According to the Union Health Ministry, 80% of the COVID-19 patients are either asymptomatic or exhibit mild symptoms.

A large percent of COVID-19 patients are asymptomatic Why this is concerningRepresentative Image
Delve Health Wednesday, April 22, 2020 - 18:05

It has been 83 days since India reported its first COVID-19 case in Kerala, on January 30. A lot has happened since — the number of cases crossed the 20,000-mark; testing strategies are slowly changing; flights and trains have been suspended; and a nationwide lockdown is in place.

While many states have managed to control the spread and are working on improving containment measures, recent findings have officials and the public equally concerned: a large percentage of asymptomatic individuals are testing positive for the novel coronavirus (SARS-CoV-2). The Union Health Ministry said 80% of the COVID-19 patients are either asymptomatic or have mild symptoms. According to Dr Raman Gangakhedkar, a senior scientist of the Indian Council of Medical Research (ICMR), 69% of those who tested postive for coronavirus are asymptomatic

“Some did not have a fever, no cough or no shortness of breath. They did not know they had coronavirus,” said Delhi CM Arvind Kejriwal as he announced that all 186 patients who tested positive for SARS-CoV-2 on Sunday had been asymptomatic, showing no symptoms of COVID-19. “This is more worrying,” he added.

Maharashtra, too, presented a concerning figure: 85% of 3,062 COVID-19 patients admitted in hospitals are asymptomatic and 65% of the total COVID-19 cases (4016) were asymptomatic as on Wednesday.

This has been the trend across the globe. The World Health Organisation (WHO) stated that 80% of COVID-19 infections are mild or asymptomatic, while 15% are severe infections.

Sample these:

  • When the outbreak was reported onboard the Diamond Princess cruise ship in early February, 712 (19.2%) of 3,711 passengers and crew were tested positive for coronavirus. Of this, 331 (46.5%) people were asymptomatic when tested.
  • At a hospital in New York, 15% of pregnant women tested positive for coronavirus, of which, 88% of them did not exhibit any symptoms of the disease, reported The Washington Post.
  • As part of his research, Sergio Romagnani, a professor of clinical immunology at the University of Florence, collected swab samples and tested all 3,000 inhabitants of Vò, a village 50 km west of Italy’s Venice. In an open letter to the authorities in Tuscany, he pointed out that between 50% and 75% were asymptomatic.

Why many are asymptomatic

In a span of just four months, scientists have discovered the new virus in the coronavirus family and have also made significant leeway in understanding it.

“So far, we know its genome sequence, the tests to find it and the receptor-binding domain (specific site on the human cell where the coronavirus attaches itself). But what we do not know yet is its reaction in the body,” says Pavithra Venkatagopalan, a Chennai-based microbiologist who did a PhD in virology and specialised in coronaviruses. 

According to Pavithra, coronaviruses usually cause mild infections. “Most of the symptoms are the body’s reaction to the infection. Maybe human bodies, which have stronger immune systems, are able to quickly produce antibodies and that is why the inflammatory reaction of the body is lesser. Severe cases occur when there is an overreaction of the immune system,” she explains.

According to experts, every viral infection has four levels of symptoms: no symptoms (asymptomatic), mild symptoms, severe symptoms and then fatality. 

Many believe that for COVID-19, a majority of individuals are more likely to be “pre-symptomatic”, wherein they may show very mild symptoms, such as a mild sore throat or an itchy throat, which is almost negligible in terms of how it physically manifests. Truly asymptomatic individuals are ones who do not exhibit any symptoms at all.

“There is speculation that most people who have been termed ‘asymptomatic’ may actually be pre-symptomatic,” states Dr Srinath Reddy, a public health expert.

High transmission rate

In his letter to Tuscany officials, Italian immunologist Sergio Romagnani underscored that the asymptomatic individuals are “a formidable source of infection.”

Dr T Jacob John, professor emeritus at Christian Medical College (CMC), Vellore, Tamil Nadu, agrees. “The asymptomatic individuals are the ones who are likely to infect others more readily than the symptomatic patients, who will be diagnosed quickly,” he said.

But how do asymptomatic individuals who do not exhibit any symptoms, including coughing or sneezing, transmit the disease?

