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Science: New research shows that SARS-CoV-2 infection levels are roughly the same on all age groups
Sicence: SARS-CoV-2 infection levels on all age groups are roughly same. It started as a preliminary analysis of routine laboratory data, and later developed into the largest ever study of the viral load levels of SARS-CoV-2 patients.
In a new study, researchers from the Charlotte Medical University in Berlin, Germany analyzed PCR samples from more than 25,000 COVID-19 patients. They determined the viral load of each sample and used their results to estimate the level of infectivity.
This provides a clear concept of disease infectivity under different age groups and different severity of diseases. It also provides new insights into the SARS-CoV-2 virus variant B.1.1.7. The relevant research results were published online in the Science Journal on May 25, 2021, with the title of the paper “Estimating infectiousness throughout SARS-CoV-2 infection course”.
According to the number of reproduction (R0), a person infected with SARS-CoV-2 will spread the virus to three to five other people on average. Although this is a useful indicator in an epidemiological setting, R0 is not suitable for estimating the risk of transmission at the individual or group level. Once normal social and environmental factors are eliminated, the infectivity of individuals and the length of time they take the initiative to spread the virus will be significantly different.
To better understand and estimate the infectivity of specific groups, these authors analyzed PCR samples from more than 25,000 COVID-19 cases to determine the viral load of these samples. The viral load of each sample—the total number of copies of the SARS-CoV-2 genome contained in the sample—provides a rough estimate of the number of viruses present in the patient’s throat, and is therefore useful for estimating personal infectivity index.
To further improve their estimates, they also applied the results of studies on the minimum viral load threshold usually required to successfully isolate SARS-CoV-2 (isolation indicating the presence of an infectious virus) in cell culture. Among the cases studied, more than 4,300 cases provided continuous samples. Using these samples to track viral load data in the throat, they were able to model typical changes in viral load during infection. They then looked for obvious differences in their data, especially those related to different age groups, disease severity, and virus variants.
There is no significant difference in the viral load levels of SARS-CoV-2 positive individuals between the ages of 20 and 65: the average throat swab sample contains approximately 2.5 million copies of the SARS-CoV-2 genome. The lowest viral load was found in very young children (0 to 5 years old). Viral load levels start with approximately 800,000 copies of the viral genome, increase with age, and approach adult levels in older children and adolescents.
These authors pointed out that although these numbers may look very different at first glance, the key thing to remember is that the viral load results are displayed on a logarithmic scale; in fact, the difference in viral load found in the youngest children is only lower than theirs. Generally considered clinically relevant thresholds. It is essential that you also understand how they arrived at these values and take them into account when interpreting them.
While emphasizing the differences in sample collection methods between children and adults, they also pointed out that the size of children’s swabs is significantly smaller, and the amount of sample collected is less than half of what is usually available for PCR testing. In addition, the degree of discomfort involved in this sample collection procedure means that deep nasopharyngeal swabs are often replaced by simple throat swabs. This will of course further reduce the amount of viral material collected. For this reason, these authors fully expected that in children, the same level of virus replication would produce a lower viral load result in PCR testing.
When comparing the peak viral load of laboratory samples, these authors estimated that the youngest child (0 to 5 years old) had an infectivity level of approximately 80% of that found in adults. As before, the values for school-age children and adolescents were found to be close to those for adults. This shows that viral load is not directly proportional to infectivity and cannot be directly converted. Even these data-based estimates of infectivity must be revised upwards because children use different sample collection methods. All of these form part of the overall evaluation of clinical virologists. Their initial hypothesis that the level of infectivity is roughly the same for all age groups has been confirmed by this study and other studies.
PAMS (pre-symptomatic, asymptomatic, and mildly-symptomatic, pre-symptomatic, asymptomatic, and mildly-symptomatic) subjects, hospitalized subjects and other subjects’ age and first-time positive viral load distribution, picture from Science, 2021, doi:10.1126/science.abi5273.
The symptom-based comparison confirms previous observations in COVID-19 cases that even people without symptoms can have very high viral loads. These authors found that throughout the course of the disease, people requiring hospitalization had a higher viral load than others. Based on their new model of the viral load over time, they estimated that people infected with SARS-CoV-2 reached the peak viral load in the throat as early as 1 to 3 days before the onset of symptoms.
Of the COVID-19 cases tested, approximately 9% showed extremely high viral loads: 1 billion copies of the SARS-CoV-2 genome per sample or higher. Among these potentially highly infectious individuals, more than one-third have no symptoms or only mild symptoms. These data provide a virological basis for the idea that a few infected people cause most of the transmission. This includes so many people without any related symptoms, a fact that highlights the importance of pandemic control measures such as social distancing and mandatory wearing of masks.
In samples collected from individuals infected with the British variant of B.1.1.7, it was found that the average viral load increased by a factor of 10, while laboratory-based infectivity was estimated to have increased by a factor of 2.6. To derive these data, the authors obtained viral load data from approximately 1,500 cases infected with B.1.1.7 and compared it with data from approximately 1,000 people infected with other variants who were at the same time at the same time. The testing center, outpatient department and clinical ward accept testing. Laboratory research may not yet provide a clear explanation, but one thing is clear: B.1.1.7 is more contagious than other variants.
These authors plan to continue their research on viral load throughout the pandemic. They hope to learn more about the changes that have occurred with the emergence of new variants, as the virus has adapted to the ever-increasing level of immunity of the population.
(source:internet, reference only)