截至上周五，加州的新冠肺炎感染率是所有州中最低的，该州拥有全国最严格的口罩和疫苗接种规定，已成功扭转了作为美国前疫情中心的局面。

波士顿儿童医院首席创新官、美国广播公司新闻撰稿人、流行病学家约翰·布朗斯坦博士说:“他们在疫苗接种要求和授权的政策方面非常具有前瞻性。

根据数据，截至周五晚上，加州过去一周的感染率为每10万人中有61.1例数据来自疾病控制和预防中心。该州曾一度是仅仅被归类为“中度”传播率，即7天内每10万人中有10至49.99例，然后回到“严重”，即7天内每10万人中有50至99.99例。

在前一周，夏威夷佛罗里达州、路易斯安那州、康涅狄格州、新泽西州、密西西比州、马里兰州、佐治亚州和华盛顿特区也都报告了“可观”的利率。

每100，000人中至少有100例的比率被标记为“高”截至上周五，蒙大拿州、怀俄明州、爱达荷州和阿拉斯加州的发病率是过去七天中最高的，每10万人中有450例以上。美国平均的这个跨度是150.9。

专家表示，加州从最差到最差的旅程第一很可能是多种因素综合作用的结果。

布朗斯坦补充说:“我们无法确定哪些因素是造成这种情况的原因，但我们知道，干预和政策的结合可以在推动传播方面发挥真正强大的作用。”

“结束这场大流行”

超过70%的加州人口至少接种了一剂疫苗，根据疾病预防控制中心的数据此外，该州还在接种疫苗总数上领先，约为5300万剂。截至周五，全国平均利率为66.2%。

作为第一个宣布作为学校接种新冠肺炎疫苗的要求，加州有一些美国最严格的规定疫苗为国家雇员,保健工作者,教师在大型室内聚会。

“疫苗接种运动越积极越好，”南海岸健康中心传染病主任托德·埃勒林博士告诉美国广播公司新闻。

鉴于疫苗是我们遏制新冠肺炎病毒传播的最有力工具，专家们表示，他们认为加州对免疫接种的重视带来了巨大的红利。

美国国立卫生研究院院长弗朗西斯·柯林斯博士在一份报告中写道:“这些结果非常清楚地表明，疫苗为缺乏免疫力的个人提供了保护，对最终结束这场大流行具有重要意义。”博客文章星期二。

爱达荷州和怀俄明州有一些最低的疫苗接种率和最高的传播率。

从那以后，美国所有三种新冠肺炎疫苗都上市了显示在预防新冠肺炎死亡方面非常有效，截至周五，加利福尼亚州夸口说全国死亡率最低，前一周每10万人中有0.3人死亡。美国的平均水平为每10万人中有2.7人。

此外，专家告诉美国广播公司新闻，在经历了去年冬天最严重的感染浪潮和7月份的三角洲激增后，以前感染的加州人可能获得了抗体和对新冠肺炎的更多自然免疫力——这两个因素与疫苗一起，可能会降低病例数。

贾斯汀·沙利文/盖蒂图像公司

一名赛弗威药剂师准备给陈凯注射辉瑞新冠肺炎公司的强化疫苗..

还有更多工作要做

除了严格的疫苗规定，加州在口罩、检测和社交距离政策上也毫不动摇。

“虽然加州接种的疫苗比其他任何一个州都多，但我们还有更多工作要做，”加州公共卫生部在给美国广播公司新闻的一份声明中说。

每个人，无论接种状态如何，都是需要在公共交通工具、医疗机构、教育和教养机构佩戴口罩。所有未接种疫苗的人都需要口罩，在剧院、零售店、餐馆和酒吧等场所，建议接种疫苗的人戴口罩。

尽管疫苗接种率高于加州，但像佛蒙特州这样的州在传播率排名上仍然落后，专家表示，这表明了距离和掩蔽的重要性。佛蒙特州出现的突破性案例可以用它的更高的比例65岁以上的居民。

机动性

研究显示流动性，或人口流动，可以是疾病传播的一个预测因素，特别是通过航空旅行或公共交通。这一点通过新冠肺炎病例激增美国在假期看到的。

加文·纽瑟姆州长辩护在加州医院面临“前所未有的压力”的时候，加州居民在平安夜避免假日聚会

加州城市有经验的假期后病例和住院人数激增，尽管此后比率有所下降。

加州政府对家庭订单并不陌生强迫几次封锁和限制，以遏制病毒的传播。它在6月中旬庆祝了最近一次重新开放，用更类似于“一切照旧”的条件取代了长达15个月的公共卫生限制

布朗斯坦告诉美国广播公司新闻，“在大流行最严重的时候，人们被锁在家里，他们根本不动，他们呆在家里。“人们走动越多，这通常是传播增加的一个指标。”

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伟大的户外活动

这种病毒已被证明在室内传播得更多，而且人与人之间的距离更近。加州阳光充足的气候意味着更多的人花更多的时间在户外。

埃勒林说:“当我们大部分时间都待在室内时，就是传播最大的时候。

布朗斯坦补充道:“在室内与家庭以外的人进行更密切的接触会带来额外的风险。”

研究表明，冬季病毒有效繁殖数量的增加也加剧了这种影响建议温度下降会促进病毒生长。

布朗斯坦说，与许多其他州相比，尤其是随着冬天的临近，加州处于“相当好的位置”。“很有可能，我们看到的趋势是现实的，在很大程度上是由遵守掩蔽和其他干预措施，加上该州覆盖面广的疫苗接种战略所驱动的。”

How did California go from the epicenter of the US pandemic to the lowest statewide transmission rate?

