California officials have recently placed multiple regions back into much more restrictive reopening tiers after a surge of coronavirus cases plagued several counties, stifling most residents’ hopes of going starting to rebuild their economy.

Governor Gavin Newsom on Monday said that 40 counties would be placed into a more restrictive tier. The official also announced that some areas, such as San Francisco, would be moved back several tiers due to the threat of the virus.

San Francisco was initially placed in the yellow tier, which allowed most businesses and establishments to reopen due to the minimal spread of the virus. However, recent statistics showed a sudden spike in infections around the city.

Officials use two key metrics to decide what tier an area would be placed in: the case rate, which shows the number of new cases for every 100,000 residents), and the positivity rate, which shows the percentage of positive cases among a tested group. Authorities also use a health equity metric but are only used when a county is to be moved to a less restrictive tier.

The case rate in San Francisco is the key factor that forced officials to move the city into the red tier. Authorities observed an average of 10 new cases every day for every 100,000 residents from a previously recorded 3.7. Officials were also concerned with the speed of the rise, which all happened in about three weeks.

The positivity rate in San Francisco has also gone up to 1.8% and coincides with the requirements of the yellow tier. However, the severity of the daily cases forced officials to overlook the relatively low positivity rate and place the city into a more restrictive tier, the San Francisco Gate reported.

Officials revealed that nine of the Bay Area counties averaged over 1,000 new daily cases last week, an increase of about 36% over the previous week. It had been 67 days since the region recorded over 1,000 new cases daily before Monday, where a total of 1,342 cases were reported.

CDC COVID-19 Incident Manager Henry Walke said they were deeply concerned with the numbers the Bay Area was showing. He noted that the region had seen a sudden spike in cases, hospitalizations, and fatalities.

The agency also advised residents who travel to always wear masks and maintain six feet of social distancing. Medical professionals also recommended washing hands with soap and water after going out.

The CDC released new and more detailed guidelines to help citizens avoid the spread of the virus. Infectious disease expert Dr. George Rutherford, of the University of California, San Francisco, said that everyone should avoid doing what they want to this Thanksgiving, which is to visit their families and friends who are away, the San Francisco Chronicle reported.

Data showed that San Francisco had seen the fastest rate of infections since the beginning of the pandemic. The sudden burst of new cases has spread across California to several other cities.

San Francisco’s Health Director Dr. Grant Colfax said on Monday that the threat of the sudden rise in infections forced officials to move the city back two tiers. Colfax noted that the recorded number of total cases jumped from 11,000 to 12,000 in less than 30 days. And this week, San Francisco reported 13,000 total cases in less than 20 days.

However, hospitalization rates remain relatively low, with only a 4% increase in coronavirus-related admissions in the last seven days. Hospitals also have 24% acute care beds available for those who need them, but officials warn that rates could skyrocket because hospitalizations lag behind the number of cases by several weeks.