Transmission of Covid virus is quite complicated because of the large numbers of Presymptomatic, Asymptomatic, and Mildly symptomatic (PAMS) patients. S a team of researchers did a study of viral RNA load and infectivity in cell culture.
Approximately 400,000 individuals, mostly from Berlin, were tested from February 2020 to March 2021 and about 6% tested positive. Of the 25,381 positive subjects, about 8% showed very high viral loads. People became infectious within 2 days of infection, and in hospitalized individuals, about 4 days elapsed from the start of virus shedding to the time of peak viral load, which occurred 1 to 3 days before the onset of symptoms. Overall, viral load was highly variable, but was about 10-fold higher in persons infected with the B.1.1.7 variant. Children had slightly lower viral loads than adults, although this difference may not be clinically significant.
The results show that PAMS subjects have viral RNA loads (pictured) and predicted infectiousness, at the first positive test, slightly less than those of hospitalized patients. Children had similar viral RNA loads and predicted cell culture infectivity.
A small fraction of individuals’ first-positive viral RNA loads – 8%- had 10^9 RNA copies per swab. This observation is in line with previous findings that 15% of index cases are responsible for 80% of transmission, and that 8-9% of infected individuals carry 90% of the total viral load.
Time-course analysis revealed that viral RNA load peaks 1 to 3 days before onset of symptoms. From this peak, viral RNA loads decline 0.17 log10 units per day. Previous studies have shown that infectious virus is typically not recovered in cell culture beyond 10 days from symptom onset.
This study also examined viral RNA loads in 1533 patients with alpha variant infections. Compared with non-alpha infections, patients infected with the alpha variant had 10 times higher viral RNA loads and a 2.6 fold higher estimated cell culture infectivity. However, the impact of an increase in viral RNA load is, as the authors write, ‘dependent on context’ and the increase in cell culture infection probability is a ‘proxy indication of potentially higher transmissibility’. Complicating this analysis is the possibility that the correlation of viral RNA loads with cell culture infectivity might differ among variants.
The most important outcome of this study is that PAMS subjects – who in this study were tested at walk-in centers – are as infectious as hospitalized patients. As these individuals circulate in the community they can clearly trigger outbreaks of infection.
The B.1.1.7 is only about 40% to 50% infectious as the wild type... Just imagine what the Delta is with 1250 times infectiousness.
Approximately 400,000 individuals, mostly from Berlin, were tested from February 2020 to March 2021 and about 6% tested positive. Of the 25,381 positive subjects, about 8% showed very high viral loads. People became infectious within 2 days of infection, and in hospitalized individuals, about 4 days elapsed from the start of virus shedding to the time of peak viral load, which occurred 1 to 3 days before the onset of symptoms. Overall, viral load was highly variable, but was about 10-fold higher in persons infected with the B.1.1.7 variant. Children had slightly lower viral loads than adults, although this difference may not be clinically significant.
The results show that PAMS subjects have viral RNA loads (pictured) and predicted infectiousness, at the first positive test, slightly less than those of hospitalized patients. Children had similar viral RNA loads and predicted cell culture infectivity.
A small fraction of individuals’ first-positive viral RNA loads – 8%- had 10^9 RNA copies per swab. This observation is in line with previous findings that 15% of index cases are responsible for 80% of transmission, and that 8-9% of infected individuals carry 90% of the total viral load.
Time-course analysis revealed that viral RNA load peaks 1 to 3 days before onset of symptoms. From this peak, viral RNA loads decline 0.17 log10 units per day. Previous studies have shown that infectious virus is typically not recovered in cell culture beyond 10 days from symptom onset.
This study also examined viral RNA loads in 1533 patients with alpha variant infections. Compared with non-alpha infections, patients infected with the alpha variant had 10 times higher viral RNA loads and a 2.6 fold higher estimated cell culture infectivity. However, the impact of an increase in viral RNA load is, as the authors write, ‘dependent on context’ and the increase in cell culture infection probability is a ‘proxy indication of potentially higher transmissibility’. Complicating this analysis is the possibility that the correlation of viral RNA loads with cell culture infectivity might differ among variants.
The most important outcome of this study is that PAMS subjects – who in this study were tested at walk-in centers – are as infectious as hospitalized patients. As these individuals circulate in the community they can clearly trigger outbreaks of infection.
