Medics arrange samples of a new fast test (Rapid test) for coronavirus at Rajiv Gandhi Government General Hospital during a nationwide lockdown on 18 April 18 | R Senthil Kumar | PTI
File photo | Medics arrange samples of a new fast test (Rapid test) for coronavirus at Rajiv Gandhi Government General Hospital in Chennai during a nationwide lockdown | R Senthil Kumar | PTI
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Several countries, including India, are reporting cases of people testing positive for Covid-19 but not showing symptoms. This has raised questions on the explanations and the implications of the novel coronavirus infection. The tests involved are both the antigen tests for viral presence in the body and the antibody tests for the immune response developed by an infected individual.

Was a test a false positive? Was the antigen test done early in the pre-symptomatic phase and later symptoms were not documented? Were minor symptoms overlooked? Or was the test result truly positive and the person did not develop any of the Covid-19 symptoms? What are the implications of a true positive or a false-positive test result, for the person tested and for others? How will this affect the prospects for herd immunity at the population level?


Also read: Why there is a big debate over accuracy and results of Covid anti-body tests


The case of false positives

Two case studies from the US, using two different types of testing, reveal similar observations on persons who were classified as test positive. The first was conducted in a homeless shelter in Boston. Of the397 people tested, 146 were positive for antigen presence. All of them were asymptomatic and remained so under observation.

The Santa Clara study in California invited volunteers to get tested for viral antibodies. The researchers estimated that the number of infected persons was likely to be 50 to 85-times more than previously reported cases. The symptomatic cases appeared to be merely the tip of the iceberg. Using uncertainty limits around the estimated point prevalence (‘the proportion of a population that has a characteristic at a specific point in time’), the authors concluded that between 2.5 per cent to 4.2 per cent of the population in Santa Clara had been infected by early April. A later study in Los Angeles County also suggested 28 to 55 times more cases identified by the antibody tests than revealed by earlier antigen tests.

Were the tests technically faulty? If that were the reason, false-negative tests would have been more likely. Was there a temporal disconnect between the time of testing and the time of symptoms? The Boston cohort was closely observed and later symptoms would have been recorded. The Santa Clara and Los Angeles volunteers would have had the infection a few weeks ago and symptoms, if any, would have been reported.

Were they false-positive results? The probability of a false positive test increases in persons who are asymptomatic and have no history of contact with a Covid-19 case, because of low prior probability of exposure. This is explained by the Bayesian conditional probability, which demonstrates that the post-test probability is a product of both the the pre-test probability and the test result, not just the test result. Those tested in the Boston shelter had possible contact with a few cases reported there earlier but the Santa Clara volunteers had no verifiable history. The RT-PCR (reverse transcription polymerase chain reaction) or the antigen test has a very high specificity for the viral antigen and false-positive tests are unlikely. The antibody tests have less certainty built into them, with a higher sensitivity but lower specificity than the RT-PCR kits. So, there is a higher risk of false-positive results when the rapid antibody test kits are used. That may translate to a large number of false positives in an asymptomatic population.

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Also read: China’s data on asymptomatic Covid cases shows most people never get sick


Rise of asymptomatic people in India

Many of these asymptomatic Covid test positive individuals, in India and abroad, are under 50 years of age. Their greater social mobility may have exposed them to the virus without their knowledge. Therefore, lack of contact history does not necessarily increase the probability of a false-positive result in them. So, the likelihood of a large number of persons in this group having actually had exposure to the virus remains high. It is best to assume so, for purposes of Covid-19 control. For the false positive-labelled individual, however, there is a misplaced sense of security in assuming acquired immunity.

What are the implications of these studies? It is good news that many infected younger adults are recovering without developing symptoms, perhaps because of greater fitness and better immune status. The infection fatality rate (IFR) in the overall population would be much lower than the case fatality rate (CFR) estimated from symptomatic and diagnosed cases. If the infection yields a strong immune response in such persons, they can become plasma donors to potentially save the lives of seriously ill patients.

At the population level, however, the challenge is to contain the danger of virus dissemination by the infected, unaware and freely mobile person. We cannot repetitively search and sieve the entire population to identify such persons through testing. So, the best way to limit spread is to practice social distancing and personal hygiene to prevent spread between the unaware virus hosts and the unprotected potential recipients.

