The third serological survey conducted by the Delhi government in September showed that 25.1% of those tested had developed antibodies to the coronavirus, Health Minister Satyendar Singh said on Thursday, ANI reported. This is an almost 4% drop from the 28.1% reported in August.

However, the Hindustan Times reported the fall to be from 29.1% among 15,000 people in August, to 24.8% among 17,197 people tested in September.

This was the third sero survey conducted in the Capital. Singh said the fourth edition of the survey would begin in 15 days.

Antibodies are immune molecules produced by the body to fight pathogens. The presence of antibodies in the blood typically suggests that people infected with a virus would gain immunity for some period of time. Although having antibodies is not the same as having immunity to the virus, research shows that antibody levels are closely linked with the ability to disarm the virus.

The presence of antibodies is only expected to grow within a population with an outbreak, according to the Hindustan Times. But the Delhi government’s submissions show the numbers have dropped in two months.

The report, submitted by the state health department to Delhi High Court, stated that the “fall was particularly localised” in three districts – North East, Central and North Delhi, according to The Indian Express. Meanwhile, a slight increase was seen in South, East, West, and North West districts.

Experts said the decrease in antibodies may be due to a change in the sampling method to represent the socio-economic conditions of the people.

“The sampling method that we have used this time is more representative; samples were collected based on where people lived – planned colonies, unauthorised colonies etc,” said Pragya Sharma, professor of community medicine at Maulana Azad Medical College, told Hindustan Times. Sharma was part of the team that designed the survey, led by the college’s Dean Nandini Sharma.

Lalit Kant, former head, epidemiology, and communicable diseases division, at the Indian Council of Medical Research, said the duration of antibodies has always been in question.

“Studies abroad have indicated that the antibodies begin to disappear in four months,” he told The Indian Express. “So, if a person had caught the disease in April or May, the antibody level might be coming down now. There may be people who had antibodies earlier but the antibodies may have disappeared when they were tested.”

“Secondly, you also need to see where the samples were taken from. If they are collecting more samples from the area where the prevalence of the infection is low, then they will get a composite number which will be very less.”  

— Lalit Kant to The Indian Express

The latest survey sampled people based on whether they lived in five types of spaces – planned colonies, resettlement colonies, urban slums and JJ colonies, urban and rural villages, and unauthorised colonies. The survey found that the prevalence of antibodies was lower among those living in planned colonies at 22.9%, as compared to 25.9% in other areas, according to the Hindustan Times.

Shobha Broor, former head of the department of microbiology at the All India Institute of Medical Sciences, expressed concern and said “there should not be a dip in the seroprevalence”.“We need to look at the minutiae of how the samples were collected and how the data was analysed,” she added.

However, an unidentified Delhi government spokesperson told the Hindustan Times that the drop in antibodies was because the latest edition of the survey “was much more representative of the antibodies prevalence at population level”.

The spokesperson said the sampling for the second survey was done at the district level, while for the third, samples were selected from 280 divisions – 272 Municipal Corporation of Delhi wards and four divisions each of the New Delhi Municipal Corporation and the Delhi Cantonment Board.

What is a sero survey?

According to the US’ Centers for Disease Control and Prevention, serology tests are “those that look for antibodies in blood”. “If antibodies are found, that means there has been a previous infection,” it says. “However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”

Serological surveys reveal how many people may have been infected with the novel coronavirus in an area but they do not show how many people are immune to the virus. It still isn’t clear how long antibodies last in infected persons and how many antibodies are needed to protect a person from reinfection.