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Wednesday, June 23, 2021

Explained: How far can someone without symptoms spread the Covid-19 infection?

The signals so far are that some people spread COVID-19 before showing symptoms themselves. Can some also spread the disease without ever showing symptoms? A look at the emerging evidence.

Written by Abantika Ghosh | New Delhi |
Updated: April 24, 2020 10:47:11 pm
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Two recent figures given out on separate days by the Indian Council of Medical Research (ICMR) about people without symptoms testing positive for novel coronavirus disease (COVID-19) has created fear about the possibility of unsuspecting individuals suddenly falling very sick, and of their infecting others. A look at the evidence and the guidance on asymptomatic cases of COVID-19:

What has the ICMR said on this?

On April 20, Dr R R Gangakhedkar, head of epidemiology and infectious diseases at ICMR, said: “I have explained that 80% people will have infection without symptoms. They will be asymptomatic but if you test them, there is a chance that they will be positive… We need to understand that RT-PCR test comes positive only after a person is symptomatic. People take time to be symptomatic. This means if I test him when asymptomatic, the chances of him/her testing positive are small.” This, he said, is a limitation of the test: the closer one is to the point of infection — when the disease is contracted and not manifested — the slimmer are the chances of testing positive. That is also why, he said, India’s testing strategy, which banks heavily on whether a person is symptomatic (unless one is in a hotspot with a known history of contact with a confirmed case), does not need any immediate revision.

The following day, after this led to apprehensions, Dr Gangakhedkar clarified that in India, 69% of the people who had tested positive are asymptomatic. “What I said yesterday about 80% people is based just on one study,” he said.

What is the evidence on this?

It is now widely understood that around the world, the bulk of the people who get infected will not show symptoms, and will probably not even know that they have contracted the virus unless they are subjected to a serological test at some point and are found to have developed antibodies. Last week, a study from China concluded that 44% of the cases who had contracted the disease had caught it 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.

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“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% (95% confidence interval, 25-69%) 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,” reported the researchers, including from Guangzhou Medical University and the World Health Organization (WHO) Collaborating Centre for Infectious Disease Epidemiology and Control, at the University of Hong Kong. The study was published in Nature Medicine.

But what is “presymptomatic stage”?

The WHO recognises three stages of COVID-19 transmission — asymptomatic, presymptomatic and symptomatic. “Symptomatic transmission refers to transmission from a person while they are experiencing symptoms. Data from published epidemiology and virologic studies provide evidence that COVID -19 is primarily transmitted from symptomatic people to others who are in close contact through respiratory droplets, by direct contact with infected persons, or by contact with contaminated objects and surfaces,” the WHO says. The incubation period of the virus ranges between 5-14 days. Presymptomatic transmission is when a person spreads the disease before symptoms appear, and eventually develops symptoms themselves.

On asymptomatic transmission, the WHO says it “refers to transmission of the virus from a person, who does not develop symptoms. There are few reports of laboratory-confirmed cases who are truly asymptomatic, and to date, there has been no documented asymptomatic transmission. This does not exclude the possibility that it may occur. Asymptomatic cases have been reported as part of contact tracing efforts in some countries.”

The US Centers for Disease Control and Prevention (CDC) is clear that both presymptomatic and asymptomatic transmission of the virus are possible. “The onset and duration of viral shedding and the period of infectiousness for COVID-19 are not yet known. It is possible that SARS-CoV-2 RNA may be detectable in the upper or lower respiratory tract for weeks after illness onset, similar to infections with MERS-CoV and SARS-CoV. However, detection of viral RNA does not necessarily mean that infectious virus is present. There are reports of asymptomatic infections (detection of virus with no development of symptoms) and pre-symptomatic infections (detection of virus prior to development of symptoms) with SARS-CoV-2, but their role in transmission is not yet known. Based on existing literature, the incubation period (the time from exposure to development of symptoms) of SARS-CoV-2 and other coronaviruses (eg MERS-CoV, SARS-CoV) ranges from 2-14 days,” the CDC says.

People line up for food in North 24 Pargana, Barasat, during the nationwide lockdown (Express Photo: Shashi Ghosh)

How does one protect oneself from asymptomatic transmission?

There is no alternative to basic safety practices such as hand-washing, wearing a mask, and social distancing, said Dr Randeep Guleria, AIIMS Director and a respiratory medicine specialist. That is, till such time that a vaccine is found. “This is why universal precautions such as social distancing hand washing are so important. Maintaining distance is very important but stigma only ends up increasing mortality and morbidity. You have to understand that many people, especially given how young our country is, would go through a bit of fever and body ache and not even know that it is COVID.”

The question of transmission from an individual who does not show symptoms is also important — while evidence on asymptomatic individuals spreading the disease may be divided, it is more or less accepted that people can spread the disease before they show symptoms.

For how long can a person spread the disease after being infected by the virus?

The reply to this question is commonly assumed to be 14 days. However, as it is a new disease, knowledge about it is evolving. In Kerala recently, a 54-year-old woman tested positive for the virus almost a month after she arrived from abroad, and had been quarantined at home ever since. Health officials told The Indian Express that there have been at least a dozen cases so far of people testing positive after having been quarantined for 28 days. On Thursday, Telangana decided to increase the home quarantine period to 28 days, from 14 days. The Union Health Ministry has advocated from the very start that patients should be followed up, at least through community surveillance, for 28 days and not 14 days.

This is also in line with some of the global experiences. In a study last month in The Lancet Infectious Diseases, researchers from Imperial College London estimated that in China the “mean duration from onset of symptoms to death to be 17·8 days and to hospital discharge to be 24·7 days.” They also pointed out that all cases were hospitalised there, not so much because they needed hospital care, but to “prevent onward transmission”.

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