# Is the R number the best way to predict the spread of COVID-19?

## Researchers have suggested that the R number may not be the best method of predicting the spread of COVID-19 due to national lockdowns.

The traditional mathematical model, which is used to help project the contagiousness and spread of infectious diseases like the seasonal flu, may not be the best way to predict the continuing spread of coronavirus, especially during lockdowns that alter the normal mix of the population, says Dr Arni Srinivasa Rao, a mathematical modeller at the Medical College of Georgia at Augusta University.

In a letter published in Infection Control and Hospital Epidemiology, corresponding author of the study Dr Rao argues that, while it is never possible to track down every single case of infectious disease, the lockdowns that have become necessary to help mitigate the COVID-19 pandemic have complicated predicting the disease’s spread.

### Using mathematical models with care

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 and 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.

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.

Rao and his co-authors have argued that a more dynamic, moment in time approach using a model called the geometric mean can more accurately predict likely numbers for the short term, despite not being able to predict long-term trends.

The model uses the current number of COVID-19 infections to predict the number of infections there might be tomorrow.

Rao 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. 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 world, and the researchers say the R-naught did not predict the current, third wave of the pandemic. The authors say that better models have implications for mitigating the spread of COVID-19 and for future planning.

“Different factors continuously alter ground-level basic reproductive numbers, which is why we need a better model,” said Rao.

“Mathematical models must be used with care and their accuracy must be carefully monitored and quantified,” the authors write. “Any alternative course of action could lead to wrong interpretation and mismanagement of the disease with disastrous consequences.”

Co-authors include Dr Steven Krantz, a professor of mathematics and statistics at Washington University and Dr Michael Bonsall, a professor in the Mathematical Ecology Research Group, at the University of Oxford.