This time the survey was conducted by the Delhi government in association with Maulana Azad Medical College. State health minister Satyendar Jain said the result was a good sign and added that a third serological survey will be conducted in the capital from September 1. Another will follow in October.
An average of 25% growth in antibodies was found among people as compared to the last sero survey. The prevalence was higher in women (32.2%) than men (28.3%). In age-groups, the highest prevalence was found among people below 18 years (34.7%) and lowest in the 18-49 age-group (28.5%).
The city is still far from acquiring herd immunity, which is at least 40%, though experts disagree on the level required. The government is taking heart from the fact that between the first sero survey, which started on June 27, and the second one, which took off on August 1, the antibody prevalence in the capital has increased significantly.
“On the other hand, 70% of people have not developed antibodies, which means that we are yet to reach the stage of herd immunity. The people of Delhi are very careful in taking precautions against Covid, which is very positive. The pandemic is not spreading exponentially in Delhi like before,” said Jain, addressing a press conference.
The survey, however, reveals that the spread of the infection has not been uniform in the city and the change between the two surveys varies from 6.8% in North-East district to 50.2% in South-East district. Seroprevalence is also highest in the South-East district (33.2%), where every third resident has got infected and recovered. It’s lowest in South-West district where only 16.3% people have antibodies. The survey was conducted in all 11 districts of the capital.
A total of 15,000 samples were collected from the districts between August 1 and 7. Of these, 25% of the samples were collected from people in the 5-17 age group, 50% from the 18-49 age group and 25% from those above 50 years. The prevalence has been found to be 34.7%, 28.5% and 31.2%, respectively.
“The sample size was decided on the basis of a study conducted by scientists. People belonging to every class were included — those living in slums, middle class and upper middle class. There are people from all age groups and genders,” said the minister.
A health official said that for the first serological survey, teams of health workers had visited different areas and chosen every fifth house from a selected locality. “For the second survey, however, we selected mostly residents of identified locations through a randomised sampling technique and asked them to visit their nearest dispensary for giving blood samples,” explained the official.
The health minister said the antibodies stay in the body from a period of 3-4 months up to 7-8 months. “Along with this, T-cells also develop and their life is much longer. They are called memory cells, and because of them, if people have been infected by the coronavirus in the past, it is not probable that they will get infected again,” said Jain, attributing his remarks to scientists.
The government will compare the data of the previous survey with the latest one and decide if any change of approach is required. “It is also reassuring to see that the positivity rate in Delhi has reduced to nearly 7%, which is much lower than the national average. The situation in Delhi is improving,” he said.