Delhi sero survey: Results of third round expected this week

The results of the third round of the serological surveillance in Delhi is likely to be released within the week, with the team of researchers having analysed all the 17,000 samples that had been collected during the first five days of the month.

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“The result of the third round of sero survey will likely be out within this week,” said one of the researchers.

The serological survey helps the government in estimating the proportion of the population that might have been exposed to the virus causing coronavirus disease (Covid-19), and developed immunity to it. This is done by checking for antibodies against Sars-CoV-2 in blood samples.

After the first round of the serological survey was conducted from June 27 to July 10 with assistance from the National Centre for Disease Control (NCDC), the Delhi government decided to conduct the surveys at the beginning of every month to map the actual spread of the infection.

The first round of sero survey in Delhi had revealed that an estimated 22.6% had antibodies against Covid-19, which translates into about 4.5 million people in Delhi having had the infection. At the time, Delhi had reported just over 100,000 positive cases through testing. A representative sample of 20,000 people had been tested by the government for the study.

The second round of sero surveillance, headed by the Maulana Azad Medical College, was conducted between August 1 and 7 when 15,000 samples were collected district-wise to check the burden of disease. For this survey, 29% of the total samples had antibodies against Covid-19, which translates to about 5.8 million of Delhi’s 20 million population. Through testing for the infection using nasal swabs, just over 142,000 positive cases had been detected.

The results for the third round of sero survey, for which 17,000 samples had been collected ward-wise – will show whether Delhi is close to achieving herd immunity. “Many people with Covid-19 are asymptomatic and hence do not go to a healthcare facility or get tested. Sero survey helps to estimate the true burden of disease and what percentage of the cases were asymptomatic. There are some studies which show with 40 to 60% of the population being infected, the transmission goes down,” Amit Singh, associate professor at the Centre for Infectious Diseases in Indian Institute of Science had told HT earlier.