Wapo: The mystery of asymptomatic coronavirus cases.
At first it was thought that asymptomtic people who tested positive for SARS-CoV-2 were generally in the incubation phase, and would develop symptoms later. Now it is assumed that atleast 40% of people infected never develop symptoms. The guess that it was only the young and health who remained asymptomatic, was questioned early on, when half the passengers on the Diamond Princess did not develop symptoms. The results from individual clusters are baffling (see table below).
Forty percent of people with coronavirus infections have no symptoms. Might they be the key to ending the pandemic?
... The theory that has generated the most excitement in recent weeks is that some people walking among us might already have partial immunity.
When SARS-CoV-2 was first identified on Dec. 31, 2019, public health officials deemed it a “novel” virus because it was the first time it had been seen in humans who presumably had no immunity from it whatsoever. There’s now some very early, tentative evidence suggesting that assumption might have been wrong.
One mind-blowing hypothesis — bolstered by a flurry of recent studies — is that a segment of the world’s population may have partial protection thanks to “memory” T cells, the part of our immune system trained to recognize specific invaders. This could originate from cross protection derived from standard childhood vaccinations. Or, as a paper published Tuesday in Science suggested, it could trace back to previous encounters with other coronaviruses, such as those that cause the common cold.
“This might potentially explain why some people seem to fend off the virus and may be less susceptible to becoming severely ill,” National Institutes of Health Director Francis Collins remarked in a blog post this past week.
... [One] group was researching blood from people who were recovering from coronavirus infections and wanted to compare that to samples from uninfected controls who were donors to a blood bank from 2015 to 2018. The researchers were floored to find that in 40 to 60 percent of the old samples, the T cells seemed to recognize SARS-CoV-2.
https://www.washingtonpost.com/health/2020/08/08/asymptomatic-coronavirus-covid/
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