Covid-19: Why Delhi needs to ramp up testing again
A rising positivity rate typically suggests that a region is testing inadequately. The positivity rate should drop to 5% or below if the testing programme is adequate and is keeping the outbreak in control, according to the World Health Organization.Updated: Oct 27, 2020, 09:02 IST
After appearing to drop for a second time, new infections of the coronavirus disease (Covid-19) in Delhi have started rising again over the past two weeks. This spike in cases has come hand-in-hand with a steady rise in the positivity rate, suggesting that the outbreak may be expanding again.
A rising positivity rate typically suggests that a region is testing inadequately. The positivity rate should drop to 5% or below if the testing programme is adequate and is keeping the outbreak in control, according to the World Health Organization. That is why testing enough, and using the right kind of tests, is the key to Delhi being able to control the outbreak in the next few weeks.
1. A distinct third surge in new infections
The case trajectory in Delhi shows three distinct surges. The first started in mid-June, and peaked when the seven-day average of daily cases, also known as the case trajectory, touched around 3,400 in the last week of June. This dropped to about 1,000 a day by the end of July. It remained in that range for a month, and the second surge started towards the end of August, rising till the middle of September, when average daily cases touched 4,174 – the highest the case trajectory has been so far.
A strong upward trend is now visible for a third time. Average cases have been rising for 13 straight days – from 2,661 on October 12 to 3,663 on Friday. The rising graph of cases in the city – days after it was starting to come under control – has come right at the heels of the festive season, and is being attributed to more people coming out of their homes, poor mask discipline, and lack of adherence to safety protocols due to Covid fatigue.
2. Low testing caused drop between 2nd, 3rd surges
After the first peak ended,and the daily cases went down to 989 (August 5) and positivity rate to 5.8% (August 13), Delhi appeared to make a crucial mistake by easing off on testing, instead of ramping up further to suppress the virus.
This led to a gradual increase in positivity rate over time, and consequently in daily cases, prompting the situation to reach a point on August 26 (1,693 cases, 8.5% positivity rate), when chief minister Arvind Kejriwal held an emergency meeting and decided that the rate of testing would be doubled from 18,000-20,000 daily.
This was a crucial intervention -- on average there were a just little over 20,000 tests being conducted daily at the start of September, and by September 20 this was up to nearly 60,000 tests -- leading to a spike in cases (since more tests were being conducted) but bringing down the positivity rate.
The average positivity rate over time dropped to as low as 5.3% for the week ending October 8.
But after touching its peak, tests started to drop again – for the week ending October 9, daily tests were down to 47,612 from the September 20 high of nearly 59,368. October 9 was also the day when the case trajectory dropped to its lowest – 2,574 new cases a day. When placed alongside each other, it is evident that the dip in average daily testing has heavily influenced the drop in average daily infections being reported.
3. Low RT-PCRs led to numbers being suppressed
A key concern even as the Delhi government increased its testing at the start of September was that much of it was through rapid antigen tests, which are cheap, easy to deploy, and give results within 15 minutes. Due to this, the proportion of reverse transcription-polymerase chain reaction (RT-PCR) tests, considered by experts as the “gold-standard”, dropped significantly in the overall test numbers.
In the first 15 days of September, while overall testing rose, the share of RT-PCR tests nearly halved – from 32.3% for the week ending September 1 to 17% for week ending September 16. This may have suppressed the real positivity rate because rapid tests gives high “false negatives” (labelling infected people as uninfected). .
The share of RT-PCR tests, however, has started rising again. In the past week, 28.2% of all tests have been RT-PCR. And this rise has overlapped with the rise in daily cases in the city since fewer positives are being missed. According to the Harvard Medical School, the reported rate of false negatives in antigen tests could be as high as 50%.
4. Rising positivity rate points to a big problem
The positivity rate for Covid-19 has started rising again, with 6.8% samples tested in the past week coming back positive (this is the highest in nearly a month, since 6.95% on September 27). The number was 5.88% the week before, and 5.49% the week before that.
On Sunday and Monday, it went as high as 8.17% and 8.23%.
Positivity rate is a crucial metric as experts say it shows how widespread the virus is in the community, and when coupled with an increase new cases, indicates that the virus is spreading fast.
Testing needs to be increased again to flatten the curve
Delhi’s rising case rate has come right at the start of the festive season. The fact that it is coupled with the positivity rate increasing is further indication that cases may continue to rise.
The silver lining is that testing is moving in the right direction. Both total tests and RT-PCR tests have increased – in fact the city has set new records for RT-PCR tests three out of the last five days.
But if the objective is to bring the overall positivity rate below 5%, and keep it there for at least two weeks to suppress the virus, the time to increase tests in now. Any figure above 7% means that,even if tests may be up, Delhi is again not testing enough. Unless the number of tests is scaled up, with the share of RT-PCR tests staying high, matters could get out of hand again for the first hot spot in India that almost managed to contain its outbreak.