“Hypothetical case based on some reality:” you’re doing OK in relative isolation, hardly ever go out except for essentials, but you know the household is still vulnerable. Then someone ,”C,” a person who does visit you, has gone back to work, wearing a mask; you know and trust “C” is mindful of hand hygiene.
But “B,” a co-worker of “C”, has also been back to work, and after a week finds out that “A”, a family member, has now tested COVID-19 positive. That co-worker and that family member had worked together on a home project over the previous week, fitting the CDC definition of close contact (15 minutes within 6 ft; also presumably face to face, with no masks, and “A” might have had mild symptoms).
So “C” had been with the co-worker over the week, and both were asymptomatic. Late in the week, before the news about the positive test, another someone, “D” goes to stay at “C” ‘s home for a few days, overnight. After that stay, “C” the potential 1st visitor couldn’t make a visit, but “D,” the 2nd visitor, came to your vulnerable household, “E”, wearing a mask, but at fairly close quarters.
If this were a chain, the most vulnerable would be the fifth link: 1) “A”, the COVID positive person, basically asymptomatic when exposing –>2) “B”, the co-worker, with possible asymptomatic exposure of–>3) “C”, the 1st visitor, with the possible asymptomatic exposure of–>4) “D”, the 2nd visitor, with possible asymptomatic exposure of–>5) “E,” the vulnerable CareGivingOldGuy household.
Sorry for the bad graphic and sketch below, but blue (purple?) is supposed to be the asymptomatic/pre-symptomatic period, possibly infectious, while green represents the start of symptoms. A test might be done then, with early results by then end of the bar. Basically, it’s trying to show the overlap of asymptomatic transmission [yeah, I’m bad at spreadsheet charts and graphic editors!]
Worst case scenario: the virus has 100% attack rate even after slightest exposure; all five levels get the damn thing. But the attack rate is not that high. Attack rates reported seem to range from ~ 80% (that choir practice in nearby Skagit County) to ~50% in a church in Arkansas (both in CDC/MMWR) to ~15% in Chinese households, to even lower in Taiwanese households (both in Lancet, I think).
So……what facts pertain in public messages? From the CDC (June 30, 2020):
“…The incubation period for COVID-19 is thought to extend to 14 days, with a median time of 4-5 days from exposure to symptoms onset. One study reported that 97.5% of persons with COVID-19 who develop symptoms will do so within 11.5 days of SARS-CoV-2 infection….
…Increasing numbers of epidemiologic studies have documented SARS-CoV-2 transmission during the pre-symptomatic incubation period,…The proportion of SARS-CoV-2 transmission due to asymptomatic or pre-symptomatic infection compared to symptomatic infection is unclear…”
For close exposures: “…It is important to remember that anyone who has close contact with someone with COVID-19 should stay home for 14 days after exposure based on the time it takes to develop illness….”
“Close contact” definition: “Someone who was within 6 feet of an infected person for at least 15 minutes starting from 2 days before illness onset (or, for asymptomatic patients, 2 days prior to specimen collection) until the time the patient is isolated….” Also, CDC on contact tracing click here.
image from pixabay
