Los Angeles, California - Faced with rising coronavirus cases, California is ordering a statewide mask mandate for indoor public spaces to go into effect on Wednesday.

The order will affect roughly half the state’s population, including San Diego and Orange counties, the Inland Empire, the Central Valley, and rural Northern California. The statewide indoor mask mandate order will last a month and will expire on January 15.

Los Angeles County, Ventura County, and most of the San Francisco Bay Area have their own indoor mask mandates that were implemented in the summer and have no end dates.

The move comes as coronavirus case rates have risen by 50% in the last two and a half weeks, and county health officials across the state say they suspect they may be seeing the start of a winter jump in the caseload.

The US Centers for Disease Control and Prevention (CDC) considers California as having a high level of transmission of the coronavirus, the worst tier in the federal agency's four-tier scale.

California's announcement came on the same day New York enacted its own statewide mask requirement in indoor public spaces, excepting only settings where everyone inside must be vaccinated. Officials in the UK have also reordered an expansion of indoor mask mandates.

The new mask orders arrive as the Omicron variant of the coronavirus – discovered only last month – has spread rapidly around the globe. Britain has recorded its first death of someone infected with Omicron variant.

In addition, many states elsewhere nationally have been struggling with a winter Covid-19 surge to the still-dominant Delta variant. "We see other states in the United States struggle with overwhelmed hospitals, and a high number of cases," Dr Mark Ghaly, the California health and human services secretary, told reporters Monday.

Ghaly said he's concerned that hospital capacity is still pressed and challenged, particularly in San Bernardino and Riverside counties, across the Central Valley and in the eastern Sierra and the rural north.

A number of hospitals throughout the state are busier than usual for this time of year, where staff are still exhausted from battling a historic pandemic over nearly two years, and there's still plenty of pent-up demand for health care needs that were postponed during earlier parts of the pandemic.

The evidence is there that masks still make a difference, Ghaly said. The coronavirus is airborne and can also spread silently from infected, asymptomatic people.

"Even a 10% increase in indoor masking can reduce case transmission significantly," Ghaly said.

"This is a critical time where we have a tool that we know has worked and can work. We are proactively putting this tool of universal indoor masking in public settings in place to ensure we get through a time of joy and hope without a darker cloud of concern and despair," Ghaly said.

"Californians have done this before. And we of course believe we can do it again."