With mounting evidence that asymptomatic Covid-19 cases are spreading the infection, India’s top medical research body is grappling with the problem posed by “silent spreaders” and examining the feasibility of starting ‘door-to-door’ surveillance to monitor and test older people at high risk who dont have symptoms. Some experts, however, believe this is a costly and unachievable objective.“Of 100 people with infection, 80 do not have symptoms,” Raman R Gangakhedkar, chief epidemiologist, Indian Council of Medical Research ( ICMR ) said on Monday.The new revelation of asymptomatic or mildly symptomatic infection in India has sparked concerns. While ICMR officials do not recommend mass testing in India, given the size and scale of the country, experts familiar with the development said the door-to-door surveillance model is being examined.“It’s not easy to test everyone. But the model of door-to-door surveillance used for polio can be replicated here. The fact that India continues to be polio free indicates the robustness of the surveillance system and that can easily be done in Covid-19 too,” said a person with knowledge of the situation. He added that this plan is still at discussion stage and no final decision has been taken.Under the model for polio in India, the vaccination teams go door to door, and check households.The teams make sure to vaccinate those children who haven’t been so far. In case of Covid-19, the possibility of monitoring people aged 50 years and more is being considered. “The silent spreaders are unknowingly infecting others. There is a need to strictly monitor the 50 plus population, which has the majority of positive cases,” added this person. Any such exercise will take a long time and be expensive. Door-to-door surveillance will prove to be “costly and unachievable” in a country like India, said Dr Anoop Misra, chairman, Fortis C-DOC (Centre of Excellence For Diabetes, Metabolic Diseases and Endocrinology).“The strategy could be multi-pronged for a country like India. We need to have intensive testing in areas which are hotspots, we can use random sampling in areas with less cases,” Misra said, adding that door-to door surveillance will not work. “It is just not possible. Some people will refuse and hide. We have to keep changing the strategy as transmission is certainly going up,” he said.Globally, too, there is enough evidence that suggests that people who never develop symptoms can transmit the virus. Studies from around the world published in the past weeks reveal the spread by “invisible sources” as a fresh challenge in the fight against Covid-19. This makes it much harder to trace and isolate everyone with the disease before they start transmitting it to others. Public health experts say these invisible sources of contagion must be factored in while deciding when to begin rolling back physical distancing measures.“Asymptomatic persons who tested positive in the antigen test, if shown to be truly positive for viral exposure by other criteria, may be a source of infection to others as long as they harbour the virus. Usually they are young, have a good immune response and may get rid of the virus quickly. But isolation till they do so is needed,” said K Srinath Reddy, president of Public Health Foundation of India.Reddy said social or physical distancing should continue. “Since a population wide search through mega scale testing is neither advisable or affordable, possible danger from such persons must be minimised by continued social distancing, personal hygiene, masks and protection of the elderly. Persons with co-morbidities should be shielded from close contact with mobile members of the family,” he said. Earlier the WHO had said that pre-symptomatic or asymptomatic transmission of the new coronavirus was “relatively rare”. However, new studies from Japan, Italy, South Korea and Washington state — have suggested that silent spreaders can be just as dangerous to a community.