The number of confirmed Covid-19 cases increased faster in San Francisco alone in the past week, as compared to the Bay Area and entire state of California, according to new data.

This was a rare occurrence for the city, which has had among the lowest virus rates in major US metropolitan areas throughout the pandemic, Xinhua news agency quoted the San Francisco Chronicle as saying in a report on Friday.

According to the California Data Coalition, the total number of new cases recorded over the past seven days in San Francisco was up 81.6 per cent from the previous seven-day period.

New cases in California over the past seven days were up 71 per cent from the previous period, and across the Bay Area, that number was 61 per cent.

Presently, San Francisco's seven-day average of new case rate was 13 per 100,000 people, while the California seven-day average case rate was 13.7 and the Bay Area's was 12.5.

San Francisco is also witnessing higher "breakthrough" case rates among vaccinated people than California. 

In San Francisco, there are 5.8 cases per 100,000 vaccinated residents and 15 cases per 100,000 non-vaccinated persons, according to county officials.

These rates are both higher than the state, which reported 2 cases per 100,000 vaccinated residents and 13 for the non-vaccinated.

The San Francisco Public Health Department said in a statement that the city has tested more people than many other places in California, which could be a factor in its higher case rate.

"As a city, we are prioritizing testing in our hardest-hit areas to detect disease. As the second densest city in the country, we know viruses that are highly transmissible will spread quicker within the community," the statement said on Friday.

More than a month after the state and the city reopened, the delta variant is spreading rapidly, experts said.

In San Francisco, 69 per cent of residents are fully vaccinated, while in California, the number is 52.3 per cent, according to Centers for Disease Control and Prevention data.