As many as 2.23 lakh people in Bengaluru may have already been infected with Covid-19, according to epidemiological calculations, which reflect badly on the measures taken to curb the outbreak in the city.
Bengaluru's population is 1.3 crore.
The massive scale of infections has opened the possibility of the government considering herd immunity, a source said, although experts in the State Task Force cautioned that the onset of such immunity was still a long way away and would come with an indeterminate number of deaths.
As of Monday evening, the total number of active cases in Bengaluru Urban was 15,052. But this number is deceptive, experts said. On Monday, for example, the city reported 1,315 new cases. Epidemiologists said this number actually reflected the city’s new cases from at least seven days ago.
“Considering that it takes four to five days for a person to develop symptoms, after which it takes nearly five days for the test result to come back to show that they are positive, the new people revealed as being infected on Monday were actually infected between seven and 15 days ago,” said Dr Giridhar Babu, a senior member of the expert committee and noted epidemiologist.
One expert likened the daily Covid figures to astronomers looking into the cosmos to study celestial bodies. Such observations are dated because of the amount of time light takes to reach the observers. Similarly, the new virus figures disclosed every day are described as a window into the past.
What is unknown is the true number of new infections today, said Dr Pradeep Badanur, another epidemiologist with the expert committee.
In the absence of a proposed and pending 'Sentinel Survey' — which would see Asha workers and other government staff test select groups across the city using RT-PCR, antigen-based and serological testing to determine the true spread of the outbreak — the committee has been carrying out epidemiological studies. These reveal that over two lakh Bengalureans currently have the disease.
The calculations are based on active cases as per data from the State Covid War Room for the last 14 days (which is the average timeframe in which a person is first infected and when he/she starts shedding the virus).
Dr Giridhar Babu explained the math: From June 30 to July 13, there have been 11,136 new cases in Bengaluru Urban. Adjusting for R0 (1.29), we get 31,978 cases. But these are new cases only seven days ago. By assuming that each person infected one other person (a most conservative estimate) for the last seven days, there would be 2,23,846 active infections in Bengaluru.
Some 60% of the population would have to be infected in order for herd immunity to set in, added Dr Badanur. “The city’s population is 13 million,” he said.