New York, April 23 : It is not herd immunity that is causing fresh peaks in Covid caseloads in several countries, including India, but a time-dependent social activity and changing people behaviours due to pandemic fatigue that is causing the lethal second wave of infection and deaths, novel research has revealed. For instance, individuals who self-isolated during the first wave -- staying home, not having visitors over, ordering groceries online -- subsequently started relaxing their behaviours. Any increase in social activity means additional exposure risk. The outcome can be that there is a false impression that the pandemic is over, although there are more waves to come, the researchers noted in a paper published in Proceedings of the National Academy of Sciences. According to scientists at the US Department of Energy's (DOE) Brookhaven National Laboratory and the University of Illinois Urbana-Champaign (UIUC) in the US, the concept of herd immunity doesn't apply in practice to Covid-19. "People's social activity waxes and wanes, especially due to lockdowns or other mitigations. So, a wave of the epidemic can seem to die away due to mitigation measures when the susceptible or more social groups collectively have been infected -- something we termed transient collective immunity," said Nigel Goldenfeld, Swanlund Professor of Physics at UIUC. "Once these measures are relaxed and people's social networks are renewed, another wave can start, as we've seen with states and countries opening up too soon, thinking the worst was behind them," he suggested. A temporary state of immunity arises because population heterogeneity is not permanent. In other words, people change their social behaviour over time. The team of scientists has developed a new mathematical model for predicting how epidemics such as Covid-19 spread. This model not only accounts for individuals' varying biological susceptibility to infection but also their levels of social activity, which naturally change over time. Herd immunity is the percentage of the population who must achieve immunity in order for an epidemic to end. "Herd immunity is a controversial topic. Since early on in the Covid-19 pandemic, there have been suggestions of reaching herd immunity quickly, thereby ending local transmission of the virus. However, our study shows that apparent collective immunity reached in this way will not last," said Sergei Maslov, a CFN user and professor and Bliss Faculty Scholar at UIUC. In follow-on work, the scientists are now feeding statistics from "superspreader" events into the model. /IANS