Updated: April 17, 2020 7:32:44 am
Even as India’s testing strategy banks heavily on testing symptomatic people, a study from China has come to the conclusion that 44 per cent of those who tested positive contracted the disease from an asymptomatic person. They estimated that viral shedding — which is when a person infects another — starts happening two to three days before the onset of symptoms.
The study, published in Nature Medicine on April 15, looked at 94 patients with laboratory-confirmed COVID-19 admitted to Guangzhou Eighth People’s Hospital and said, “We observed the highest viral load in throat swabs at the time of symptom onset, and inferred that infectiousness peaked on or before symptom onset. We estimated that 44%… of secondary cases were infected during the index cases’ presymptomatic stage, in settings with substantial household clustering, active case finding and quarantine outside the home. Disease control measures should be adjusted to account for probable substantial presymptomatic transmission.”
Even higher proportions of presymptomatic transmission of 48% and 62% have been estimated for Singapore and Tianjin, noted the researchers who included those from Guangzhou Medical University and the World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control at the University of Hong Kong.
Since most cases were isolated at the onset of symptoms, postsymptomatic transmission was far more limited.
India’s testing strategy focuses overwhelmingly on testing of symptomatic cases. Currently, India is testing “all symptomatic individuals who have undertaken international travel in the last 14 days, all symptomatic contacts of laboratory-confirmed cases, all symptomatic health care workers, all patients with Severe Acute Respiratory Illness (fever AND cough and/or shortness of breath)” and symptomatic people with influenza-like illness in the hotspots and clusters as defined by the ministry of health.
The only asymptomatic people who are tested are those with history of contact with a confirmed positive case between day 5 and day 14 of exposure. When the Indian Council of Medical Research randomly tested 5,911 SARI patients for COVID-19 from February 15 to April 2, a total of 104 (1.8 per cent) tested positive for COVID-19 across 52 districts in 20 states and UTs. Forty of these cases in 15 states and 36 districts had no history of contact with a known case or any international travel.
Health officials in India have maintained that transmission by asymptomatic people is very limited and there is no need to further revise testing strategy. However the Nature Medicine article concluded: “For a reproductive number of 2.5, contact tracing and isolation alone are less likely to be successful if more than 30% of transmission occurred before symptom onset, unless >90% of the contacts can be traced. This is more likely achievable if the definition of contacts covers 2 to 3 days prior to symptom onset of the index case, as has been done in Hong Kong and mainland China since late February. Even when the control strategy is shifting away from containment to mitigation, contact tracing would still be an important measure, such as when there are super-spreading events that may occur in high-risk settings including nursing homes or hospitals.”
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