Delhi reported 2,312 new Covid-19 cases on Tuesday — the highest in 59 days — according to the Delhi government. This is the second time in a week that the number of new cases has breached the 2,000-mark in the city.

The rise in cases came on a day when the city conducted 24,198 daily tests — the second-highest tests ever recorded since August 8, when the city tested 24,592 samples. The number of cases had declined for a day in between due to fewer tests — just over 14,000 — being conducted on Sunday. On an average, Delhi conducted at least 20,691 tests per day, in the last seven days.

Along with the rising number of cases, the positivity rate – the percentage of people who test positive for the virus of those overall who have been tested – has also been increasing over the last two weeks. Delhi has recorded a seven-day average positivity rate of 8.9%, with a one-day high of 9.9% on August 30, when 2,024 new cases had been recorded. The lowest daily positivity rate — 5.1% — was recorded on August 7.

“The number of cases is on the rise across India. And, if we keep testing more people, chances are we will find more cases. With how densely populated we are and the sheer volume of our population, the absolute numbers are going to be huge. But what is encouraging is that our CFR (case fatality ratio) is close to 2%,” said Dr Jugal Kishore, head of the department of community medicine, Safdarjung Hospital.

Delhi recorded 18 new deaths due to the infection on Monday, taking the total fatalities in the city to 4,462. This puts Delhi’s case fatality ratio – fraction of people who die among those who test positive — at 2.52%. Delhi’s CFR has been on a decline since June third week, when it peaked at 4.11% after the government reconciled its data adding 437 deaths in a single day. It remains higher than the national average of 1.8%.

The Delhi government has started collecting blood samples for the third round of serological surveillance on Tuesday with samples of about 17,000 people collected from 272 wards in the city in over five days. The sero survey will help the government estimate the prevalence of antibodies against Sars-CoV-2, the virus that causes Covid-19, and determine the spread of the disease.

“Unlike the last two sero-surveys, in which we ensured that samples were collected from all 11 revenue districts, this time we shall ensure that samples are collected from all 272 municipal wards in the city,” said Delhi health minister Satyendar Jain. For each municipal ward, the sample size would depend on its area, population density and existing Covid-19 caseload, among other factors, said senior health officials.

The health minister said that samples from all wards will be collected between September 1 and 7 and the results will likely be shared in another 10 days.

“There are several models that have estimated that the transmission of Covid-19 will slow down when about 40 to 60% of the population has had exposure. We might get to see whether Delhi has reached close to 40% in the sero-surveillance and if so the number of new cases might start coming down soon,” said Amit Singh, associate professor at the Centre for Infectious Diseases in Indian Institute of Science, Bangalore.

The third survey will also ensure an adequate mix of people living in rural and urban areas and in recognised and unrecognised colonies. “The decision to collect samples ward-wise has resulted in more clarity on where the samples will be picked up from. We have been asked to ensure an adequate mix of population from rural and urban areas, and from recognised and unrecognised colonies. Also, we have to collect 25% samples from 1-17 years of age, 50% from those between the ages of 18-49, and 25% from those over the age of 50, just like the previous survey” said a senior official from south-west district. The district has to collect a total of 1,600 samples.

The official said samples would not be collected from those who were covered under the two previous surveys.

(With inputs from Abhishek Dey)