New Delhi: Symptomatic patients have been the main driving force behind Covid-19 transmission in Karnataka, suggests a study released Friday.
The study, which is yet to be peer-reviewed, says symptomatic and asymptomatic patients can both transmit Covid-19, but the risk of transmission is higher with the former.
This argument has been backed primarily by two yardsticks, the secondary attack rate (SAR) and dispersion rate, which is described as “being useful in the evaluation of specific health strategies designed to reduce intraregional geographical inequality in the distribution of health status indicators in a specific period”.
The research is a joint initiative of the Public Health Foundation of India (PHFI), a New Delhi-based non-profit public-private initiative, the Department of Virology at the National Institute of Mental Health and Neurosciences (NIMHANS), and the Karnataka government’s Directorate of Health and Family Welfare.
It involved an examination of all the 3,404 Covid infections reported in Karnataka between 8 March and 31 May. It was posted on the preprint portal MedRxiv Friday and has been submitted to The Lancet Public Health.
According to the researchers, the study has major implications for testing policies.
As of 19 September, Karnataka had recorded 5,02,982 Covid-19 cases, with 7,808 deaths and 3,94,026 recoveries.
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What the study found
According to the study, approximately 91 per cent (3,096) of the cases reported between 8 March and 31 May were asymptomatic, while the rest were symptomatic. Of the 3,404 infections, the source could not be ascertained for 267.
Among the remaining 3,137 infections, the source was attributed to either domestic (2,136 or 68 per cent) or international travel (128 or 4 per cent), or coming into contact with a Covid patient (873 or 27.8 per cent).
Of the 873 cases where the source of infection was a known Covid-positive contact, 822 could be epidemiologically linked to 144 source cases. The remaining 51 could not be precisely linked to a known source.
Among these, 90 symptomatic source cases were found to have transmitted infection to 645 people, at a secondary attack rate of 7.1, while 54 asymptomatic ones infected 177 people, at a secondary attack rate of 3.1, the researchers said.
“The proportion of secondary cases manifesting overt clinical illness was higher when the index case was symptomatic,” the researchers added.
“Also, the mortality was higher among the symptomatic, especially among the aged and with comorbidities,” they stated.
The researchers admit there are limitations to the study, including that viral shedding through the upper respiratory tract is higher during the time of symptom onset, and that it’s difficult to distinguish between asymptomatic and pre-symptomatic patients.
However, they say the findings have serious implications for devising testing, tracking and isolation strategies, saying infection containment policy should focus on “active search for symptomatic cases, subjecting them to testing and treatment”.
“…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,” they said.
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