High daily testing allowed isolation of Covid patients
- Delhi tested 955,849 samples per million people till May 26, nearly four times the national average.
With vaccines in short supply, testing and isolating people quickly is the best way to keep infections under control. Experts say more testing helps trace and isolate Covid patients, set up containment zones, and track the size and spread of an outbreak.
Delhi is the runaway leader in testing among the country’s 20 most populous regions, shows an HT analysis, highlighting another important factor that helped the city get control over its deadliest Covid wave. The Capital tested 955,849 samples per million people till Wednesday, nearly four times the national average of 252,809. Jammu & Kashmir, in second, tested 633,133 per million.
Delhi has tested twice the proportion of its population compared to other states with good testing strategies, such as Kerala, Karnataka, and Telangana.
In all, the city has conducted over 19 million tests till Friday.
What turned out to be crucial was that even at the height of the fourth wave, when every third person was testing positive and labs were overwhelmed, the city administration did not let the number of daily tests drop below 60,000. Moreover, the city kept conducting a high number of the more accurate RT-PCR (reverse transcription polymerase chain reaction) test.
At least 65% of the over 257,000 tests in April and 76% of the 193,000 in May were RT PCR, considered the gold standard in testing, even as nationally the proportion of the less accurate rapid antigen tests (RAT) increased as governments ramped up testing during the second wave of infection. At the peak of the infection, when the positivity rate was 36% and about 25,000 new cases were emerging every day, it also meant that the 25,000 people were being isolated from the rest of the population every day to break the chain of transmission.
“Delhi has been a model state nationally in terms of testing since the first Covid wave. Against the national average of 1,500 tests per million population per day, Delhi has consistently been doing 4,000 tests for several months now, which is the highest nationally and among the highest globally.”
Experts said this played a role in Delhi controlling the wave.
“The rapid antigen tests are known to miss one out of every two positive cases. Once a person gets a negative report, albeit using an RAT, they are likely to go out in the society and infect others if they are asymptomatic carriers of the infection. Ideally, all tests should be conducted using RT-PCR but it is not possible to do so in such a populous country,” said Dr Harsh Mahajan, president of healthcare association NatHealth and founder of Mahajan Imaging that does RT-PCR tests in Delhi.
“What is good though is that Delhi maintained the 70:30 ratio for RT-PCR tests. Had that not been the case, more people with the infection would have been out and about and infected others,” he added.
RT-PCR and other molecular tests amplify the genetic material of the virus extracted from the swab sample to detect an infection. They are, therefore, more accurate than the rapid antigen tests that detect the presence of viral proteins without any amplification.
When Delhi first imposed a weekend lockdown on April 17, the seven-day average of daily samples had touched a peak of 99,776 – the most recorded in the city. The number of samples tested crossed the 100,000-mark on five occasions in the first two weeks of April.
However, the imposition of the lockdown brought down the number of tests, which Delhi health minister Satyendar Jain attributed to a stop in testing at transit points like railway stations and interstate bus terminals. Still, the city on average tested over 70,000 samples daily, including in the city’s rural areas.
“During this wave, we have seen a high number of cases being reported from the rural areas of Delhi as well. This was not the case during the last wave. To ensure all cases get detected quickly, a high number of tests – nearly 10,000 to 12,000 a day – is being conducted in districts with rural pockets such as Southwest,” Jain said last week.
Another challenge the city faced as cases rose in April was lab technicians getting infected, leading to long delays in sample collection and results, resulting a crucial lag period in which people could unknowingly infect others. The government, on April 22, reiterated its May 2020 order asking labs to release reports within 48 hours of collection to check the delay in infected patients getting isolated.
“Testing has to be done aggressively even when the number of cases is low. And, this is not just from the government’s point of view, people should also be willing to come out and get tested. If that happens, then the government will know in which locations the number of cases are on the rise and localised lockdowns can be implemented to stop the spread of the infection to the rest of the city,” said Dr Amit Singh, associate professor, Centre for Infectious Disease Research, Indian Institute of Science, Bengaluru.
In addition to tests, experts have suggested the government boost genomic sequencing in view of the looming threat of new variants. So far, Delhi has sequenced the third highest number of samples after Kerala and Maharashtra, both states with a much higher population. Of the 19,000 samples sequenced by Indian Sars-CoV-2 Genomic Consortia (INSACOG), 1,605 are from Delhi.
With only 10 institutes across India sequencing genomes, the numbers, however, are low as compared to the recommended 5% of total positive samples. In April alone, Delhi reported 487,000 positive cases; 5% of this would have translated to over 24,000 samples sequenced.