The counties moved from the red tier to the most restrictive purple tier, which means indoor activities will be shut down for churches, gyms, movie theaters and restaurants. Forty-one counties are now in the purple tier, including all of Southern California.
On Sunday, Orange County reported 639 new COVID-19 cases, its highest number of daily coronavirus cases since mid-August. Ventura's case rate on Sunday was at 7.2%, but it needs to be below 7% in order to remain at the red level.
For Orange County, the possibility of the county reverting back to more restrictions had been looming due to a recent surge. Officials in both counties warned the high number of cases almost guaranteed they would slide back tiers. Business owners have until Tuesday to implement the changes.
RELATED: Newsom considering curfew as CA COVID-19 cases spike, 40 counties move to more restrictive tiers
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County officials have been urging residents to follow safety protocols, including wearing masks and avoiding large gatherings, in the hopes of reducing those numbers.
Orange County's positivity rate, which is reported each Tuesday, actually declined from 3.6% to 3.3% last week, and the daily case rate per 100,000 population decreased from 6 to 5.6.
The county's case rate per 100,000 might jump this week to about 8, which exceeds the 4 to 7 rate in the red tier, County CEO Frank Kim has said.
California's reopening framework is broken down into four tiers (purple, then red, orange and yellow) and determines if businesses must operate outdoors or reduce capacity.
Last week, 11 counties, moved back to more restrictive tiers in the state's reopening system as the nation struggles containing the pandemic.
Gov. Gavin Newsom on Monday said during a coronavirus briefing that he was pulling the "emergency brake" on the state's efforts to reopen its economy as coronavirus cases surge more dramatically than they did during a summer spike.
The measures announced will impose more restrictions on businesses across most of the state. Health officials say the troubling rise in cases in November has come at a faster pace than a spike in mid-June and could quickly surpass the peak of the hospitalizations at the time.
California has surpassed 1 million cases.
WATCH | Here's a breakdown of each tier in Newsom's reopening framework
City News Service and the Associated Press contributed to this report.