Covid-19 Latest Updates: When Does Herd Immunity Kick In for the Virus?

At what point does a country achieve herd immunity? What portion of the population must acquire resistance to the coronavirus, either through infection or vaccination, for the disease to fade away and life to return to normal?

Since the start of the pandemic, the figure that many epidemiologists have offered has been 60 to 70 percent. That range is still cited by the World Health Organization and is often repeated during discussions of the future course of the disease.

Although it is impossible to know with certainty what the limit will be until we reach it and transmission stops, having a good estimate is important: It gives Americans a sense of when we can hope to breathe freely again.

Recently, a figure to whom millions of Americans look for guidance — Dr. Anthony S. Fauci, an adviser to both the Trump administration and the incoming Biden administration — has begun incrementally raising his herd-immunity estimate.

In the pandemic’s early days, Dr. Fauci tended to cite the same 60 to 70 percent estimate that most experts did. About a month ago, he began saying “70, 75 percent” in television interviews. And last week, in an interview with CNBC News, he said “75, 80, 85 percent” and “75 to 80-plus percent.”

In a telephone interview the next day, Dr. Fauci acknowledged that he had slowly but deliberately been moving the goal posts. He is doing so, he said, partly based on new science, and partly on his gut feeling that the country is finally ready to hear what he really thinks.

Hard as it may be to hear, he said, he believes that it may take close to 90 percent immunity to bring the virus to a halt — almost as much as is needed to stop a measles outbreak.

Asked about Dr. Fauci’s conclusions, prominent epidemiologists said that he may be proven right. The early range of 60 to 70 percent was almost undoubtedly too low, they said, and the virus is becoming more transmissible, so it will take greater herd immunity to stop it.

Dr. Fauci said that weeks ago, he had hesitated to publicly raise his estimate because many Americans seemed unsure about vaccines, which they would need to accept almost universally for the country to achieve herd immunity.

Now that some polls are showing that many more Americans are ready, even eager, for vaccines, he said he felt he could deliver the tough message that the return to normal might take longer than anticipated.

“When polls said only about half of all Americans would take a vaccine, I was saying herd immunity would take 70 to 75 percent,” Dr. Fauci said. “Then, when newer surveys said 60 percent or more would take it, I thought, ‘I can nudge this up a bit,’ so I went to 80, 85.”

“We need to have some humility here,” he added. “We really don’t know what the real number is. I think the real range is somewhere between 70 to 90 percent. But, I’m not going to say 90 percent.”

Doing so might be discouraging to Americans, he said, because he is not sure there will be enough voluntary acceptance of vaccines to reach that goal. Although sentiments about vaccines in polls have bounced up and down this year, several current ones suggest that about 20 percent of Americans say they are unwilling to accept any vaccine.

Traffic remained largely at a standstill at the Port of Dover on Thursday, more than 24 hours after France lifted a ban on freight traffic imposed over fears about a variant of the virus circulating in Britain. Truck drivers must be tested for the coronavirus before boarding ferries, and many have been waiting in their rigs for days.

Ferries working to get freight and drivers to continental Europe will sail on Christmas and Boxing Day, said Grant Shapps, Britain’s transportation secretary, adding the military and contact tracers were working with French firefighters, who brought 10,000 tests to Dover, to clear the backlog from the border closure. Trying to assuage earlier concerns that some drivers would be trapped before the Christmas holiday, Mr. Shapps said that borders at the Eurotunnel, Dover & Calais would remain open through Christmas to help hauliers and citizens return home.

A ferry company, P & O Ferries, said Thursday morning that the area remained “heavily gridlocked,” adding that a road to the port was still blocked, but the first convoys of freight had been released during the night. They added the that authorities were working on increasing testing capabilities, and that ferries were scheduled to depart. Officials have cautioned truck drivers not already in the Dover area to avoid the region.

A team of British scientists released a worrying study on Wednesday of the newly discovered coronavirus variant sweeping Britain. They warned that the variant was so contagious that new control measures, such as closing down schools and universities, might be necessary.

The study, released by the Center for Mathematical Modeling of Infectious Diseases at the London School of Hygiene and Tropical Medicine, has not yet undergone review by a scientific journal. The study compares a series of models as predictors of data on infections, hospitalizations, and other variables. Other researchers are studying the variant in laboratory experiments to determine if it is biologically distinct.

The study found no evidence that the variant was more deadly than others. But they estimated that it was 56 percent more contagious. On Monday, the British government released an initial estimate of 70 percent.

Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health who was not involved in the study, said that it presented a compelling explanation of the past and potential future of the variant.

The variant, which came to the attention of British researchers this month, has been rapidly spreading in London and eastern England. It carries a set of 23 mutations, some of which may make it more contagious.

The authors of the study found more evidence that the variant does indeed spread more rapidly than others. For example, they ruled out the possibility that it was becoming more common because outbreaks started in places where people were more likely to come into contact others.

The researchers built different mathematical models and tested each one as an explanation for the variant’s spread. They analyzed which model of the spread best predicted the number of new cases that were actually confirmed, as well as hospitalization and deaths.

Closing schools until February could buy Britain some time, the researchers found, but lifting those extra restrictions would then cause a major rebound of cases.

The researchers warned that their model was based, like any model, on a set of assumptions, some of which could turn out to be wrong. For instance, the rate at which infected people die from Covid-19 may continue to drop as doctors improve at caring for hospitalized patients.

Still, they wrote, “there is an urgent need to consider what new approaches may be required to sufficiently reduce the ongoing transmission” of the coronavirus.

