Sero survey: Nearly 30% infected in Delhi – This is higher than what daily Covid testing data shows

A little over 29% of the capital’s population — or 60 lakh persons — has been, at one point or another, been infected with Covid-19, according to the latest sero survey. This is an increase of about 25% since the last one conducted in July indicated an infection level of 23.5%.

Interestingly, the Case Infection Rate (CIR) revealed by the daily testing data suggests infection levels in the capital were a much lower 12.5% — 1,42,723 persons were infected out of 11,43,703 tests done — at the time the survey was done. While the sero-survey shows Delhi’s infection levels have risen since last month, the CIR shows a fall in infection levels. Today’s CIR in Delhi is 11.5%. What this means is that Delhi’s testing strategy is-n’t very effective; this is, of cou-rse, worsened by extensive use of the Rapid Antigen Test — 62% of tests today are RAT ones — that capture less infections than the standard RT-PCR tests do.

Though there has been no all-India sero-surveys since May — that survey said 0.73% of the country’s population was Covid-positive — detailed city-wise sero-surveys suggest the all-India number would also have climbed significantly. As in Delhi, sero surveys for other cities also give much higher infection levels than the case-infection-rates (CIR) that the regular tests suggest (see graphic).

In the case of Mumbai, last month’s sero survey suggested a third of the city’s population was Covid-positive as compared to a case infection rate of just 23.4% based on the daily tests done. That CIR is down to 19.6% today. In the case of Pune, the CIR was 20.4% earlier this month compared to the sero-survey’s 51.5%. In the case of Ahmedabad, both the CIR and the sero-survey numbers were roughly the same, though the CIR has fallen dramatically since; this may be due to excessive use of RAT, as in Delhi. Ahmedabad was testing 1,500-2,200 samples daily a month ago, and is now testing 21,000 samples every day.

A sero survey is a blood sample of people pre-selected on a random basis, and the selection is done keeping in mind the characteristics of the population in the city/state. Most sero surveys test for antibodies in the blood which indicate whether a person had been infected by Covid-19 in the past. The sero survey being done in Karnataka, though, also uses RT-PCR tests that, unlike the antibody tests, indicate whether a person is an active Covid-19 case.