A team from Nagoya University (Japan) has revealed that human behavior, such as confinements and isolation measures, affect the evolution of new strains of SARS-CoV-2, the virus that causes Covid-19.
The coronavirus developed to become more transmissible earlier in its life cycle.
The findings, published in Nature Communications, provide new insights into the relationship between people’s behavior and disease-causing agents.
As with any other living organism, viruses evolve over time. Those with survival advantages become dominant. Many environmental factors influence this evolution, including human behavior.
By isolating sick people and using lockdowns to control outbreaks, humans can alter the evolution of the virus in different ways. Predicting how these changes occur is vital for developing adaptive treatments and interventions.
An important concept in this interaction is the viral loadwhich refers to the amount or concentration of a virus present per ml of a body fluid.
In the case of SARS-CoV-2, a higher viral load in respiratory secretions increases the risk of transmission through droplets. Viral load relates to the potential to transmit a virus to other people.
For example, a virus like Ebola has an exceptionally high viral load, while the common cold has a low one. However, viruses must perform a careful balancing act, as increasing the maximum viral load can be advantageous, but excessive viral load can make people too sick to transmit the virus to others.
The research group led by Professor Shingo Iwami identified trends using mathematical models with an artificial intelligence component to investigate previously published clinical data.
Thus, they discovered that the SARS-CoV-2 variants that were most successful in spreading had an earlier and higher peak in viral load. However, as the virus evolved from the variant pre-Alpha to the Delta varianthad a shorter duration of infection.
The researchers also found that the decreased incubation period and the higher proportion of asymptomatic infections recorded as the virus mutated also affected the evolution of the virus.
The results showed a clear difference. As the virus evolved from the Wuhan strain to the Delta strain, they found a five-fold increase in peak viral load and a 1.5-fold increase in the number of days before viral load peaked.
Iwami and his colleagues suggest that human behavioral changes in response to the virus, designed to limit transmission, were increasing selection pressure on the virus. This caused SARS-CoV-2 to be transmitted primarily during the asymptomatic and pre-symptomatic periods, which occur earlier in its infectious cycle. As a result, the peak viral load advanced to this period to spread more effectively in the earliest pre-symptomatic stages.
Public health
According to scientists, when evaluating public health strategies in response to Covid-19 and any potentially pandemic-causing pathogens in the future, it is necessary to consider the impact of changes in human behavior on virus evolution patterns.
«We expect that immune pressure from vaccines and/or previous infections will drive the evolution of SARS-CoV-2 -Iwami points out-. “However, our study found that human behavior can also contribute to the evolution of the virus in a more complicated way, suggesting the need to reevaluate the evolution of the virus.”
Their study suggests the possibility that new coronavirus strains evolved due to a complex interaction between clinical symptoms and human behavior.
