Symptomatic patients prime drivers of infection in Karnataka during lockdown: study

It is important to find people who have symptoms and test them on priority, says one of the authors

A free COVID-19 test centre in Bengaluru.

While both asymptomatic and symptomatic COVID-19 patients transmit infection, a study by a team of experts from Karnataka has found that the main driving force behind the spread in the State during the lockdown were symptomatic people.

The team of experts led by V. Ravi, senior professor and Head of Neuro Virology at NIMHANS, who is also part of the State’s COVID-19 task force, studied 3,404 cases reported from March 8 (when the first case was detected) to May 31.

Of these, while 3,096 (91%) were asymptomatic, 308 (9%) were symptomatic. Majority of the asymptomatic cases were aged between 16 and 50 while symptomatic cases were aged between 31 and 65. Most of those affected were males.

“As per our analysis, on an average, each symptomatic patient has spread the infection to 7.1 individuals as against every asymptomatic patient who spread the infection to 3.1 individuals. While 90 symptomatic source cases transmitted infection to 645 people, 54 asymptomatic source cases transmitted infection to 177 people,” Dr. Ravi told

The Hindu . “Symptomatic patients can spread the infection to a large number of people because when they cough and sneeze they throw out higher viral particles,” he added.

A cluster analysis revealed that symptomatic source patients contributed to 33 large clusters (more than five cases) with the mean size of each cluster being 17.03 (range five to 56 cases per cluster), he said, quoting the study that has been posted on medRxiv, the pre-print server for health sciences.

“In contrast, asymptomatic source cases contributed to 14 large clusters (more than five cases) with the mean size of each cluster being 8.1 (range five to 22 cases per cluster). Also, mortality was higher among the symptomatic, especially among the aged and those with co-morbidities,” he said.

“Therefore, our results indicate that public health actions focused on testing, tracing, tracking, and treating the symptomatic person must be accorded top priority in curbing the transmission and in reducing the mortality,” he said.

During the study period, the infection was mainly spread through travel. While 68% (2,136) of cases got the infection through domestic travel, 4% (128) was through international travel. As many as 873 (27.8%) of the cases studied got the infection through a known contact with a COVID-19 positive case. In the remaining 267 cases, the precise reason behind the acquisition of infection could not be ascertained.

Another author, R. Giridhara Babu, who is also a member of the State’s COVID-19 Technical Advisory Committee, said the findings have major implications for policies related to testing.

“It is important to actively find people who have symptoms and test them on priority. Testing for asymptomatic persons can continue but the priorities for symptomatic persons is underscored in the study,” he said.

Among the 3,404, the highest number of cases were identified in Bengaluru, followed by Kalaburagi, Mandya, Yadgir, Udupi, and Raichur.

During the study period, the State was testing approximately 4,377 per million, with a positivity rate of 1.1. The total cases per million were 2, 708 in Karnataka then.