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Silent spreaders make Covid a bigger challenge: Experts
Asymptomatic infections are higher than symptomatic infections in several states, including Karnataka and Assam, according to state government officials familiar with the matter.
Delhi chief minister Arvind Kejriwal said on Sunday that all 186 people who were diagnosed with the coronavirus disease (Covid-19) the previous day had exhibited no symptoms of illness, reinforcing evidence from other states that asymptomatic “silent spreaders” are unknowingly infecting others.
Asymptomatic infections are higher than symptomatic infections in several states, including Karnataka and Assam, according to state government officials familiar with the matter. At the INS Angre station in Mumbai, 66% of the Indian Navy personnel diagnosed with Covid-19 had no cough, fever or any other sign of the illness, according to Western Naval Command.
Experts say that the expansion of testing from only symptomatic contacts of a confirmed Covid-19 patient to testing everyone who could have been exposed to the infection, irrespective of symptoms, is leading to more asymptomatic cases being detected n India.
“People with no symptoms are either asymptomatic or pre-symptomatic. Some develop symptoms some days or a week later, but they begin shedding the virus and infecting others even before they develop symptoms. Young, healthy people are more likely to have a milder disease than older people and those with chronic illnesses,” said Dr N K Ganguly, former director general of the Indian Council of Medical Research (ICMR).
Chronic diseases such as diabetes, hypertension and lung problems, and diseases of the heart and kidney, among others, exacerbate illness and raise the risk of complications and death.
According to a study published in Nature Medicine on April 15, around 44% people with Covid-19 got infected by people who had no symptoms, with coronavirus-positive people being the most infectious two or three days before the symptoms appear.
Another study from Asia published in March had found that asymptomatic and pre-symptomatic transmission accounted for 48% of the cases in Singapore and 62% in Tianjin, China.
“Asymptomatic transmission has made containment of this pandemic such a challenge. Unlike Sars-CoV, where disease transmission started after people developed symptoms, around 40% of Covid-19 infection is spread by people before their symptoms appear. Contact tracing and quarantining people will have to continue for several months to break transmission,” said Dr Manoj Murhekar, director of the ICMR-National Institute of Epidemiology, Chennai.
According to a study published in the journal Science on March 16, undetected cases of people with “mild, limited or no symptoms” were responsible for 79% of Covid-19 transmission before the lockdown in China, because infected and contagious people continued to congregate or travel.
“This is why contact tracing is so important, where every contact of a known case is traced and tested irrespective of symptoms. Social isolation will have to be sustained for another one to two years to reduce the multiplier effect. A recent study from Stanford University has found many people who have mild or no disease remain undiagnosed but continue to infect others,” said Dr Ganguly.
A modelling at Stanford University that tested 3,300 people in Santa Clara County in California for antibodies against Covid-19 to detect past infection found actual infections outnumbered confirmed cases by a factor of 50-85.
“These prevalence estimates represent a range between 48,000 and 81,000 people infected in Santa Clara County by early April, 50-85 times more than the number of confirmed cases,” said researchers in the yet-to-be peer-reviewed study published on April 17 in the preprint server medRxiv.
Younger people who are healthy are more likely to stay asymptomatic, found a study in Nanjing in the Jiangsu Province of China, which followed 24 people who tested positive but didn’t show symptoms. In the one to three weeks after diagnosis, the seven who continued showing no symptoms had a median age of 14 years , according to the study published in Science China Life Sciences, which is the journal of the Chinese Academy of Sciences.
As India widens rapid antibody testing for community surveillance and tests everyone in Covid-19 clusters and hot spots, the number of asymptomatic cases will grow, experts say. “Now that states are increasing rapid antibody testing, those who have been infected without knowing it because they had mild disease or did not get tested will be diagnosed, which will help us better understand if there has been a sub-clinical spread of the disease,” said a former health ministry official who asked not to be named.
“Along with rapid testing, we must ramp up polymerase chain reaction (PCR) tests to detect active infection and do it at the local level to speed up results, so that there is no lag in the containment response,” she said.
Doctors say that until a treatment or vaccine is developed against Covid-19, the only protection against silent spreaders is wearing masks, social distancing and hand-washing. “Anyone can be a pre-symptomatic or asymptomatic carrier without knowing, so personal protection and social distancing must become the new norm,” said Dr Murhekar.
Covid-19 transmission can be stopped only when cases are brought down to below one per million population, said Dr Ganguly.
“To detect community transmission, we need to scale up testing, continue testing all severe acute respiratory testing for surveillance, and trace, test and quarantine all contacts of laboratory-confirmed cases, irrespective of whether they develop symptoms or not,” said Dr Ganguly.
