Uncertainty about how many vaccinated people previously were infected with COVID-19 makes it hard to gauge the population’s total immunity and predict the potential duration and severity of the recent “brisk acceleration” in new cases, said Joe Gerald, MD, PhD, an associate professor of public health policy at the Mel and Enid Zuckerman College of Public Health. Given that uncertainty, policymakers and individuals “should take immediate steps to adopt evidence-based mitigation practices to reduce transmission,” Gerald wrote Friday in an assessment of COVID-19 conditions in Arizona.