On Monday, the World Health Organization’s Covid-19 technical lead Maria Van Kerkhove claimed that asymptomatic transmission of the disease is “extremely rare.” After queries poured in from around the world, the WHO called a social media interaction in which Van Kerkhove clarified that asymptomatic transmission does happen and that, according to some modelling studies, the incidence of such transmission could be as high as 40%.
A look at why the question of asymptomatic transmission is an important factor in the Covid-19 pandemic, and where the evidence stands.
What is asymptomatic transmission?
Asymptomatic transmission is when a person who has no symptoms of Covid-19 — such as fever, body ache, cough etc — transmits the novel coronavirus to another person.
This is important because of the high infectiousness of the virus. If indeed asymptomatic transmission was rare as the WHO had initially claimed, the need for universal use of masks would reduce. In such a scenario, only people who showed symptoms would need to wear face covers to prevent onward transmission of the virus. Also, containing the virus would be easier if every person who showed symptoms simply self-isolated.
Estimates vary but, by all accounts, it does take place. A study from China published in Nature Medicine on April 15 estimated that 44% cases who had contracted the disease had caught the disease from a person who was not showing symptoms.
“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%… 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 researchers including from Guangzhou Medical University and the WHO Collaborating Centre for Infectious Disease Epidemiology and Control, at the University of Hong Kong. The study looked at 94 Covid-19 patients admitted to Guangzhou Eighth People’s Hospital.
In the second WHO interaction, Van Kerkhove said there had been a “misunderstanding” about her statement on asymptomatic transmission. She revised her position and said: “There have been two-three studies that followed asymptomatic cases over time and looked at all contacts and came to the conclusion that there was no spread but that is a very small subset of studies. In trying to articulate what we know, I used the words ‘very rare’ and there was a misunderstanding… what I didn’t report yesterday because this is a major unknown is that there are some modelling studies that estimate that 40% transmission may be due to asymptomatic models. It is a modelling study so I didn’t include in my answer yesterday.”
However, she said, it is an open question as to what percentage of people who don’t have symptoms transmit the disease. She quoted a figure of 6-41%.
At how much has India assessed asymptomatic transmission?
The Indian Council of Medical Research (ICMR) has given widely varying figures for the extent of asymptomatic cases in the country. While the head of infectious diseases at ICMR, Dr R R Gangakhedkar, had said in April that 69% of all Covid-19 patients in India are asymptomatic, a study by ICMR of all positive cases till April 30 returned a figure of 28% asymptomatic patients in a total of 40,184. This latter figure is more in line with the WHO estimates.
In general, asymptomatic patients of Covid-19 have been observed to be younger and without comorbidities.
How can an asymptomatic person, who is not coughing and sneezing, transmit a disease that spreads through droplets?
The coronavirus SARS-CoV-2 resides in the upper respiratory tract. That is what makes it such a potent traveller through droplets from the human body to a surface and then onwards to its next victim.
“Normally for symptomatic people therefore the transmission mode is obvious — coughing or sneezing. But for asymptomatic it can happen when you are singing, or in the gym breathing heavily or in a night club where you are shouting to be heard by somebody who is standing very close to you. Basically in any situation when you express air under pressure, droplet transmission can happen,” Dr Michael Ryan, Executive Director, WHO Health Emergencies Programme, said in the interaction.
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If asymptomatic COVID cases spread the disease, how does one stay safe?
Universal use of masks is a good place to start. This can prevent inadvertent spread by an asymptomatic person who is not aware of his/her positive status. It is also important to maintain a distance of at least one metre from any individual, apart from handwashing and maintenance of personal hygiene. At home, it is best to wear a mask, especially if there are elderly people in the house. but if that is not possible, it is advisable to cover one’s mouth when coughing or sneezing or to do so into one’s shirt sleeve. WHO recommends fabric masks for people who are living in areas of active transmission and cannot practise physical distancing, such as in public transport or in closed settings.
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