Benjamin Mueller contributed reporting.

The number of travelers to pass through airport security checkpoints in the United States hit nearly one million on Tuesday as the country approaches Christmas Day.

On Tuesday, 992,167 people traveled through Transportation Security Administration checkpoints, the agency reported — a slight dip compared with Dec. 18 to Dec. 20, when daily checkpoint numbers surpassed one million.

The steady stream of travelers comes as cases soar in many states: Maine and Virginia set single-day case records on Wednesday; Alabama set a single-day death record; and California surpassed two million cases since the start of the pandemic, the first state in the country to do so, according to a New York Times database.

With Christmas just a day away and New Year’s soon after, the Centers for Disease Control and Prevention urged people not to travel. The agency issued an advisory this month that “postponing travel and staying home is the best way to protect yourself and others.” It added, “you and your travel companions (including children) may feel well and not have any symptoms, but you can still spread Covid-19 to family, friends, and community.”

A patchwork of state travel restrictions exists across the United States, with some states requiring travelers to quarantine, fill out mandatory health forms or provide proof of a negative Covid-19 test, while other states have no such measures. That, along with growing alarm that a more contagious variant of the virus is spreading through Britain and beyond, is adding to the confusion and anxiety.

The C.D.C., realizing that some people may not adhere to warnings, has suggested people take several precautions before traveling, such as receiving a flu shot, bringing extra protective supplies like masks and gloves, and getting tested for Covid-19.

Days after her Thanksgiving feast was prepared, served and eaten, Maribel Rodriguez tried to muster the will to unpack the tree, lights and decorations of boisterous Christmases past.

Instead she found herself praying a rosary over the wooden urns containing the ashes of her husband, her mother and an aunt, all of whom had shared a home with her in a rural section of Edinburg, Texas.

“My husband was the one who used to set up the tree and dressed up as Santa every year,” Ms. Rodriguez said, her voice echoing around the hacienda-style home that is emptier now. “I can’t get myself to do it. I end up crying before I touch any of the ornaments.”

Her husband, Domingo Davila, 65, tested positive for the coronavirus in September. Within days, Ms. Rodriguez caught the virus, too, along with her mother, Maria Guadalupe Rodriguez, and aunt, Mirthala Ramirez.

In all, Ms. Rodriguez has lost seven relatives to the virus since it hit the Rio Grande Valley. “This virus didn’t kill me,” she said, “but it sure took my life.”

After a devastating summer along the border region where family gatherings known as pachangas accelerated the spread of the virus, many families have had two, three or more casualties per household.

The death rate here peaked at 5 percent and remains high, representing at least 2,168 funerals. Nationally, the death rate is less than 2 percent of those known to be infected.

Health officials blame a combination of poverty, lack of access to health care and a close-knit culture for the widespread infection within family clusters.

There are reasons the virus has been especially lethal here: It is common for multigenerational families to live under the same roof — and older relatives tend to have chronic conditions such as diabetes.

The situation was worse in the summer, when there were as many as 60 deaths a day. But health officials are expecting another spike after Christmas and New Year celebrations. About 2,500 people are actively battling the coronavirus, according to county data.

The statistics became personal for Ms. Rodriguez, 53, a hospice nurse until she quit because of the toll the illness had taken on her body, she said. She has been scraping by with donations and by selling tamales.

While much as the pandemic has been a story of devastation and loss, it has also been one of resilience — of individual people, families and entire communities not only surviving a deadly threat but seeing in the moment a chance to serve others. Some even dare remind us that joy is still a possibility. We asked our correspondents around the world to share stories from this year that speak to the strength of the human spirit, and to how disruption can bring out the best in us.

BRAZIL

“I am yearning for your hug …”

… wrote the third grader. A few months into quarantine, his teacher, Maura Cristina Silva, could tell that her vivacious students were starting to buckle.

They had become 57 tiny boxes on a computer screen, leaving her with shaky and poorly lit glimpses into the toll the pandemic was taking on a cluster of families in Padre Miguel, a working-class district in western Rio de Janeiro.

Students with learning disabilities were falling behind, as were those who did not have their own computers.

But the text from the unhugged student, which came four months after their public school was abruptly closed, got to Ms. Silva. The child had used the word saudade, a Portuguese term that conveys feelings of longing and melancholy.

Ms. Silva wondered if she could find a way to safely embrace her students.

Her first idea was to use a transparent shower curtain fitted out with four plastic sleeves — but sanitizing it after each embrace seemed impractical.

Then she came up with the idea of a pandemic “hugging kit” — disposable raincoats, surgical gloves, face masks and hand sanitizer.

The response from parents was resounding: How soon could she drop by?

She rolled out the hugging operation in late July, renting a sound truck and driving from door to door, blasting a classroom playlist her students loved.

“Distance can’t destroy what we have built,” Ms. Silva, 47, said on a recent rainy afternoon after visiting three students. “I needed to show them that our bonds are still alive, even if I’m not able to hold them every morning.”

The kids beamed as Ms. Silva draped herself and each student in plastic with a surgeon’s precision. Then she wrapped her arms around each one and lifted them off the ground for a long, tender embrace.

Yasmim Vitória de Oliveira said she missed the museum outings and classroom pajama parties that Ms. Silva used to organize.

“She’s playful and she lets us have fun,” the 9-year-old said.

Ms. Silva said that once the pandemic passed, she would hug her students with abandon, never again taking for granted the healing power of touch.

“In a moment of tragedy, we’ve been able to share moments of love,” Ms. Silva said. “That is very powerful.”