Almost 30% of 15,000 people surveyed in the Capital had antibodies for the Sars-Cov-2, according to the second serological survey that suggests approximately 1.2 million people more may have silently contracted, and recovered, from Covid-19 between July and early August.
The data showed that prevalence of the disease may be higher in women (32.2%) than men (28.3%), while the youngest age group (0-18) showed the highest rates of past infection (34.7%). Among the 18-50 age group, 28.5% had antibodies and for those 50, this number was 31.2%.
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The data also indicated that the outbreak may be less deadly than previously feared, and that a large chunk of infections are going undetected by the existing testing mechanism in the Capital.
“It is relieving to know that around 29% of the city’s population has recovered after being infected by the coronavirus. On the other hand, 71% of people in Delhi have not developed antibodies, which means that they are still vulnerable,” said Delhi health minister Satyendar Jain while releasing the results.
“The people of Delhi are being very careful in taking precautions against Covid-19. So, this is a very positive development. It shows that the Covid-19 infection numbers are no longer rising exponentially in Delhi,” Jain said, while adding that the Capital “is heading towards herd immunity, but experts say that should be at least 40%”.
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The minister said the higher prevalence, which also indicates a larger proportion is now immune to the disease, does not mitigate the risks of resurgence. “We cannot rule out a second wave because only yesterday India saw the highest ever spike in daily fresh Covid-19 cases,” he added.
A serological survey involves testing of blood for the presence of antibodies. The city has held two till now and the latest findings suggest the rate of silent spread has been slower than expected given that activity increased in recent weeks following the lockdown.
The first such study, involving 21,837 people, was carried out in early July and revealed close to 23% had antibodies. This number corresponded to infections that took place over roughly 120 days, an average monthly infection rate of 5.7% of the population. The latest 6.2% figure for the month is only marginally higher. “We had expected that in the second sero survey the seroprevalence would increase considerably. But it has increased by around 6-7% since July,” said Jain.
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The findings also underscore the challenge posed by the Sars-Cov-2 virus, which experts believe largely does not cause symptoms in those infected, but all infected people can pass it on to others. The July 10-August 7 period had roughly 33,500 confirmed infections, compared to the 1.2 million who may have actually had it if the survey findings are extrapolated.
It also suggests that the infection fatality rate (IFR) – a measure of how many people will succumb to the disease from all infected person, not just known cases – was at 0.07%. The CFR, based on known cases, comes to about 2.7% for the Capital.
Government officials said the latest findings that those under 18 had the maximum antibodies compared to other age groups and would play a crucial role in the Delhi government’s decisions on opening schools and colleges.
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Chief minister Arvind Kejriwal, in his Independence Day speech on August 15, said that the government will not open schools until it is fully convinced about the city’s Covid-19 situation.
Giving the district-wise break-up, the health minister said the south-east district has seen the biggest increase in people developing antibodies. The lowest increase was seen in north-east Delhi, which once had the highest number of Covid-19 cases.
“Antibodies are found in high numbers anywhere between three and six months after the recovery of a patient. In the process, the person also develops T-cells because of which the chances of contracting Covid-19 again become extremely low,” Jain said.
In the south-west district, 16.3% of the people have been found to have developed antibodies, which is the lowest of all the 11 districts in Delhi. The second-lowest was the New Delhi district (24.6%).
A day after the high court directed the Delhi government to do a comparative analysis of the two serological surveys conducted in Delhi so far, the health minister on Thursday said the government will comply with the order.
Jain said Delhi has a population of around 20 million and the findings of the latest survey suggest that around 5.9 million have contracted the Sars-Cov-2 virus and recovered from the disease. He pointed to subsequent spike in other nations to stress on the need to maintain precautions.
“In Europe or the USA, numbers have started rising again. Our country saw the highest rise in cases at 69,000 cases. So, we cannot believe that coronavirus is over in Delhi,” he said.
Jain said there was a time when the cases in Delhi were increasing by 4,000 daily and the test positivity ratio was 30%. “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.”
Charoo Hans, a former head of the microbiology department at Delhi’s Ram Manohar Lohia Hospital, said if 71% of people are still vulnerable, it is quite a high number.
“It suggests that the government should continue with its efforts on aggressive testing, contact tracing, surveillance, isolation, and providing good treatment. It also suggests that people should continue ensuring strict compliance with social distancing, wearing masks, washing their hands frequently, and following respiratory etiquettes. There is no room for any complacency,” she said.