“At the onset of the infection, the virus first settles in the throat and nasal cavities where it begins to multiply and spread. This leads to an increase in the viral load [the concentration of the virus found in the blood], but it might be localised to this particular region in the body. So while the person does not necessarily have symptoms of an active infection, the virus can still spread, at least in theory, via nasal and oral secretions,” explains Dr Deepak, a senior consultant pulmonologist from Rajiv Gandhi Institute of Chest Diseases in Bengaluru.

He also stressed that the viral load of an infected individual seems to peak right before they exhibit any symptoms. This means that the carriers of SARS-CoV-2 can still inadvertently spread the disease to a healthy person, even with no or very mild symptoms.

The virus could be transmitted while clearing the throat, saliva spray while talking, physical contact or when a healthy person touches an infected surface. That is why health experts insist on using face masks when stepping outside the house, washing hands thoroughly and maintaining a minimum distance of three to six feet from the nearest person.

In some cases, an asymptomatic person may continue to be a carrier of the virus even after the 14-day incubation period. For example, in Kerala’s Kannur district, the samples of many high-risk asymptomatic contacts, especially those who returned from Dubai and have been in home quarantine since their arrival, were collected after more than 20 days or towards the end of a 28-day quarantine period. A majority of the tests returned positive.

Many experts, however, do not have a scientific answer for why some incubation periods are exceeding the normal 14 days. 

“The incubation period of the virus is anywhere from two to 14 days. The WHO has stated that on average, most people begin to present with symptoms around the fifth day after being exposed to the virus, though this may not hold true for all. Some people begin to show symptoms around four to five days while others are showing symptoms later,” says Dr Deepak, adding, “There is still a lot of research going into understanding the variation.”

Why the sudden rise

India started universal screening of international passengers on March 6, although some airports had already begun thermal screening at the time. However, screening was largely symptom-based, where the temperature was recorded and health-related questions were asked. As a result, many asymptomatic individuals were undetected.

This has posed a risk to disease control, as the onset date of the symptoms, which is required for the epidemic curve (to track the onset of an outbreak), is not available or known.

The initial set of testing strategies focused only on symptomatic individuals with a travel history and their close contacts. Gradually, by April second week, many states started changing their testing strategies and conducting random tests.

Take the example of Kerala, which has been reporting an increase in COVID-19 cases over the last one week although it had managed to contain the spread. Many patients who are turning positive are asymptomatic despite being in home quarantine for more than 14 days.

“Many high-risk contacts, who had returned from Dubai and other countries, were instructed to be in home quarantine once they reached home from the airport. As Kerala recently changed its strategy again, where it has been decided to test asymptomatic persons as well, we decided to test some of these high-risk contacts who have been in quarantine,” Kannur District Medical Officer K Narayanan Nayak tells TNM.

This is something that experts across the globe have been insisting — that randomised testing is the only effective containment tool at this point to keep a check on community spread.

“Asymptomatic cases will remain hidden unless you test them for different purposes, like contact tracing and contact history,” says Dr Jacob John, who also used to head the Centre for Advanced Research in Virology of ICMR.

He further explains the difference between testing a symptomatic patient and the public. “Normally, sick people, with respiratory illness, shortness of breath or loss of smell, will be tested for diagnostic purposes. That is in the healthcare context, carried out for the benefit of these patients and the health workers treating them. But testing the contacts of an infected person is in the public welfare context. Public testing methodology is the only way to know who may or may not be infected, to identify and quarantine such primary and secondary contacts, and to ensure healthy persons are not infected,” says Dr Jacob.

States like Tamil Nadu, Karnataka and Andhra Pradesh have already started randomly testing individuals, especially since it has many containment zones or hotspots. While Tamil Nadu is expected to see a spike in COVID-19 cases in the next two weeks, Karnataka, which has increased its testing, has detected about 60% asymptomatic individuals.

According to microbiologist Dr Navin Dang, reverse transcription-polymerase chain reaction (RT-PCR) is the only way to test and identify such individuals. “It is also important to motivate people to take these tests so that officials can find if they have the viral load, have any symptoms and find their potential contacts if they have no or mild symptoms,” he said. 

While symptomatic individuals are treated based on their symptoms, asymptomatic individuals who have been tested positive are advised to remain in isolation.