A healthcare worker collects the swab sample of a resident at Malad (East) on Sunday. (Satyabrata Tripathy/HT Photo)



Of the 12,926 active Covid-19 patients in Mumbai, 9,155 (71%) are asymptomatic, displaying no symptoms before undergoing tests for the presence of Sars-CoV-2, the pathogen that causes the disease, data from the Brihanmumbai Municipal Corporation (BMC) shows. A total of 835 patients are in critical condition.

While doctors call it a good sign, as chances of transmission and spread of infection from asymptomatic patients are lower, they stress the need to conduct more reverse transcription polymerase chain reaction (RT-PCR) tests – considered the gold standard in detection of the novel coronavirus – and warn against overdependence on the quick-result rapid antigen tests (RAT).

Every day, the civic body conducts around 15,000 Covid-19 tests, of which a majority – almost 50-55% – are antigen tests. Similar to RT-PCR, rapid antigen kits detect the presence of molecular pathogens of the novel coronavirus. But RT-PCR test results takes almost eight hours as compared to within 30 minutes via RAT.

Asymptomatic transmission refers to transmission of the virus from a person who does not develop Covid-19 symptoms. As per the World Health Organization (WHO), there are few reports of laboratory-confirmed cases, who are truly asymptomatic, and there is no documented asymptomatic transmission till date.

“The infectious period of the virus that causes Covid-19 in symptomatic individuals may begin within two days. It persists for more than 10 days after symptoms show,” said Dr Pradeep Awate, state surveillance officer. “In asymptomatic patients, however, the chances of transmitting the virus are lower, as these individuals don’t sneeze or cough. Hence, having more asymptomatic patients helps break the chain of infection.”

Most asymptomatic patients are identified through contact-tracing. In Mumbai, more than 40 lakh close contacts of infected patients have been traced by the civic body since the outbreak of the virus in March.

“Asymptomatic patients don’t know they are infected because they do not have symptoms. Contact-tracing becomes important to identify this set,” said Dr Mangala Gomare, executive health officer, BMC. “Many asymptomatic patients get detected when they are admitted to hospitals for other ailments.”

However, a section of medical experts call asymptomatic patients “silent spreaders”, suggesting that more Covid-19 tests via RT-PCR be undertaken across the city that will help in early diagnosis.

Since the outbreak of the infection in March till December 5, the civic body has conducted 19,71,736 Covid-19 tests, of which 5,25,143 tests were done through rapid antigen kits. “We get less than 2% false positive reports using rapid antigen tests. Also, people get results within 30 minutes, which helps reduce the burden on labs,” said Suresh Kakani, additional commissioner, BMC.

However, doctors have highlighted a loophole in the system. As per rules of the Indian Council of Medical Research (ICMR), every positive report through RAT is treated as true positive, while those with negative report have to be mandatorily re-tested through RT-PCR only if they display symptoms. “Asymptomatic patients don’t show symptoms and as per ICMR, the sensitivity of rapid antigen kits is only 50%. So, thousands of asymptomatic patients are slipping through the loophole in diagnosis, as the civic body is relying more on rapid antigen tests,” said a senior epidemiologist. “While there is no scientific evidence that asymptomatic patients can’t infect others and the spread of infection rate may be less, they could still transmit,” added the doctor.

“Post Diwali, many migrants who had gone back to their hometowns are returning to the city, and many will be asymptomatic. So, it is essential to run tests to find silent carriers. Considering they live in densely populated slums, they can contribute further to the spread of the virus,” said Dr Om Srivastava, epidemiologist and member of the state’s Covid-19 taskforce.