California broadly reopened its economy barely two weeks ago and since then an especially contagious coronavirus variant has spread among the unvaccinated, a development that has dismayed health officials and already has prompted Los Angeles County to strongly recommend everyone resume wearing masks inside.

The nation’s most populous state is averaging close to 1,000 additional cases reported daily, an increase of about 17 per cent in the last 14 days.

But public health officials raised concern this week with the more transmissible delta variant spreading among the unvaccinated, who comprise the vast majority of new infections.

The county Department of Public Health on Thursday reported 506 new cases, the highest number in a day since mid-April and more than double the figure from two weeks ago.

“Given that 4 million residents in L.A. County are not yet vaccinated, the risk of increased spread is very real,” a department statement said.

Gov. Gavin Newsom lifted a series of pandemic-related restrictions on 15 June after a final push to get more people vaccinated.

In Contra Costa County, where 72 per cent of eligible residents are fully vaccinated, officials recently began publishing virus case rates by vaccination status. Since the county has a high inoculation rate, the number of new coronavirus cases is generally low but unvaccinated residents remain at risk, said Dr. Chris Farnitano, the county’s health officer.

For example, the seven-day average of new virus cases per 100,000 people in the county was recently 7.0 for those who are unvaccinated, and 0.4 for those who are vaccinated.

Dr. Tomás Aragón, the state’s public health officer, said the state would continue to offer $50 cards to residents who get vaccinated and that some vaccination sites are offering free amusement park tickets.

Vaccination rates vary widely across California. A San Francisco ZIP code reports more than 95 per cent of those eligible are fully inoculated, while one in rural Modoc County has a 37 per cent rate, according to state data.

Health officials said areas with low vaccination rates are especially at risk as the delta variant, first reported in India, spreads across the US. The variant was found in 15 per cent of specimens sequenced in California in June, up from 5 per cent in May.

In Orange County, the variant accounts for 45 per cent of sequenced cases in the most recent week, said Dr. Regina Chinsio-Kwong, the county’s deputy health officer.