With the lowest COVID-19 infection rate among all states as of Friday, California, which has some of the strictest mask and vaccination mandates in the country, has managed to flip the script as the former U.S. epicenter of the pandemic.

"They've been very much forward-thinking in terms of policies around vaccination requirements and mandates," said epidemiologist Dr. John Brownstein, chief innovation officer at Boston Children's Hospital and an ABC News contributor.

As of Friday evening, California had an infection rate of 61.1 cases per 100,000 over the past week, according todatafrom the Centers for Disease Control and Prevention. The state was briefly theonlyto be classified with a "moderate" rate of transmission -- 10 to 49.99 cases per 100,000 over seven days -- before it went back up to "substantial," meaning 50 to 99.99 cases per 100,000 over seven days.

Over the previous week,Hawaii, Florida, Louisiana, Connecticut, New Jersey, Mississippi, Maryland, Georgia and Washington, D.C., each also reported "substantial" rates.

A rate of at least 100 cases per 100,000 is labeled as "high." Montana, Wyoming, Idaho and Alaska, as of Friday, had the highest rates over the previous seven days, with each above 450 cases per 100,000 people. The U.S.averageover that span was 150.9.

Experts said California's journey from worst tofirstis likely due to a combination of things.

"We can't know for sure which factors are responsible," Brownstein added, "but we know that the combination of interventions and policies can play a really strong role in driving transmission."

'Ending this pandemic'

More than 70% of California's population has received at least one dose of vaccine,according to the CDC, and the state also leads in total vaccinations administered, roughly 53 million doses. As of Friday, the national average rate was 66.2%.

Being the first state toannouncea COVID-19 vaccine requirement for schools, California has some of the strictest mandates in the U.S. The state has mandatedvaccinesforstate employees,health care workers,teachersand atlarge indoor gatherings.

"The more aggressive the vaccination campaign, the better," Dr. Todd Ellerin, director of infectious diseases at South Shore Health, told ABC News.

Given that vaccines are the most powerful tool we have in curbing the spread of COVID-19, experts said they believe California's emphasis on immunizations has provided big dividends.

"These results show quite clearly that vaccines offer protection for individuals who lack immunity, with important implications for finally ending this pandemic," Dr. Francis Collins, director of the National Institutes of Health, wrote in ablog postTuesday.

Idaho and Wyoming have some of the lowest vaccination rates and the highest transmission rates.

All three COVID-19 vaccines available in the U.S. have sinceshownhigh effectiveness in preventing death due to COVID-19, and as of Friday, Californiaboastedthe lowest death rate in the country, with 0.3 per 100,000 over the previous week. The U.S. average over that span was 2.7 per 100,000.

Additionally, experts told ABC News, having endured the harshest waves of infections last winter and the delta surge in July, formerly infected Californians may have acquired antibodies and more of a natural immunity to COVID-19 -- both factors that, in concert with vaccines, could be driving down case numbers.

'More work to do'

In addition to strict vaccine mandates, California has been unwavering on face-coverings, testing and social distancing policies.

"While California has administered more vaccines than any other state, we have more work to do," the California Department of Public Health said in a statement to ABC News.

Everyone, regardless of vaccination status, isrequiredto wear a mask on public transit, in health care settings and at educational and correctional facilities. Masks are required for all unvaccinated people and recommended for vaccinated people in settings such as theaters, retail stores, restaurants and bars.

Despite having a higher vaccination rate than California, states like Vermont are still falling behind on transmission rate rankings, which experts said points to the importance of distancing and masking. Breakthrough cases seen in Vermont can be further explained by itshigher proportionof residents 65 and older.

Mobility

Researchshows mobility, or movement in populations, can be a predictor of disease spread, especially via air travel or mass transit. This became evident through thesurge of COVID-19 casesthe U.S. saw during holiday season.

Gov. Gavin Newsompleadedwith California residents on Christmas Eve to avoid holiday gatherings at a time when hospitals in the state were under "unprecedented pressure."

Californian citiesexperienceda huge surge of cases and hospitalizations following the holidays, although rates have since declined.

No stranger to stay-at-home orders, the California governmentimposedseveral lockdowns and restrictions to curb the spread of the virus. It celebrated its most recent reopening in mid-June, which replaced 15-month-long public health restrictions with conditions more akin to "business as usual."

"In the height of the pandemic, people were locked down, they weren't moving at all, they were staying at home," Brownstein told ABC News. "The more people move around, that's usually an indicator for increased transmission."

The great outdoors

The virus has been shown to spread more indoors and with people in close proximity to one another. California's sunny climate means more people spend more time outside.

"When we spend most of our time indoors, that's when transmission is greatest," Ellerin said.

Added Brownstein: "More close contacts with people outside your household indoors is what creates additional risk."

This effect is also compounded by the rise in the virus' effective reproductive number during the winter season with researchsuggestingthat a drop in temperature encourages virus growth.

Compared to many other states, especially as winter draws nearer, California is in "quite a good spot," Brownstein said. "In all likelihood, the trends that we're seeing are realistic and very much driven by compliance with masking and other interventions, plus the well-covered vaccination strategy in the state."