Model for flu spread may not work in Covid-19: Study


NEW YORK: A mathematical model that can help project the spread of infectious diseases like the seasonal flu may not be the best way to predict the spread of Covid-19, researchers, including one of Indian-origin, have reported.
Called the R-naught, or basic reproductive number, the model predicts the average number of susceptible people who will be infected by one infectious person.
It is calculated using three main factors — the infectious period of the disease, how the disease spreads and how many people an infected individual will likely come into contact with.
Historically, if the R-naught is larger than one, infections can become rampant and an epidemic or more widespread pandemic is likely.
The Covid-19 pandemic had an early R-naught between two and three. In a letter published in the journal Infection Control and Hospital Epidemiology, researchers have argued that lockdowns that have become necessary to help mitigate the Covid-19 pandemic have complicated predicting the disease’s spread by altering the normal mix of the population.
Arni Rao, a mathematical modeller at the Medical College of Georgia at Augusta University in the US and his co-authors instead suggested more of a dynamic, moment in time approach using a model called the geometric mean.
That model uses today’s number to predict tomorrow’s numbers. Current number of infections — in Augusta today, for example — is divided by the number of predicted infections for tomorrow to develop a more accurate and current reproductive rate.
While this geometric method can’t predict long term trends, it can more accurately predict likely numbers for the short term, the researchers said.
« The R-naught model can’t be changed to account for contact rates that can change from day to day when lockdowns are imposed, » Rao explained.
« In the initial days of the pandemic, we depended on these traditional methods to predict the spread, but lockdowns change the way people have contact with each other. »
A uniform R-naught is also not possible since the Covid-19 pandemic has varied widely in different areas of the country and world.
Places have different rates of infection, on different timelines — hotspots like New York and California would have higher R-naughts. The R-naught also did not predict the current third wave of the Covid-19 pandemic.
« Different factors continuously alter ground-level basic reproductive numbers, which is why we need a better model, » Rao says.
Better models have implications for mitigating the spread of Covid-19 and for future planning, the authors said.

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