Over 50% exposure to Sars-Cov-2 in 10 of 11 city districts: Sero survey in Delhi
The fifth serological survey in Delhi to detect Covid-19 antibodies showed that 56.13% of the 28,000 people sampled tested positive, state health minister Satyendar Jain said on Tuesday, releasing findings that suggest more than half of the Capital’s population on average may have been exposed to the virus.
The data released by Jain showed that except for the north district, the other 10 districts had a seroprevalence of more than 50%. Of these, six had a seroprevalence of more than 55%. South-east district had the highest number at 62%.
These numbers now appear to fall in a territory that experts believe is similar to the herd immunity threshold – a point in seroprevalence (estimated to be 60-80% of the population in case of Sars-Cov-2) when there are too few people left for the virus to spread to, slowing an outbreak down.
But Jain cautioned that this should not be seen as having hit herd immunity, something that experts have separately said is uncertain in the case of natural infections since such immunity could wane and people still spread the disease while they are infected (contrary to what happens when someone is vaccinated).
“Some say herd immunity is achieved at 50%, others say it should be 60%. I don’t think we should get into the debate of herd immunity now. This is a new form of virus or disease, we should not get lax with our precautionary measures because in October, we had thought the virus was brought under control, but then we got the third wave,” said Jain.
According to Jain, the latest round – which was conducted between January 15 and January 23, and included 100 samples from each of the city’s 272 wards – was the largest in the Capital yet. “It is also the largest in the country so far. Delhi has largely won over Covid-19, but we should continue to practise Covid-19 appropriate behaviour,” Jain said.
The latest figure is an over 30 percentage point jump over the last sero survey that was held in October, before the city entered the third wave of infections.
The high seroprevalence is also in sync with the state of the outbreak in the Capital. Jain stated that over the past 10-12 days, Delhi has been reporting less than 200 Covid-19 cases every day and the positivity rate, which at one point had touched 15%, is now stable at lower than 1% for the past one month.
“Still, I want to urge everyone to continue wearing masks for a few more months. In the past two months, the compliance has improved substantially, the result of which is in front of everyone -- Covid cases have drastically come down. If people continue to wear masks this way, we will be able to totally control the virus in the next few months,” the minister said.
The minister said the full report of the latest serological survey will be released in a day or two after further examination of the findings.
“Like the results in the previous surveys, this time too the number of females having antibodies was more than that of males. There means that all these people had Covid at some point and now have fully recovered. We can say that antibodies have been found in 49.09% to 62.18% of the people in Delhi, and the average percentage of people who have been found positive is 56.13%,” Jain said.
Dr Nutan Mundeja, director general of health services (Delhi state health mission) said this time, the sero survey was not just bigger in scale, but also better technology was used.
“We have used CLIA technology this time, which is more sensitive than the ELISA technology, as a result of which results come out more accurate. All the samples were tested at the ILBS hospital to ensure uniformity. The survey was conducted in association with Maulana Azad Medical College. The samples were collected from people of all socio-economic strata and from across all kinds of population and age groups,” said Mundeja.
Delhi’s first serological survey was done in the city in June-July and it had shown that 23.4% of people surveyed had developed antibodies against the virus. Similar surveys in August showed that 29.1% of people had antibodies, 25.1% in September, and 25.5% in October. A positive result implies that the person has developed antibodies against the virus. This means that the person was infected at some point and developed immunity against it.
Jain said earlier, the Delhi government was conducting sero survey every month.
“But, now we are doing it after 1.5-2 months. The next survey will also be done after about 2 months,” he said.
T Jacob John, former head of clinical virology at Christian Medical College, Vellore said the latest sero survey result is a representation of the third wave of the Covid-19 pandemic which had hit the national Capital in mid-November when the peak daily cases had touched around 8,500.
“This sero survey does not include those who got infected in the first and second wave because the tests detect antibodies which lasts only for about 2-3 months. So, in general more than 56.13% people in Delhi have got infected and recovered. Also, loss of antibodies, which is not the last word on immunity, does not mean loss of protection,” he said.
John said herd immunity will be better achieved once vaccination is rolled out in a larger way.
“Senior citizens and those with co-morbidities, school staff and essential service providers should be vaccinated at the earliest so that Delhi can inch towards herd immunity,” he said.
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