New Delhi: The third Covid-19 serological survey in Delhi, conducted last month, has found coronavirus antibodies in 25.1 per cent of the participants.
The seroprevalence detected in the survey, whose results have been accessed by ThePrint, is lower than that found in August — 28.36 per cent. Experts say it could be because of many reasons, from survey limitations to the disappearance of Covid-19 antibodies after a certain amount of time. The first serosurvey, in July, had found a seroprevalence of 22.83 per cent.
The latest serosurvey report was submitted in the Delhi High Court Wednesday. This, after the court asked the Delhi government, at a hearing on 16 September, to submit the report in court before making it public, unlike the previous two serosurveys.
A total of 17,409 samples from each of Delhi’s 11 districts were studied in the survey, which took place between 1 and 15 September. The August and July rounds involved 15,046 and 22,853 samples, respectively.
The study was based on a multistage random sampling method where participants from each district were selected on the basis of seroprevalence detected in the August survey — that is, participants were selected from areas where the seroprevalence was found to be high in the last exercise.
The second and third surveys have been conducted by the Delhi government-run Maulana Azad Medical College and Hospital, while the first was carried out by the National Centre for Disease Control (NCDC), which is overseen by the Union Health & Family Welfare Ministry.
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Highest and lowest prevalence
The third serosurvey found the highest prevalence of antibodies in Northwest district, at 31.8 per cent, while the lowest was detected in Northeast at 12.2 per cent.
In the August survey, the highest prevalence was found in Southeast district at 33.2 per cent, and the lowest in Southwest at 17.4 per cent. In the first serosurvey, the highest prevalence was found in the central district, 27.8 per cent, with Southwest again at the other end of the spectrum with a seroprevalence of 12.9 per cent.
The latest survey has found seroprevalence of above 30 per cent in only three districts — Northwest, East (31.1 per cent) and South (30.1 per cent).
The others, in descending order of seroprevalence, are Shahdara (28.7 per cent), West (27.9 per cent), Southeast (27 per cent), North (24.1 per cent), Central (21.7 per cent), New Delhi (18.6 per cent), Southwest (14.6 per cent).
Highest prevalence among adolescents
Like in the August exercise, the September survey, too, has found the highest prevalence among the 5-17 age group, at 27.2 per cent, followed by the 50-and-above age group, where prevalence was recorded at 26 per cent.
The 18-49 age group recorded the lowest seroprevalence at 23.5 per cent. In the August survey, too, the 5-17 age group recorded the highest seroprevalence at 31.1 per cent, followed by the 50-and-above age group at 30.7 per cent. The 18-49 age group recorded the lowest seroprevalence in the previous survey, too, at 26.5 per cent.
Higher prevalence among women
In another finding similar to the August survey, the study found a higher seroprevalence among women than men across all three age groups. The highest prevalence was found among women in the 50-and-above age group at 28.5 per cent, followed by the 5-17 age group, with a seroprevalence of 27.9 per cent, and the 18-49 age group, at 24.7 per cent.
Among men, seroprevalence was found to be highest in the 5-17 age group, at 26.7 per cent. This was followed by the 50-and-above age group, where seroprevalence was 23.9 per cent, and the 18-49 age group, at 22.5 per cent.
The survey also sought to study exposure trends with respect to the participants’ socioeconomic status. Covid seroprevalence, the survey found, was higher among those on the lower end of the socioeconomic spectrum, with per capita incomes of less than Rs 5,000. Among the participants with an income of less than Rs 5,000, an estimated 26.1 per cent had had exposure to the novel coronavirus, as opposed to 20.5 per cent among those with incomes above Rs 5,000.
The survey also found a lower seroprevalence in planned colonies in comparison to resettlement colonies, urban slums, unauthorised settlements — 22.9 per cent against 25.9 per cent.
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‘Focus on elderly’
Asked about the survey trends, experts told ThePrint that the discovery of a higher seroprevalence in the 5-17 age group should not take the focus away from the elderly, for whom Covid-19 is believed to constitute a graver, more lethal risk.
“Although infections may be more among the 5-17 age group as the serosurvey shows, we need to keep focus on the elderly as mortality rate is higher among them,” said Dr Sanjay Rai, professor of community medicine at AIIMS Delhi.
“Even if they (the elderly) develop antibodies, they may be going away faster than in adolescents, which makes them more vulnerable,” he added.
Experts also said that serosurveys have limitations on account of sampling methods, and not every finding should be taken at face value.
“Serosurveys are conducted in stratified random samples of the population. If the sampling frame varies or sampling methodology varies, repeat surveys can yield variations in estimates,” said said Dr K. Srinath Reddy, a member of the Indian Council of Medical Research (ICMR) Covid-19 taskforce and president of the Public Health Foundation of India (PHFI).
“Also, Covid-19 antibodies tend to decline or even disappear by three months. So, persons infected during March to July may test negative even if they were infected but tested several weeks later. Further, any estimate must be reported with 95 per cent confidence limits to represent the range that can be associated with that estimate,” he added.
Only if samples are repeated, experts said, will a trend emerge on whether seroprevalence has decreased or not.
“Serosurveys are all estimates based on samples. It’s difficult to say that prevalence has gone down. Only if we repeat it over time can we say if it’s a trend or not as data suggests that infections are increasing, so we are definitely not out of the woods,” said Dr Anant Bhan, researcher in global health and bioethics.
ThePrint reached Delhi Director General Health Services Dr Nutan Mundeja but she refused comment. Asked if the serosurvey findings with respect to adolescents may effect a shift in treatment strategy, Maulana Azad Medical College and Hospital Dean Dr Nandini Sharma said antibodies don’t determine treatment strategies.
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