As much as 90 percent of Marin County residents are still susceptible to contracting the novel coronavirus, the county’s public health officer said Tuesday.

Marin County’s rate of new cases has slowed in recent weeks, allowing the county to move into a less-restrictive tier of the state’s pandemic reopening process.

As of Tuesday, 4,791 cases have been confirmed in Marin County, which does not include the 2,239 cases that have been confirmed at San Quentin State Prison. Many of those cases were confirmed during the state’s summer wave of infections.

“July was a rough month for us,” county public health officer Dr. Matthew Willis told the county Board of Supervisors Tuesday. “We had several outbreaks happening simultaneously in our long-term care facilities. Our highest mortality rate was in July. We’ve seen significant progress since then.”

Despite the summer wave of cases, Willis suggested that roughly 80 to 90 percent of the county’s population has not built up an immunity to the virus, making it vulnerable to a second or third wave before a vaccine is available early next year.

Willis suggested that a second wave of cases in the county could be even worse than the surge of cases in July, much like the second wave of the 1918 influenza pandemic.

“We’re seeing now, in other parts of the world and other parts of the country, similar experiences where case rates that are happening now are actually some of the highest case rates, in some areas, ever,” Willis said.

Willis echoed the advice of other local and state health officials that now-common public health guidance like wearing a face covering and physically distancing from others will be the county’s biggest asset in preventing a second or third wave of new cases.

County residents should also get a flu shot sooner rather than later and avoid traditional celebrations of holidays like Halloween, including trick-or-treating.

Alternative Halloween celebrations, such as vehicle parades, would be much safer, according to Willis.

“Conventional trick-or-treating, where children would move through the neighborhood house-to-house, is not a safe practice this year,” Willis said.