These 3 California counties haven't seen new COVID-19 cases in weeks

SAN FRANCISCO (KGO) -- COVID-19 is undoubtedly on the decline in California, but in three counties it has practically disappeared.

Alpine, Sierra and Trinity counties all reported zero new COVID-19 cases in the past week of data available from the California Department of Public Health.

Alpine County hasn't reported a COVID-19 case since March 29. Trinity County actually hasn't seen a single new coronavirus case in over a month. Sierra County only had one case reported three weeks ago.

RELATED: California mask mandate: When you still have to wear a mask, what will change on June 15

What's their secret?

First off, all these counties are either small or very small. The population of Alpine County is only about 1,000 people. Trinity County is the largest of three at about 12,700 residents (but to put that in perspective, San Francisco County has about 875,000 people -- 69 times more residents). Smaller populations generally mean fewer people contracting COVID-19 and testing positive for it.

Not only are the counties small, they're all also sparsely populated and mountainous. Many of the Bay Area's hot spots for coronavirus transmission over the past year have been dense areas like the Mission in San Francisco or the Canal in Marin County. There aren't any neighborhoods like that in Alpine, Sierra or Trinity. Social distancing becomes a lot easier with all that space.

One of the counties is also ahead of the game when it comes to vaccinations. Alpine County already has 79% of its residents with at least one dose of the vaccine, the highest vaccination rate in the state.

VACCINE TRACKER: How California is doing, when you can get a coronavirus vaccine

Having trouble loading the tracker above? Click here to open it in a new window.

"We are close," reads a recent public health bulletin from the county on herd immunity. "Let's get to the finish line!"

Sierra County is in the middle of the pack, with about 45% of its population vaccinated. Trinity County is at 34%.

All three counties are firmly in the yellow reopening tier.

MAP: CA counties that can, can't reopen under new rules

For a better experience, click here to view the full map in a new window

RELATED STORIES & VIDEOS:

• Cheat sheet: What you can and can't do after being fully vaccinated
• COVID-19 vaccine: Everything to know about your 2nd Pfizer or Moderna shot
• These CA counties are way ahead in vaccinations
• How to register for a COVID-19 vaccine in every Bay Area county
• Map shows everywhere you can get a COVID-19 test in the Bay Area
• Interactive map shows what's closed and what's reopening in the San Francisco Bay Area
• Data tracker: Coronavirus cases, deaths, hospitalizations in every Bay Area county
• COVID-19 Diaries: Personal stories of Bay Area residents during pandemic
• Get the latest updates on California EDD, stimulus checks, unemployment benefits
• Coronavirus origin: Where did COVID-19 come from?
• What is a COVID-19 genetic, antigen and antibody test?
• What does COVID-19 do to your body and why does it spread so easily?
• Coronavirus Timeline: Tracking major moments of COVID-19 pandemic in San Francisco Bay Area
• Coronavirus Doctor's Note: Dr. Alok Patel gives his insight into COVID-19 pandemic