The second wave outbreak of the pandemic in Odisha is on the wane. The signs of high transmissibility of the virus strain in the State have turned weaker. In the last fortnight, the cycle threshold (Ct) value of Covid-19 positives in the State has been measured very higher now.
As per data provided by Institute of Life Sciences (ILS) Director, Ajay Parida, during the peak month of May, the Ct values measured in Covid-19 positives in the State have been very low at around 14-15. However, during the last fortnight, the Ct values measured in positive samples stood higher at around 29-30.
The rise in Ct values document the waning of an outbreak, says a recent study published in Clinical Infectious Diseases. Moreover, it claimed that higher Ct values (around 30) show the infectiousness of positive patient is on the wane.
What Is Ct Value?
When detecting SARS-CoV2, a real-time PCR (RT-PCR) test is conducted in a lab. The sample will be termed positive by accumulating the fluorescent signals. And the Ct (cycle threshold) is defined as the number of cycles required for the fluorescent signal to cross the threshold (i.e exceeds the background level).
The Ct levels are inversely proportional to the amount of target nucleic acid in the sample (ie the lower the Ct level the greater the amount of target nucleic acid (viral load) in the sample). The real-time test undergoes 40 cycles of amplification.
An RT-PCR test positive after 12 rounds of the cycle. Therefore, the Ct value will be then 12 for the sample. If the fluorescent signal is not detected till 35 rounds of amplification, then the sample is declared as negative.
Simply put, a sample has the highest viral load if its Ct value is 12, and the lowest viral load when the Ct value is around 30-34. The range of viral load given below:
- Ct value 12-21 - Viral load high
- Ct value 22-30 - Viral load moderate
- Ct value 31-35 - viral load low
As per a study published in Clinical Infectious Diseases, the infectiousness of a person depends on its viral load. When the viral load is low, then the chances of infecting another person appear negligible.
The data by ILS, thus, points to decreasing transmissibility of the virus in the State now.
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What Is Viral Load?
Viral load measures the number of viruses in a host cell. A load of virus increases gradually, after the first infection. The load increases in the body following replication of the virus. On day 1, the load may be above 30. But studies have confirmed that by day 7, the viral load has been measured at around 20.
Ct Locates What?
In order to detect the novel coronavirus, the World Health Organisation (WHO) recommends screening with an E gene followed by a confirmatory test using the RdRp (RNA-dependent RNA polymerase) gene to avoid cross-reaction with other endemic coronaviruses due to the genetic drift of SARS-CoV-2. RdRp is unique to SARS-CoV2. therefore, WHO specifies detection of RdRp to confirm Covid-19.
In every RT-PCR report, three Ct values are given:
- E gene - the outer or envelope protein of the virus.
- N gene - the spike protein of the nucleic acid
- RdRp - this is the gene that helps the lethal virus to replicate in the human body.
Low Ct Value Linked To High Severity?
According to ILS director, high viral load cannot be used to determine the severity of the disease. "The Ct value is not an absolute scale to measure severity. It is only indicative of the viral load in a sample," observed Dr Parida.
However, as per The Lancet Infectious Disease Study, the viral load might be a useful marker for assessing disease severity ( oxygen requirement and inflammation of human cells) and prognosis. The study by Lancet finds that high viral load is seen in patients having severe symptoms vis-a-vis ones having mild signs.
Though there is no uniformity of views among the scientist community over high load and severity, the more certain unanimity among them is - high viral load means high infectiousness.
The Bottom Line: Since recent data in Odisha has shown RTPCR samples having higher Ct values, the certain conclusion is the transmissibility is slowing down and the outbreak is on the wane in Odisha.