California, the nation's most populous state, has confirmed 827 monkeypox cases as of Monday, the second-largest state tally after the 1390 infections documented in New York, according to the US Centres for Disease Control and Prevention.
The first case of monkeypox in the United States was confirmed in Massachusetts on 20 May, with the first case in California, in a person who had travelled abroad, confirmed five days later.
Monkeypox, which spreads through close physical contact, tends to cause flu-like symptoms and pus-filled skin lesions, though it is rarely fatal.
Newsom's proclamation came nine days after the World Health Organization (WHO) declared a global health emergency, the agency's highest level of alert, over the rapidly spreading viral disease.
Around 70 countries where monkeypox is not already considered endemic have reported outbreaks, with global cases surpassing 22,100, according to a Reuters tally published on Monday. A WHO report from 22 July said only five monkeypox deaths had been reported worldwide, all of them in Africa.
In order to expand inoculation against the disease, California's proclamation enables emergency medical services personnel to administer federally approved vaccines, similar to the statutory authority recently enacted for pharmacists to give vaccines.
The state's monkeypox response builds on programs developed during the Covid-19 pandemic, the governor's office said in a statement announcing the declaration.
Last month, California public health leaders urged the federal government to make more vaccine doses available to the state as quickly as possible, and 25,000 does have so far been administered in California.
New York Governor Kathy Hochul declared a monkeypox emergency last week in her state, which accounts for one in four US cases documented to date.