Change in method of recording leads to dip in Covid antibodies: Experts

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The findings of the third serological survey in the national capital conducted in August revealed a reduction of approximately four per cent of antibodies against the novel coronavirus in the population.

A first glance at the findings suggests a decline in antibodies developed against the Covid-19. However, the medical experts believe that the dip accounted for the antibodies was done due to a changed method of recording the seroprevalence.

Dr. Akhila Kosuru, senior physician and general manager for quality and training clinical services at Apollo Telehealth said a change in the sampling method could have attributed to the lesser recording of antibodies in the general population.

"Ideally there should not be a dip in the seroprevalence. Reason for the dip could be a drop in antibodies among people who may have been infected a long time ago or due to a change in sampling method this time," she added.

Dr. Gauri Aggarwal, the founder of Genestrings laboratories which is empanelled by the Delhi government to test the Covid-19 samples, said that the results are surprising but not unfathomable.

"At a time when infection prevalence has been increasing, the seroprevalence must have increased as compared to the previous survey that came out in August. However, it dropped by nearly four percentage points in the latest survey. The difference in results is surprising but not completely unfathomable given the fact that there has been a change in the survey sample method," she commented.

She presumed that the locations where sampling for the serosurvey was conducted would have impacted the outcome of the study. "The location from where samples have been taken will determine the level of seroprevalence achieved," Dr. Gauri said.

According to the latest survey, a significant decrease in seroprevalence was seen in Northeast, North and Central districts as per the survey report, while a slight increase was seen in South, East, West, and North West districts.

The report, submitted by the state health department to the Delhi High Court, stated that the "fall was particularly localised" in three districts -- North East, Central and North Delhi.

Dr. Kosuru also said that the variation in the type households for sample pooling might have led to the contradictory result. "The latest survey sampled people based on whether they lived in five accommodation types -- planned colonies, resettlement colonies, urban slums and JJ colonies, urban and rural villages, and unauthorised colonies. The study has also revealed that seroprevalence was highest among participants of low socioeconomic status with per-capita income below Rs 5,000," she explained.

The survey found a lower seroprevalence of 23 per cent in planned colonies in comparison to resettlement colonies, urban slums, unauthorised settlements where the seroprevalence was found at 26 per cent.

Dr. Akshay Budhraja, Consultant Department of Pulmonology, Aakash Healthcare Super Speciality Hospital, said there is a possibility that a particular area where the study was conducted has a population who was recently exposed to the virus and the titer of antibodies were found less in them. "Besides, age also plays a factor. The elderly population has lesser antibodies in comparison to the younger population," he added.

However, does the decline in antibodies amounts to reinfection? Answering this query, Dr. Budhraja stated that it is difficult to conclude with the serological study.

"While it is a matter of worry that the antibodies are reduced by a slight percentage, but does it mean that those people recovered from the Covid-19 are susceptible to reinfection, is difficult to answer based on this particular study," he said.

It is a novel virus to determine the pattern and sustainability of antibodies would take months and years of study, he concluded.