San Francisco,  Experts have warned of a potential "tripledemic" in California this winter, with spikes in cases of Covid-19, flu and respiratory syncytial virus.

The warning came amid the latest signs of statewide resurgence of coronavirus cases, with a daily case rate reported at 6.1 per 100,000 residents, said the California Department of Public Health in a weekly update on Thursday.

That's down 4 per cent from the previous week, but the rate of decline has slowed, Xinhua news agency quoted a local media report as saying.

California, home to around 40 million residents, is now conducting only about 72,000 Covid-19 tests a day, its lowest testing volume since the first three months of the pandemic.

The state was processing more than 250,000 daily lab tests as recently as in early June, the report added.

As a result, statewide testing positivity is resurging in California, up from 4.1 per cent last week to 4.5 per cent as of Thursday.

Covid-19 hospitalizations are also creeping back up, the report said.

It pointed out that California's latest positivity and hospital numbers remain well below the peaks of more than 16 per cent and nearly 5,000 in hospital beds in July, but the recent growth comes as a new family of omicron subvariants starts to make up a rapidly increasing portion of cases, and as colder weather starts to drive more people to gather indoors.

California, once an epicentre of the pandemic in the country, reported a total of 10,519,175 confirmed cases to date, along with 96,185 deaths.