A study conducted by a team of doctors from Bangalore Baptist Hospital has revealed a high seroprevalence of 62.7% in Bengaluru Rural district in July 2021 after the second wave.
The team led by Carolin Elizabeth George, head of Community Health and Research Division at the hospital, conducted a cross-sectional study in a representative sample to determine the seroprevalence of COVID-19 infection among unvaccinated population in the district post the second surge. The study, which has been sent for publication in a peer-reviewed journal, found that despite the advantages of low population density and well-ventilated rural landscapes, 62.7% of the population had been exposed to the SARS-CoV-2 virus.
The team conducted a house-to-house survey and collected samples of 831 unvaccinated adults in July 2021. “We estimated an overall prevalence of 62.7%, which is more than five times the seroprevalence of 12.4% found during a similar survey done on the same population during the first surge in October last year,” Dr. George said.
The State’s second seroprevalence survey, which was carried out in 290 healthcare facilities spread across Karnataka from January 25 to February 18 this year, had found a prevalence of 20.3% in Bengaluru Rural. The State’s average seroprevalence was 15.6% then.
According to the Baptist Hospital study, the case-to-undetected-infected ratio (CIR) was 1:8.65, which means that for every confirmed COVID-19 case, there were 8.7 cases not detected. This means when the district reported 59,112 confirmed cases, the reality was 5,11,722 cases. “This is of no surprise due to many reasons. Firstly, most of the people infected were asymptomatic and hence not detected. Secondly, the health-seeking behaviour of people in rural areas is generally poor compared to urban counterparts. Thirdly, the limited capability of the rural health system to identify COVID-19 infections. The CIR in the district was almost similar in the first (1:7) and second waves (1:8.65), though the caseload during the second surge was almost 10 times more,” he said. This shows that the district was able to ramp up the diagnostics to meet the increased testing demand during the surge crisis, the doctor explained.
The Infection Fatality Rate (IFR) was 16.27 per 10,000 infections as of July 13, 2021, when compared to 12.38 per 10,000 infections as on October 22, 2020, during the hospital’s first study in the district. A history of at least one symptom suggestive of COVID-19 or a positive COVID-19 test of self or a family member in the past was significantly associated with seropositivity.
Dr. George said the findings were consistent with the preliminary reports of the fourth sero survey by the Indian Council for Medical Research (ICMR) that reported an overall national seroprevalence of 67.6%. This survey had found a seroprevalence of 69.8% in Karnataka in June and July 2021. To estimate the State’s seroprevalence, ICMR sampled three districts of the total 30 districts, and Bengaluru Rural was not part of the survey.