India, then, has to dutifully ensure that the poor have housing and occupational environments where they too can achieve social distancing. A worrisome aspect is that, despite the manifold increase in numbers of the infected, obtained by combining the un-enrolled asymptomatic Covid graduates with the registered class of Covid cases, infected persons ranged only between 2-4 per cent of the population. Herd immunity goal of 60-70 per cent infectivity in the population appears far away. A vaccine is also unlikely to be at hand until at least 2021. Till either of these is available to handcuff the virus, we stay two metres apart and compulsively clean our hands.

The author is President, Public Health Foundation of India. Views are personal.

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6 Comments Share Your Views

6 COMMENTS

  1. Why doesnt anybody think of the possibility of ‘false positive’ cases? Knowing how clean our govt hosp labs are, cross contamination of samples is a definite possibility. And the inherent limits of a test can lead to significant number of false positives when the number of tests are high.
    Govt shd stop testing the asymptomatic irrespective of their contact history and shd simply resort to monitored self isolation of contacts.

  2. This is high on science and yet low on logic. When we have a very high asymptomatic population, it simply means that the virus is benign and incapable of causing disease and when this is the case then why worry and maintain unnatural and impracticable social distance. Have our scientists, doctors and health reporters and journalists all been brain washed into accepting something which is not the case as per their own statements and write ups and research. What is going on?

  3. Please note that those who are not symptomatic (i.e. asymptomatic) will carry the infection, infect others and the infection may surface in them later. This is similar to when the Chicken Pox attack becomes asymptomatic, the virus does not die but remains dormant and surfaces later in old age, or when high stress or other complications such as diabetes, psoriasis or urticaria prompt its resurfacing as “shingles”

    Asymptomatic carriers are part of the bio engineering carried out on its bio weapon COVID 19 by China to make it more difficult to deal with its rapid spread and tenacity.

    In short, the only possible solution to COVID 19 is for India to obtain 1.5 Billion dozes of COVID 19 vaccine, and administer it to 1.5 Billion humans resident in India in very quick time and not allow anybody to enter India without a COVID 19 vaccine certificate like a Yellow Fever certificate. Certainly not by Modi leading the Nation to clap, bang pots and pans, or light lamps to frighten away the Koran Virus or “arresting” it by imposing a Police State and sentencing everybody to either house imprisonment or long, sometimes 1000 KM marches without food, water or shelter, to die on the way

    Part of the Chinese design also relies on the inability of target countries to implement such a solution to avoid debilitation as the disease spreads rapidly and entrenches itself in the population.

  4. Asymptomatic people would represent such people who have sufficient resistance in their body against infection and would not succumb to the virus. Should that be a worrying factor? Awaiting herd immunity is dangerous. Look where it has taken the UK and US with 20K and 50K deaths. Lakhs have to be infected and survive it, thousands have to die. What is the guarantee that a person who has survived one round of infectin won’t be infected again?

  5. Why is asymptomatic cases are worrying ? If the Indian population is 100% infected and are asymptomatic. Would that matter ?

  6. The author is prescribing an idealist environment when he says : “ India, then has to dutifully ensure that the poor have housing and occupational environments where they too can achieve social distancing. “. This is an impossible pre-condition which cannot exist in reality. Where do we accommodate the lakhs of poor from Dharavi slums.? And if we had such accommodation had these slums existed in the first place ? Similar situation exists in all Metro and big cities in India. In fact, you will not find a city without a slum. When we are fighting a war, there is no point in complaining about what we cannot have. As 3rd May approaches, it is time we re-evaluate what we have achieved and it is time to re-strategies. The lockdown theory too needs re-evaluation, as our options close fast. Reopening of economy will become inevitable and mandatory , no matter what is the situation regarding flattening of the Covid-19 curve. At some point, India will have to take the plunge. Lockdown cannot be prolonged indefinitely. Reducing infection rate and achieving flattening of the curve seems to be far away. What do we do now? This is a million dollar question, that defies an easy answer. Perhaps we should focus more on controlling deaths rather than striving to reduce infections. These means identifying the vulnerables amongst the infected and treat them well.

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