In the last 15 days, the Bengaluru Urban district has seen a 61% decline in the number of daily Covid-19 cases. Experts believe this precipitous decline is due to a combination of good vaccination coverage and a large population of people who recovered from the disease in the second wave.
Since the second wave began on March 12, as many as 7.94 lakh people in Bengaluru officially contracted and recovered from Covid-19, comprising 6.23% of the total city population. Recoverees generally enjoy the benefits of natural antibody protection for a period of up to three months. In addition, 58.94% of 91.1 lakh city residents above the age of 18 have been inoculated once.
The test positivity rate is also low enough to suggest that the decline in cases is real. However, experts and municipal officers agreed that this is not yet enough to ensure "herd immunity".
The municipal chief commissioner, Gaurav Gupta, was blunt: "There is no herd immunity — cases would rise if there is risky and indiscriminate conduct by people — following the Covid-appropriate behaviour is essential."
It is a statement that the noted virologist, Dr V Ravi, formerly of Nimhans, can agree with. While Dr Ravi scuppered the idea of "herd immunity" as wishful thinking, he specified two reasons driving the current dip in cases: One, an increased amount of mask-wearing in public and secondly, less crowding in closed spaces.
"You need one or two super-spreading events to ignite the fire. The sparks are not flying yet. But the moment you do, you can expect a wave to begin," he added.
Although it has been 21 days since Unlock 1.0 began on June 14, experts pointed out that gatherings in closed spaces such as theatres, malls, markets and other places have only started to increase since last week.
"We need two more weeks to determine if the high mobility seen as of late will trigger a surge in cases," said Dr Pradeep Badanur, Additional Professor and Principal Investigator, Department of Epidemiology, Nimhans.
"The notion of herd immunity can only be gauged after congregations of people increase in cinema halls, malls and schools. At that point, if cases continue to be small, then we can say herd immunity has set in," he added.
Reinfection problem
Another element preventing epidemiologists and others from determining when and if India can achieve herd immunity is the problem of reinfections. One prominent genomic scientist who did not want to be named pointed out that the baseline 'Delta' variant is known to cause both reinfections and breakthrough infections.
"Many such cases are likely to be asymptomatic or mildly symptomatic and therefore might not even be tested," the scientist said, adding that there is a lack of systematic surveillance to reinfections across the country.
This means that existing sero-prevalence studies may no longer be reliable. "We have to assume that the sero-prevalence of the second wave is significantly higher than the 29% of population infection determined by the second sero-survey in January. However, nobody can tell you at this stage what the sero-prevalence is," Dr Badanur said.
The Karnataka government is planning a third sero-prevalence study. "Right now, the protocols are being looked at," Dr Ravi said.