疾控中心主任罗谢尔·瓦伦斯基(Rochelle Walensky)周三表示，起源于英国的传染性更强的冠状病毒变体B-117变体已成为美国的主要病毒株。

瓦伦斯基在白宫的一次简报会上告诉记者:“根据我们最近对疾控中心监测的估计，B-117变体现在是美国最常见的谱系。”

瓦伦斯基的评论是按计划进行的，此前有预测称，B-117变体将在3月底或4月初成为更具优势的菌株。

瓦伦斯基周三没有具体说明B-117变体的优势如何，但疾控中心的一名发言人表示，她指的是尚未公布的初步数据，但显示B-117是最主要的菌株。

这与疾控中心截至3月13日的最新可用病例数据相一致，该数据显示，从2月底到3月中旬，B-117病例的百分比增加了一倍多。

2月底，B-117变异体占所有病例的11%左右，但到3月中旬，这一比例已升至27%。

专家们认为，B-117变异体更具传染性，可能更致命，这可能有助于解释为什么尽管疫苗接种速度很快，但它在美国的流行率却在上升，疫苗接种已被证明对这种变异体有效。

瓦伦斯基在本周早些时候的白宫简报会上说:“我们知道，这些增加部分是由于更高传播性的变异，我们正在密切监测。”周三，疾控中心报告说，在过去的七天里，平均每天有62000例病例。

斯潘塞·普拉特/盖蒂影像公司

人们走过新冠肺炎检测诊所和新冠肺炎疫苗接种点的标志

除了新的、传染性更强的变种外，专家表示，新冠肺炎放宽限制和增加旅行也是病例数量上升的原因。

瓦伦斯基说，虽然近一半的美国成年人已经部分接种了疫苗，但病毒现在正在没有接种的年轻人中传播。她指出，医院收治的30多岁和40多岁的人越来越多，而关于集群的报告大多来自日托中心和青年体育。

美国授权的三种疫苗——辉瑞、莫德纳和强生——已被证明能有效对抗B-117变体。

该国首席传染病专家安东尼·福奇博士在周三的简报会上说，这些疫苗预计还将提供至少六个月甚至更长时间的免疫力。

虽然这些疫苗都非常有效，但专家说只有时间才能证明这种保护能持续多久。福奇周三又增加了一层希望，称研究显示免疫力可能会持续“远远超过”六个月。

瓦伦斯基敦促美国人“卷起袖子”，加入45%已经接受过一针疫苗的成年人的行列，并警告说，每一例都给病毒带来更多变异和变得更成问题的机会。

“65，000个病例，也就是说有65，000个机会发生突变，更多的变异得以传播。所以，我们只是非常仔细地观察，”她说。

“最后，我想承认这一时刻的两个重要事实，”她说。“我们仍然有理由担心病例数的上升、报告的变异数的上升和住院人数的上升，而且有如此多的理由抱有如此大的希望。”

UK variant has become most dominant COVID strain in US, CDC says

The more contagious variant of coronavirus that originated in the UK, the B-117 variant, has become the dominant strain in the U.S., CDC director Rochelle Walensky said on Wednesday.

"Based on our most recent estimates from CDC surveillance, the B-117 variant is now the most common lineage circulating in the United States," Walensky told reporters at a White House briefing.

Walensky’s comment was on schedule with previous predictions that the B-117 variant would become the more dominant strain by late March or early April.

Walensky did not specify on Wednesday just how dominant the B-117 variant is, but a spokesperson at the CDC said she was referring to preliminary data that has not yet been released but shows B-117 is the most dominant strain.

That tracks with the latest available CDC data on cases through March 13, which shows the percentage of B-117 cases more than doubled from late February to mid-March.

In late February, the B-117 variant accounted for around 11% of all cases but by mid-March that had jumped to 27%.

Experts believe the B-117 variant is more contagious and likely more deadly, which could help explain why its increasing prevalence in the U.S. has coincided with a rise in cases despite the speed of vaccinations, which have proven effective against the variant.

"We know that these increases are due, in part, to more highly transmissible variants, which we are very closely monitoring," Walensky said at a White House briefing earlier this week. On Wednesday, the CDC reported an average of 62,000 cases a day over the last seven days.

In addition to new, more contagious variants, experts say relaxed COVID-19 restrictions and increased travel are also to blame for the rising case numbers.

While nearly half of U.S. adults have been partially vaccinated, the virus is now spreading among young people who haven’t been, Walensky said. She noted that hospitals are seeing more people in their 30s and 40s admitted, while reports of clusters are mostly from day care centers and youth sports.

All three vaccines authorized in the U.S. -- Pfizer, Moderna and Johnson & Johnson -- have proven to work effectively against the B-117 variant.

The vaccines are also expected to provide immunity for at least six months, and likely longer, Dr. Anthony Fauci, the nation’s chief infectious disease expert, said at Wednesday’s briefing.

While the vaccines are all highly effective, experts say only time will tell how long that protection lasts. Fauci added another level of hope on Wednesday, saying studies showed immunity was likely to last "well beyond" six months.

Walensky urged Americans to "roll up their sleeves" and join the 45% of adults who have already received one shot of the vaccine, warning that each case brings more opportunity for the virus to mutate and become more problematic.

"Sixty-five thousand cases, that’s 65,000 opportunities for mutations to occur, for more variants to spread. And so, we're just watching this very carefully," she said.

"I want to close by acknowledging the two important truths of this moment," she said. "There is still reason for us to be concerned with rising case counts, rising variants reported and increasing hospitalizations, and there is so much reason for so much hope."