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At the most basic level, they use the reproductive value of a virus - a model itself, that predicts how infectious it is - to determine how it might spread through a city based on its population size. Infectious disease models are computer simulations built to mimic the spread of a pathogen through a population. Its application has also been somewhat unprecedented - never before have models been used so consistently, and at such a local level, to inform crisis policy. “But I remember tearing my hair out and thinking, ‘I hope this isn’t wrong.’”ĭisease modeling has played a vital, if at times rudimentary, role in the coronavirus pandemic. “Our best take from the model was that San Francisco had acted soon enough,” Petersen said in a recent interview. And then, of course, she realized: No one knew anything about COVID-19. “I don’t know anything about COVID,” she recalled thinking. Petersen, an epidemiologist at UC Berkeley’s School of Public Health who had been studying HIV for more than a decade, immediately thought no, surely there must be someone better suited to the job. Maya Petersen got a surprising and urgent request: Health officials wanted to know whether she could put together a model that would help them forecast what was shaping up to be a horrifying pandemic. San Francisco was a few days into sheltering in place in March 2020 when Dr. Stephen Lam/The Chronicle Show More Show Less Stephen Lam/The Chronicle Show More Show Less 3 of3īoth Petersen and Schwab helped build the mathematical model that provides COVID forecasts for San Francisco and other counties during the early stages of the pandemic. Maya Petersen helped build build a computer model that provides COVID forecasts for San Francisco and other counties. Stephen Lam/The Chronicle Show More Show Less 2 of3 Maya Petersen and husband Joshua Schwab helped build a mathematical model that provides COVID forecasts for San Francisco and other counties.