Only seven per cent of coronavirus samples have a “high viral load” — meaning these people would, on average, transmit the infection to 6.25 other people. The vast majority (84%) have a low infectiousness, transmitting to only 0.8 persons on average. Nine per cent have a moderate viral load.
Viral load refers to the quantity of a virus in an organism and is a reflection of how quickly the virus is replicating.
These are new findings by ICMR’s National Institute of Occupational Health (NIOH), Ahmedabad, based on a study of over 1,000 samples collected in the state in April and May. The results will be submitted to ICMR’s Indian Journal of Medical Research (IJMR) this week and underline the need to include viral load figures in Covid test reports.
This assumes significance given that many who test positive are asymptomatic.
Said a top official of the institute: “With the burden we are seeing in Gujarat, we should test for viral load and quarantine the ‘super-spreaders’. We have observed that viral load is a very strong determining factor. Few people spread most infections and most people spread very few infections — this has been observed in our study.”
One indicator of viral load is “Ct value” or cycle threshold.
RT-PCR tests for Covid use fluorescent signals to detect nucleic acids. Ct value is the number of cycles required in the machine for the fluorescent signalling to cross the base level of the machine. A low value, therefore, means a high viral load and vice versa.
“RT-PCR results as of now give positive and negatives but we also should report the Ct (cycle threshold) value. With that value, we can identify high, moderate, or low viral load. It is automatically given in the RT-PCR test anyway, but we need to change our testing policy so this value is also collected by the testing labs.”
An official told The Indian Express that ICMR is seeing Ct values between 23 to 40 for Covid testing. Below 35 is positive, above 35 negative, and 35 would require a repeat testing.
Some experts have cautioned that irregularities in swab-taking could muddle viral load results. However, with standardised swab practices, the reporting would allow for a “dose-response curve” for the infection, which could map viral quantities with illness severity and infectiousness.
NIOH researchers did not integrate clinical data to observe correlations between viral loads and age, underlying conditions, or severity of illness. However, the official said that people who are more immuno-compromised and less resistant to the virus could have higher viral loads.
The official also said that research trends outside India show a correlation between viral load and severity of illness. One example of this early research is a Lancet paperin March from China that found severe patients have viral loads that were sixty times as much as mild forms of the virus.
Said the official: “For example, why are we seeing doctors getting infected at such rates? They are probably more exposed to patients with high viral loads.”
The research is still contested. A pre-print article from Lombardy in March as well as a New England Journal of Medicine article in March found no difference in viral load between symptomatic and asymptomatic patients. This would signal similar transmissibility between asymptomatic and symptomatic people.
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