wise, the seroprevalence among children ranged between 7.9% in Wayanad and 63.3% in Kasaragod.

The State claims that “the low levels of seropositivity among children is a surrogate of flattening of the curve of natural COVID-19 infection of the State and an indicator of the State’s fight against the rapidly spreading infection”.

The State utilised two assays to estimate anti-spike antibodies (signals immunity through vaccination or natural infection) and anti-nucleocapsid antibodies (signals either prior natural infection or vaccine-derived immunity through Covaxin ).

The huge difference in sero prevalence due to anti-spike antibodies (82.14%) and anti-nucleocapsid antibodies (19.26%) is quite surprising, indicates that sero conversion in the State has been contributed largely through anti-spike antibodies. It does raise questions about the efficacy or sero conversion rate of Covaxin