I feel like the Great Panic may have been way more complicated than the book illustrates. Like, after the journalist reveals the truth about Phalanx, there'd be this period of uncertainty. People would start noticing that cases were increasing, and start questioning health officials for answers. Perhaps a week after this uncertainty, health officials and news outlets would confirm the rumors about the Solanum virus and Phalanx's ineffectiveness. Society wouldn't erupt into rioting right away, there'd be this rising fear. People would flock to the stores for supplies, but stores would be relatively calm until lines get so long that some people rush out with carts full of stolen goods. Fights would break out inside stores over canned foods. Hospitals would be locked down as they fill up with Solanum victims. This is when the actual panic would break out, ATMs being broken open, businesses shutting down, people chucking molotovs and so on. Don't forget, the Great Panic is also the point where major urban areas are overrun. Hospitals are completely overwhelmed and hordes of zombies break out, spreading throughout the city and infecting rioters. Maybe around late July, there'd be this period where major cities (e.g. New York) are overrun.
Then comes the Battle of Yonkers.
That's my perspective, anyways. The book mainly focuses on the aftermath and recovery more than the initial panic and urban outbreaks.