One caveat re: duration of infectiousness is that the cell culture studies almost exclusively looked at naso/oropharyngeal specimens. 2/n
The question is, is cell culture of naso/oropharyngeal specimens an adequate surrogate measure for infectiousness? 3/n
Some studies have shown that viral loads may be higher and slower to decline in sputum, suggesting greater and potentially more prolonged infectiousness: 4/n
A greater proportion of sputum samples yielded positive cultures as compared with swabs, but no cultures from any location were positive beyond day 8: 6/n
And all positive cultures had high viral loads: 7/n
(orange=sputum, yellow=swab, grey=stool).
However, there were only 5 sputum cultures performed on samples beyond day 8 from an unclear number of patients (though there are 16 orange dots, so probably 5): 8/n
(orange=sputum, yellow=swab, grey=stool).
And the study included only 2 patients who showed signs of lung infection, in whom:
“sputum viral loads showed a late and high peak around day 10 or 11, whereas sputum viral loads were on the decline by this time in all other patients.”
When they were cultured isn't clear. 9/n
A <5% isolation success from cell culture was predicted >9.78 days or <5.4 log10 (~250k) RNA copies/mL, but 95% confidence intervals were wide: 10/n
Whether these small numbers of patients and sputum cell cultures beyond day 8 are representative is unclear and I’m not sure we can be confident about the tail end of the infectious period based on the information currently available from cell culture studies. 11/n
On the other hand, the authors of the Lancet Microbe SR&MA reference two contact tracing studies that also point to early transmission, one of which addresses the tail of the infectious period: 12/n
100 index cases in Taiwan led to 22 secondary cases/2761 contacts; no cases were identified in contacts whose exposure started more than 5 days following symptom onset in the index case (0/852 contacts). 13/n
I wish I had a bigger follow to promote this paper because I think this kind of research is so important. I just joined twitter, so I don't, but I'll try to do my part anyway. Have a look at: medrxiv.org/content/10.110…
They found that HCWs were identified as cases of COVID at a rate 5.5x nonHCWs (range 2-6 mirroring the epidemic curve), and that 9.8% of HCWs likely transmitted to a household contact. Lots of other stuff in here but these were the findings I found most interesting.
I don't think the available data allowed them to adjust for a differential rate of testing, which was almost certainly higher among HCWs, though I doubt 5.5x higher. On the other hand, the denominator used to calculate rates of identified cases in HCWs was probably over-inclusive