The Hindu Explains | Asymptomatic COVID-19 carriers, status of a SARS-CoV-2 vaccine, and risks involving reopening of schools

Coronavirus | What role do people without symptoms play in spreading COVID-19?

What is the World Health Organisation’s assessment?

The story so far: On June 8, the technical lead of the World Health Organisation (WHO), on the COVID-19 pandemic, said transmission by people without symptoms is “very rare”. Following a firestorm of protests by health experts, WHO retracted the claim on June 9, calling it a “misunderstanding”. WHO has been engaged in a series of controversial statements on the role of asymptomatics — those who have been infected with the virus but do not display symptoms — in spreading SARS-CoV-2.

What did the World Health Organisation (WHO) say about the role of asymptomatics?

On Monday, June 8, at a virtual press conference, Dr. Maria van Kerkhove, WHO’s technical lead on the COVID-19 pandemic, said in response to a question by a journalist, that current evidence seemed to suggest that many countries monitoring asymptomatic cases and their contacts were not finding enough evidence to suggest that they were actually transmitting the disease. She said, “It’s very rare and much of that is not published in the literature... We are constantly looking at this data and we’re trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward... What we really want to be focused on is following [t]he symptomatic cases.” These comments provoked opposition from infectious disease experts and doctors — primarily on social media — arguing that Dr. Kerkhove may have downplayed the significance of this mode of transmission. Even more significantly, a recommendation to “focus on symptomatics” would mean ignoring infections by a category called “presymptomatics”. These are people who carry the virus and are asymptomatic, but over time show few or mild symptoms. The next day Dr. Kerkhove appeared to backtrack. “I used the phrase ‘very rare’ and I think that it’s a misunderstanding to state the asymptomatic transmission globally is very rare. What I was referring to was a subset of studies. I was also referring to some data that isn’t published,” she said, as quoted in The Guardian. However, she maintained that significant disease spread by asymptomatics was the result of “modelling studies”, meaning they were not based on real world observations or properly vetted by peer-reviewed studies.

Also read | 28% of 40,184 COVID-19 cases in India till April 30 asymptomatic, says study

Why do WHO’s observations matter?

Asymptomatics are those people who never show symptoms of the disease but test positive for the virus. WHO and infectious disease specialists assert that asymptomatic patients exist but it is not known how many there are and in what proportion. The current understanding of the disease is that people are most infectious when symptoms start to manifest, or are “presymptomatic”.

It is not clear if asymptomatics are much less likely than presymptomatics and symptomatics to transmit the disease.

WHO’s emphasis on symptomatics could imply that asymptomatics are “safe” and this undermines the widespread adoption of face masks and social distancing. WHO moved slow on the use of face masks and formally recommended it only after it was a widely accepted practice in several Asian countries. A potentially greater role of asymptomatics and presymptomatics means that isolating and quarantining ought to be more aggressive. However, when numbers are large, as in India, the vast majority are not likely to suffer from serious infection and the focus is on treating only the manifestly ill — as is now being followed in several States—and opening up restrictions on movement. This way, many experts say, it is more feasible to focus on symptomatics and their contacts.

Also read | India should focus on symptomatic patients, say experts

Do genes have a role in determining the degree of sickness?

This is still an open question and researchers across the world are comparing the genes of those with the disease and trying to find correlations with the severity of illness. Being a novel coronavirus, by definition, a large population is susceptible and no natural immunity prevails anywhere. Genetic factors are known to play a role in severe malaria. Chronic health conditions such as diabetes and hypertension increase susceptibility to severe COVID-19 disease. A recent study, that is still being peer reviewed, associated severe respiratory failure to genes that determine blood type. Blood type A was found to be more susceptible to respiratory disease than O. There were also other genes found that influenced how the immune system responded to the virus. A lot of focus in collaborative human genome studies is on understanding the role of certain genes in the Human Leukocyte Antigen complex, which is an important part of the immune system. The virus, as it is now understood, is able to spread easily because of a mutation in its spike protein; this helps it to lodge itself better in the lung cells. It is the variation in the HLA across populations that can reveal immune response.

But there is no genetic link yet on why some people are asymptomatic.

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Printable version | Jul 13, 2020 8:59:58 AM |

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