WHO says 80% cases asymptomatic but mild; number varies in different countries
Are 80 per cent of novel coronavirus disease (COVID-19) positive cases actually asymptomatic — or those who carry the virus without showing any symptoms — specifically in India? The answer is no.
Eighty out of 100 confirmed cases were asymptomatic, R Gangakhedkar, the head of the epidemiology division at the Indian Council of Medical Research (IMCR), said at a press conference on April 20, 2020.
He did not, however, clarify that this was not a proportion specific to India.
It was a generic figure that the World Health Organization (WHO) had come up with. Data suggests 80 per cent of infections are mild or asymptomatic, while 15 per cent are severe infections requiring oxygen and five per cent are critical infections requiring ventilation, the WHO said on March 17.
Different countries with different scenarios show different proportions of asymptomatic cases. This does not mean that the figure mentioned is true for all countries at all times.
India, however, undertook no research to ascertain its own number of asymptomatic cases so far, Sartiha Nair, senior scientist and a spokesperson for ICMR, told Down To Earth on April 21.
Sixty-nine per cent of the confirmed cases during testing in India were found to be asymptomatic, said Gangakhedkar on April 21.
This was to suggest that a large number of cases may be treated without much medical intervention.
Scenario in different countries
A look at research undertaken in terms of asymptomatic cases in different countries reveals it can range from as low as five per cent to as high as 80 per cent.
The share was just five per cent for 262 patients transferred to a dedicated COVID-19 facility from different hospitals in Beijing on February 10, according to a study.
Another study from China, however, showed the proportion of asymptomatic cases that emerged after the lockdown was lifted could be as high as 78 per cent, according to the BMJ.
The number of asymptomatic cases in the early days of the disease outbreak in Italy was found to be only seven per cent.
Studies conducted later, however, said it could be as high as 40 per cent. It is, thus, important to look at not just a country, but also the date, time and settings under which calculations were done.
The number of asymptomatic cases on the Diamond Princess cruise ship increased as days progressed. Of the 634 confirmed cases, a total of 306 and 328 were reported to be symptomatic and asymptomatic, respectively.
A study on South Korea found three out of 28 patients who contracted the disease were asymptomatic, when the disease was beginning to spread in the country.
The proportion of asymptomatic cases was “not fully understood”, said the European Centre for Disease Prevention and Control (ECDC) on April 8.
Asymptomatic cases in the United States (US) may be 25 per cent, said Robert Redfield, the director of the country’s CDC, in an in interview. In a study published on coronavirus cases in children on April 6, the US CDC said the asymptomatic cases were about 1.3 per cent.
It would, thus, be difficult to arrive at any numbers regarding India, unless country-specific research was carried out.
Asymptomatic vs pre-symptomatic cases
There was also no consensus among researchers about the transmission potential of cases. The WHO consistently maintained that asymptomatic cases are not the driver of the current transmission. Its technical lead Maria Van Kerkhove said so at several press meets.
There were few reports of laboratory-confirmed cases who were truly asymptomatic, and to date, there was no documented asymptomatic transmission, said a WHO situation report released on April 2. “This does not exclude the possibility that it may occur,” it said.
The European CDC, however, hinted at a possibility of transmission from asymptomatic cases as well.
“No significant difference in viral load in asymptomatic and symptomatic patients has been reported, indicating the potential of virus transmission from asymptomatic patients,” it said, in its seventh update.
The chance of transmission potential of the disease increases with an increase in the viral load.
Pre-symptomatic transmission, on the other hand, is different from asymptomatic transmission, according to the WHO.
“The incubation period for COVID-19, which is the time between exposure to the virus (becoming infected) and symptom onset, is on average five to six days. It can, however, be up to 14 days,” the WHO said.
During this pre-symptomatic period, some infected people can be contagious, according to the WHO. Transmission from a pre-symptomatic case can occur before symptom onset, the WHO said.
An asymptomatic case, on the other hand, does not develop a symptom at all. The European CDC in its eighth update said that pre-symptomatic transmission contributed to 48 per cent and 62 per cent of transmissions in Singapore and China quoting a study.
Although transmission from asymptomatic patients has also been reported, the risk of transmission from pre-symptomatic or symptomatic patients is considered to be higher, it clarified.
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