A study conducted by a team of doctors from Bangalore Baptist Hospital in the city’s D.J. Halli slum during the last week of August and early September found an overall 57.9% seroprevalence of IgG antibody against COVID-19. This means over half the population in the slum had been exposed to the virus by September.
The findings of the study, which have been accepted for publication in Epidemiology and Infection , a peer-reviewed medical journal, are consistent with a study conducted in slums in Mumbai, where a prevalence of 57% was noted three months past the index case. Conducted two weeks after the violence in D.J. Halli, this is the first seroprevalence study from a slum in South India and the second in India.
“About one-thirds of the 499 people surveyed in the slum had co-morbidities and 95.2% of them were asymptomatic. Hypertension (19.2%) and diabetes (15.4%) were reported as the most common co-morbidities. A majority of the people reported frequent use of hand sanitisers (97.8%) and face covers (88.4%),” Carolin Elizabeth George, lead author of the study, told The Hindu .
Although seroprevalence among participants with diabetes and hypertension was 62.3% and 66.6%, respectively, the association with seropositivity was not significant. “Among the seropositive individuals, 41.6% had a history of an infected family member, and 33.3% had a history of exposure to a person with COVID-19 in the past,” she said.
Case-to-infection ratio
The study found a high case-to-infection ratio (CIR) with 195 undetected infected individuals for every RT-PCR confirmed case. The infection fatality rate (IFR) was calculated at 2.94 per 10,000 infections, said Dr. George, who heads the Department of Community Health at the hospital.
“If we extrapolate our findings to the entire D.J. Halli slum, 57,900 people would have contracted the infection, as against the 295 cases officially reported by the State. This is no surprise, as the hazardous physical environment, overcrowding, poor sanitation, and the impracticability of social distancing, hand washing, and using face covers in slums are conducive to a rapid spread of infection,” she said.
As over 95% of the surveyed individuals were asymptomatic, they harboured and dispersed the infection without being caught by tests or inviting medical attention. Although the State had also done a seroprevalence survey in all the 30 districts in September, it had not included the slums. This survey had shown that 16.4% had developed antibodies against COVID-19. The case-to-infection ratio (CIR) in the State’s seroprevalence survey was 1:40. “Our CIR being higher than that in other surveys can be attributed to the low testing rates at the beginning of the epidemic,” the doctor said.
What it means
“It suggests that the worst is over. It is a relief to realise that the virus spared most lives in this slum as a large chunk of the population here was young. With more than half of the population already being infected, the infection curve is likely to have started its journey down, and COVID-19 may cease to be a public health problem in D.J. Halli. This is confirmed by the low COVID-19 test positivity rate (1.5% to 2%) despite adequate testing (2,000 tests per million/day) from this slum in the past one and a half months,” Dr. George said.
