The recommendation that vaccinated people in some parts of the country dust off their masks was based largely on one troublesome finding, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.
美国疾病控制与预防中心(Centers for Disease Control and Prevention)主任罗谢尔·瓦伦斯基(Rochelle Walensky)表示，之所以建议美国一些地区的疫苗接种者重新戴上口罩，主要是基于一个令人不安的发现。
New research showed that vaccinated people infected with the Delta variant carry tremendous amounts of the virus in the nose and throat, she said in an email responding to questions from The New York Times.
The finding contradicts what scientists had observed in vaccinated people infected with previous versions of the virus, who mostly seemed incapable of infecting others.
That conclusion dealt Americans a heavy blow: People with so-called breakthrough infections — cases that occur despite full vaccination — of the Delta variant may be just as contagious as unvaccinated people, even if they have no symptoms.
That means fully immunized people with young children, aging parents, or friends and family with weak immune systems will need to renew vigilance, particularly in high-transmission communities. Vaccinated Americans may need to wear masks not just to protect themselves, but everyone in their orbit.
There are 67,000 new cases per day on average in the United States, as of Thursday. If vaccinated people are transmitting the Delta variant, they may be contributing to the increases — although probably to a far lesser degree than the unvaccinated.
The C.D.C. has not yet published its data, frustrating experts who want to understand the basis for the change of heart on masks. Four scientists familiar with the research said it was compelling and justified the C.D.C.’s advice that the vaccinated wear masks again in public indoor spaces.
The research was conducted by people outside the C.D.C., the scientists said, and the agency is working quickly to analyze and publish the results. The agency expects to publish the research on Friday, one official said.
Some of the research may be related in part to an outbreak in Provincetown, Mass., where Fourth of July festivities have led to 882 cases as of Thursday. Nearly three-quarters of those people were fully vaccinated.
The agency also has tracked data from the Covid-19 Sports and Society Workgroup, a coalition of professional sports leagues that is testing more than 10,000 people at least daily and sequencing all infections.
疾控中心还追踪了来自新冠体育和社会工作组(Covid-19 Sports and Society Workgroup)的数据，这一小组由职业体育联赛组成，每天检测超过1万人，并对所有感染进行测序。
It’s still unclear how common breakthrough infections are and how long the virus persists in the body in those cases. Breakthroughs are rare, and unvaccinated people account for the bulk of virus transmission, Dr. Walensky said.
Regardless, the data that the C.D.C. is reviewing suggest that even fully immunized people can be unwilling vectors for the virus. “We believe at individual level they might, which is why we updated our recommendation,” Dr. Walensky said in her email to The Times.
The new data do not mean that the vaccines are ineffective. The vaccines still powerfully prevent severe illness and death, as they were meant to, and people with breakthrough infections very rarely end up in a hospital.
The variant is twice as contagious as the original virus, and one study suggested that the amount of virus in unvaccinated people infected with Delta might be a thousand times higher than seen in people infected with the original version of the virus. The C.D.C. data support that finding, said one expert familiar with the results.
Anecdotes of clusters of breakthrough infections have become increasingly frequent, with groups of vaccinated people reporting sniffles, headache, sore throat, or a loss of taste or smell — symptoms of an infection in the upper respiratory tract.
But the overwhelming majority do not end up needing intensive medical care, because the immune defenses produced by the vaccine destroy the virus before it can get to the lungs.
“We’re still going to see a huge, huge, huge impact on severity of disease and hospitalization,” said Michal Tal, an immunologist at Stanford University. “That’s really what the vaccine was made to do.”
“我们仍将看到（疫苗）对疾病严重程度和住院治疗的极其巨大的影响，”斯坦福大学(Stanford University)免疫学家米哈尔·塔拉(Michal Tal)说。“这就是疫苗的真正用场。”
“The vaccines — they’re beautiful, they work, they’re amazing,” said Frances Lund, a viral immunologist at the University of Alabama at Birmingham. “But they’re not going to give you that local immunity.”
“疫苗——它们很漂亮，有效，非常棒，”阿拉巴马大学(University of Alabama)伯明翰分校的病毒免疫学家弗朗西斯·隆德(Frances Lund)说。“但它们不会带来那种区域性免疫。”
When people are exposed to any respiratory pathogen, it may find a foothold in the mucosal lining of the nose — without causing any harm beyond that. “If you walked down the street and swabbed people, you would find people that had viruses in their mucosa who were asymptomatic,” said Dr. Michael Marks, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Our immune system is mostly fighting these things off most of the time.”
当任何呼吸道病原体接触到人类时，它都可能在鼻腔黏膜内层找到立足点——除此之外不会造成任何伤害。“如果你走在街上拿拭子给别人做测试，你会发现黏膜中有病毒的人是无症状的，”伦敦卫生与热带医学院(London School of Hygiene and Tropical Medicine)的流行病学家迈克尔·马克斯(Michael Marks)说。“我们的免疫系统大部分时间都在与这些东西作斗争。”
But the Delta variant seems to flourish in the nose, and its abundance may explain why more people than scientists expected are experiencing breakthrough infections and cold-like symptoms.
Still, when the virus tries to snake down into the lungs, immune cells in vaccinated people ramp up and rapidly clear the infection before it wreaks much havoc. That means vaccinated people should be infected and contagious for a much shorter period of time than unvaccinated people, Dr. Lund said.
“But that doesn’t mean that in those first couple of days, when they’re infected, they can’t transmit it to somebody else,” she added.
To stop the virus right where it enters, some experts have advocated nasal spray vaccines that would prevent the invader from gaining purchase in the upper airway. “Vaccine 1.0 should prevent death and hospitalization. Vaccine 2.0 should prevent transmission,” Dr. Tal said. “We just need another iteration.”