The latest survey on prevalence of antibodies to COVID-19 among people in Delhi has shown a fall compared to the last exercise, with experts saying possible reasons could be immunity lasting for lesser time in people and "change in virulence" of the virus.

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Virulence refers to the severity or harmfulness of a disease. The findings of the survey conducted in the first week of September, with a representative sample of 17,409 people across various categories, was submitted to the Delhi High Court on September 30. It found that sero-prevalence of antibodies to COVID-19 reduced to 25 percent in September from nearly 29 percent found in the August exercise 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," he told PTI.

Sherwal conjectured this trend if persists means, when the vaccine comes for COVID-19, a patient would need to be given "power doses too" besides the "first dose". The survey also showed that there was a significant decrease in seroprevalence in northeast, north and central districts of Delhi, while there was a slight increase in south, east, west and northwest districts. The sero-prevalence was found more in women during all the rounds of the survey and 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 sero-prevalance 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.

The national capital recorded 41 COVID-19 fatalities on September 30, as the toll climbed to 5,361, while 3,390 fresh cases took the tally to over 2.79 lakh. On September 29, 48 deaths were reported, the highest number of fatalities recorded in a day since July 16, when the city saw 58 fatalities.

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 sero-positive when we went to donate plasma soon after. Recently, he again went to donate plasma but couldn't as his sero-prevalance came negative, meaning his antibodies had vanished," he said.

Chatterjee said, human bodies have two kinds of antibodies, one which is humoral or inherent one and the other one is cellular immunity which is in response to an infection. IgM antibodies last for shorter duration, and IgG antibodies are for the long run, the Apollo hospital doctor said. All three of them advised people to not be complacent about the COVID-19 situation, and still observe the same safety norms as earlier, with Chatterjee cautioning, "the WHO had already said that antibodies cannot be your passport to immunity", and therefore people can't afford to lower their guard.

The September survey report also stated that asymptomatic infections were manifested in most cases during all the sero-survey rounds and diabetes mellitus was observed to be a risk factor for subclinical infection.

Richa Sareen, consultant, pulmonology and critical care medicine at Fortis Hospital, Vasant Kunj here, though concurred with other doctors on the immunity not lasting longer post infection, but asked to take the "statistical findings with a pinch of salt". "The survey done in August and one in September were not done on same set of people, so one should not immediately extrapolate the findings," she said.

Sareen and Sherwal suggested holding sero-prevalance surveys with a wider interval than every month. "In fact, it should be done every three months to get a better idea for analysis," she said. The next round of 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 sero survey rounds and future surveillance should focus more on "ensuring 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 sero survey.