Why is the number of asymptomatic COVID-19 cases rising in India?

[Wrichik Basu]

Local and national authorities in India have stated that there is a rise in the number of asymptomatic COVID-19 cases. Around 80% of the cases are asymptomatic according to the Indian Council of Medical Research (ICMR). A news article mentioning the same can be found here:

https://www.ndtv.com/india-news/cor...dical-research-body-icmrs-scientist-t-2214799

Without symptoms, detection would become difficult, no question about that. But why is the number of asymptomatic cases rising? Has the virus changed its RNA which is suppressing the symptoms till the infection becomes severe and the patient faces a difficulty in breathing? Or is it the immune system of the victim that suppresses the symptoms?

Or have these detections been made so early that the virus didn't get time to multiply enough so as to show symptoms?

 

[atyy]

It is hard to say, unless asymptomatic is defined. Some of these people may have been tested before they show symptoms. Or it could be that because the illness is very mild in many people, they may not think they are ill.

There are some people who seem to be asymptomatic by a quite strict definition (ie. no symptoms noticeable by oneself, even if one is alert for even mild symptoms). The younger one is, the more likely one falls in this category. India has fairly young population compared to most parts of the world, so there may be more of these in India.

 

[jim mcnamara]

Typically age class distributions are considered a good way to see what @atyy is referring to:

From indexmundi

Compare this to Italy:

 

[Ygggdrasil]

Here's a nice summary from Scripps collecting data from a variety of studies that tested a large population for the virus, summarizing the number of positive but asymptomatic individuals found:

In the studies that we have summarized in the table, the range of infection rates is wide: from 0.76% for residents of Iceland to 36% for residents of a Boston homeless shelter. It is striking, however, that the proportion of individuals who test positive for SARS-CoV-2, but who have no symptoms of COVID-19, remains consistently high: from approximately 31% to 88%, with a mean of 56%. Because of various limitations in the summarized studies, this likely overstates the overall population mean, which some observers have suggested is around 40%.

It should be noted that the summarized studies are essentially convenience samples. They do not purport to depict anything more than the circumscribed populations from which data were collected. Large, well-designed studies with representative samples are desperately needed to accurately assess the prevalence of those who are infected with SARS-CoV-2, yet are asymptomatic for COVID-19—and to determine their impact on the pandemic.

https://www.scripps.edu/science-and...te/about/news/sarc-cov-2-infection/index.html

In some of these cases, it is possible that the positive tests could be pre-symptomatic individuals (because the average incubation for the virus is ~5 days from initial infection with the virus to showing symptoms, many individuals who test positive and are asymptomatic may later show symptoms).

 

[atyy]

https://www.scripps.edu/science-and...te/about/news/sarc-cov-2-infection/index.html

In some of these cases, it is possible that the positive tests could be pre-symptomatic individuals (because the average incubation for the virus is ~5 days from initial infection with the virus to showing symptoms, many individuals who test positive and are asymptomatic may later show symptoms).

In the other thread, you posted a critique of the Santa Clara serological study, in which one concern was the false positive rate. The tests in the Scripps summary are PCR tests, which should have a lower false positive rate - but is the false positive rate (or specificity) for the PCR test known, and has it been calculated to have a negligible effect on the results of the estimation of the asymptomatic ratio?

 

[Ygggdrasil]

In the other thread, you posted a critique of the Santa Clara serological study, in which one concern was the false positive rate. The tests in the Scripps summary are PCR tests, which should have a lower false positive rate - but is the false positive rate (or specificity) for the PCR test known, and has it been calculated to have a negligible effect on the results of the estimation of the asymptomatic ratio?

That is a very good point. Presumably the raw counts presented have not been corrected for potential false positives. PCR tests have a much higher specificity than antibody tests, though there is still some rate of false positives. In many of the studies (e.g. the Iceland study where 0.3% of those tested fall into the positive and asymptomatic bin), false positives could be contributing quite significantly to the count of asymptomatic cases.

 

[Laroxe]

We have to consider that all infectious diseases have an associated level of people who remain asymptomatic and there are lots of things which effect this "asymptomatic fraction." In the 2014 influenza season it was estimated only 23% of those infected became symptomatic in 2016 the estimate was around 80% became symptomatic. Unlike flu, in Covid 19 we only have a few months of data and little idea about the factors having the biggest influence on disease progression, so I'd be wary of jumping ahead to much.