15k samples to form basis of Delhi’s 2nd sero survey
First survey done from June 27-July 10; 22.86% had antibodies
nNEW DELHI Around 15,000 blood samples – half of which will be from people between the ages of 18 and 49 -- will be collected from across Delhi between August 1 and 5, according to guidelines released by the state government that detail how the city’s second serological survey for Covid-19 will be carried out, setting a target that is slightly lower than the previous such study.
The exercise is an epidemiological survey covering a representative population – the number of people, where they reside, their age and gender is meant to reflect the city’s demographics and who the outbreak affects.
The findings will help determine how much closer Delhi has gotten to the herd immunity threshold after the first serological survey covering close to 21,800 people between June 27 and July 10, showed 22.86% of those tested had antibodies and were thus possibly immune to the Sars-cov-2 virus that causes Covid-19.
“On the first day (Aug 1), tests will be conducted in four revenue districts. From the second day (Aug 2), it will be done in all 11 revenue districts. The sample size has been set at 15,000 in adherence to recommendations of experts working with the health department,” said a spokesperson from the chief minister’s office.
According to the standard operating procedures released to district-level health staff, the remaining of the samples will be collected evenly from people in two remaining age groups: those under 18 and those above 49.
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As per the document, a copy of which is with the HT, the highest number of samples has to be collected by the Northwest district (2,200), followed by West (2,145), and Central (1,548). The fewest has to be collected from New Delhi district, from where 885 have to be collected.
Unlike the first round of the surveillance, where all of the samples had been collected doorto-door, the government has also allowed collection at selected spots and health centre.
“Sample collection may be undertaken in community (through visits of teams) of at the designated spot/ health facility appropriate for public and the team. In both cases sample of only the identified persons (as per the age group) from the identified household will be collected,” said the SOP accessed by HT.
Delhi health minister Satyendar Jain had on July 22 announced that a sero-surveillance will be undertaken in Delhi during the first week of every month will be conducted to determine the silent spread of the infection in the city.
HT reported on July 29 that more than a hundred teams of three-four members each will be involved in the exercise. According to the SOP that will now be followed, households included during the first round of surveillance will be excluded in order to eliminate selection bias.
District and ward-wise sample distribution for the survey has been planned by researchers from Maulana Azad Medical College. For the first round, this was done by the National Centre for Disease Control (NCDC).
Each team, which can collect 25 to 40 samples a day, will consist of ASHA (Accredited Social Health Activist) workers, an auxiliary nurse midwife (ANM), a phlebotomist or a lab technician, and a community volunteer.
Samples will be processed in 18 laboratories across the Capital, in Delhi government and central government hospitals, including the NCDC laboratory.
“If you consider a prevalence of 23% — and I expect the number must have gone up by now — we need as few as 5,000 samples to estimate the population-level presence of antibodies statistically. A smaller sample would make analysing results quicker as well. However, a large sample size such as 15,000 would be beneficial to establish the prevalence in different groups such as children or vegetable vendors, provided it is a representative sample,” said Dr Jugal Kishore, head of the department of community medicine at Safdarjung hospital, who was also consulted by the Delhi government for the upcoming survey.