The latest round of sero survey conducted in Delhi to find the prevalence of antibodies to COVID-19 has shown a decline from 29% to 25%, reported PTI. 

Experts have pegged the reason behind the decline to lesser lasting time of immunity in people and “change in virulence” of the virus, as per the PTI report. Virulence indicates the severity and harmfulness of the disease.

The Delhi High Court on Wednesday had informed about the reduction in seroprevalence of antibodies to COVID-19 in September. 

These are the findings of the third round of sero surveillance carried out by the AAP government in the national capital between September 1 to September 7.  

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The findings of the survey with a representative sample of 17,409 people across various categories were submitted to the Delhi High Court on Wednesday. 

It found that seroprevalence of antibodies to COVID-19 reduced to 25% in September from nearly 29% found in the August in the national capital.

Medical director of Rajiv Gandhi Super Speciality Hospital, a dedicated COVID-19 facility, said the fall in figures indicate that “immunity to the virus attack is not lasting beyond two months or so.”

“It also means that we are far away from the desired herd immunity level. The decline in figures means the antibodies developed in response to the infection, symptomatic or asymptomatic, are vanishing far earlier than expected. So, people need to more careful now,” reported PTI quoting the medical director of Rajiv Gandhi Super Speciality Hospital.

Sherwal opined that if the current trend persists then when the vaccine comes for COVID-19, a patient would need “power doses too” besides the “first dose”.

The survey has shown that there was a significant decrease in seroprevalence in northeast, north and central districts of Delhi, while there was a slight increase in the south, east, west and northwest districts.

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The seroprevalence was found more in women during the rounds of the survey, while it was the least in the 18-49 years age group, as compared to age groups of less than 18 years and more than 50 years, the report said.

The survey also found that nearly one-third of the participants with a past history of COVID-19 infection, “did not have detectable IgG antibodies”.

“However, there is evidence that antibodies to COVID-19 may deplete over time,” it added.

Medical director of LNJP Hospital, another dedicated COVID-19 facility run by the Delhi government, concurred with Sherwal that duration for which antibodies last, “seems to be reducing”.

“The fall in seroprevalence level, if extrapolated, means people had antibodies earlier but it has disappeared now. Also, one other possible reason could be the change in virulence of the virus, so we don’t know how it going to behave now. Hence, people need to be more careful,” he said.

Dr Suranjit Chatterjee, senior consultant, Internal Medicine at Apollo Hospitals here, said, “fall in duration for which antibodies last seems to be a factor”.

“One of my patients, who had tested positive for COVID-19 in July and recovered had been found seropositive when we went to donate plasma soon after. Recently, he again went to donate plasma but couldn’t as his seroprevalence came negative, meaning his antibodies had vanished,” he said.

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The next round of the survey was scheduled to start from October 1 but it will be delayed a bit, officials had earlier said.

The surveyors have recommended that there is a need to space the serosurvey rounds and future surveillance should focus more on “ensuring the representativeness of the sample in terms of settlement type and socio-economic status”.

The Delhi government told the high court bench that it will inform the court on the next date of hearing on October 7 as to when it will conduct the next round of serosurvey.