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“The Trump administration’s decision to hire a private vendor and then cloak that vendor in a nondisclosure agreement raises numerous questions about their motivations and risks the ability of our public health experts to understand and effectively fight this virus,” Mr. Schumer said in a statement Friday.

The controversy over the contract stems from the administration’s abrupt order in July for hospitals to stop reporting coronavirus information to the C.D.C.’s National Healthcare Safety Network — a longstanding government data system — and instead send it to TeleTracking for inclusion in a coronavirus database overseen by H.H.S. officials in Washington. H.H.S. has said the switch was necessary because the C.D.C.’s system was slow and incomplete; the government uses the hospital data to make critical decisions about how to allocate scarce supplies, like ventilators and the drug Remdesivir.

The contract — and in particular the sudden switch in reporting from C.D.C. to TeleTracking — generated objections from public health experts and outside advisers to the health agency, who say that the new system is burdening hospitals and endangering scientific integrity by sidelining government experts.

TeleTracking is majority owned by its chairman and chief executive, Michael Zamagias, a Pittsburgh real estate developer.

Ms. Tillipman said it is also unusual for the government to keep the names of bidders a secret.

It’s only the latest sign of upheaval at the Post Service prompted by the pandemic and the new postmaster general, Louis DeJoy, who is a Republican megadonor, an ally of President Trump and now under investigation by the Post Service’s inspector general.

As many states turn to vote-by-mail operations to carry out elections safely amid the coronavirus pandemic, Thomas J. Marshall, general counsel for the Postal Service, warned that “certain deadlines for requesting and casting mail-in ballots are incongruous with the Postal Service’s delivery standards.”

Mr. Marshall urged the states — including electoral battlegrounds like Pennsylvania, Florida and Michigan — to require residents request ballots at least 15 days before an election rather than just the four days allowed under some state laws.

The Palm Beach County elections department in Florida has prepared mail ballots for this Tuesday’s primary election for the president and the first lady, according to local registration records.

How and where a president votes is typically a footnoted formality. But Mr. Trump has broadly questioned the legitimacy of voting by mail, without evidence of significant voter fraud, and made statements suggesting that he views expansion of mail voting as a threat to the Republican Party.

It was a remarkable addition to the body of guidance from the agency, and its first indication that immunity to the virus may persist for at least three months. Scientists have said that it could last even longer but no definitive evidence exists yet. Even so, the C.D.C. still recommends physical distancing, mask-wearing and other precautions.

Doctors have reported some cases of people who seemed to be infected a second time after recovery, but experts have said those are more likely to represent a re-emergence of symptoms from the initial bout.

“There is no evidence to date of any reinfection within 90 days of the initial diagnosis,” said a C.D.C. spokesman.

Researchers at a dozen clinical trial sites said that testing delays, staffing shortages, space constraints and reluctant patients were complicating their efforts to test monoclonal antibodies — man-made drugs that mimic the molecular soldiers made by the immune system.

And Eli Lilly’s chief scientific officer, who had said in June that its antibody treatment might be ready in September, said this week that he now hopes for something before the end of the year.

“Of course, I wish we could go faster — there’s no question about that,” said the executive, Dr. Daniel Skovronsky. “I guess in my hopes and dreams, we enroll the patients in a week or two, but it’s taking longer than that.”

One major hurdle has been testing. According to the rules of the Regeneron trial, a patient must be treated with the antibodies within seven days of the onset of symptoms. Both the Regeneron and Eli Lilly trials require giving the drug within three days of taking a test that comes back positive, but with turnaround times in some areas lagging for five days or more, keeping within those time frames has proved difficult.

A study by researchers in South Korea last month suggested that children ages 10 to 19 spread the coronavirus more frequently than adults — a widely reported finding that influenced the debate about reopening schools.

The incident — just the latest example of science about the virus unfolding in front of our eyes — underscores the need to consider the preponderance of evidence, rather than any single study, when making decisions about children’s health or education, scientists said.

The disclosure does not negate the overall message of that study: Children under 10 do not spread the virus as much as adults do, and the ability to transmit seems to increase with age.

“It’s indisputable that the highest risk of becoming infected and being detected as being infected is in older age groups,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “I think you have to be really careful before you decide to open high schools.”

The earlier study was not intended to demonstrate transmission from children to adults, only to describe contact-tracing efforts in South Korea, said Dr. Young June Choe, assistant professor of social and preventive medicine at Hallym University College of Medicine and an author of both studies.

But Dr. Choe and his colleagues reported an odd finding in the group of 124 children aged 10 to 19: They appeared significantly more likely than adults to spread the coronavirus. Experts told The Times at the time that the finding was likely to be a fluke.

In the latest study, the researchers found only one undebatable case of transmission among older children, from a 16-year-old girl, who had returned from Britain, to her 14-year-old sister. The remaining infected contacts of the older children could all be explained by a shared exposure.

In detailing the base’s health capabilities, Matthew P. Donovan, an under secretary of defense, said the detention operation had acquired the ability do rapid on-site testing in addition to airlifting samples to U.S. military labs.

The military acknowledged two cases of the virus on the base, in March and April, before the Pentagon ordered installations to stop disclosing any new cases for “operational security” reasons.

The prison has the capacity to isolate each of the 40 prisoners, including with space for four in regular inpatient rooms, two in intensive care and two more in rooms that have negative pressure and can control the flow of infectious particles.

Doctors who looked at the capabilities noted that the prison’s Covid-19 Care Team lists only four I.C.U. nurses, far below a standard of care that requires one such nurse per ventilated patient around the clock.

This article was produced in partnership with the Pulitzer Center on Crisis Reporting.

France on Friday declared Paris and the Marseille region in its southeast to be high-risk zones, granting the local authorities powers to restrict the movements of people and vehicles, limit access to public transportation and public buildings and close restaurants and bars.

Britain added France to its list of countries that visitors arriving from must quarantine for two weeks. Britain also added the Netherlands, where cases have doubled every two weeks since early July, along with Aruba, Malta, Monaco and Turks and Caicos.

Authorities in Britain unveiled the expanded list with little more than a day’s notice, prompting an instant scramble from vacationers to return there before the quarantine is imposed at 4 a.m. on Saturday.

France’s rising caseload reflected not only an increase in the number of tests, which stand at more than 600,000 per week, but also a higher infection rate, especially among young people, the health authorities said. The country’s total caseload has risen to 209,365, with 30,388 deaths, according to the Times database.

In other news from around the world:

• North Korea lifted a lockdown it had imposed last month on the city of Kaesong, near its border with South Korea, on government suspicions that a runaway from South Korea had brought the virus with him. The North said the reversal was “based on the scientific verification and guarantee by a professional anti-epidemic organization” but without saying whether the nation has a coronavirus outbreak. Its leader, Kim Jong-un, has said it is facing “twin perils” — the virus and flooding from an unusually long monsoon season.

• Spain ordered bars and clubs to close by 1 a.m. and banned drinking on the street on Friday, according to Reuters. Virus cases have risen steadily since July when the country emerged from a strict lockdown that only allowed residents to leave their home to walk their dog or grocery shop. Spain reported more than 5,400 new cases on Friday, according to a New York Times database.

• President Rodrigo Duterte of the Philippines has delayed opening schools until Oct. 5, his chief aide said. The Philippines has the highest number of infections in Southeast Asia, with 153,660 confirmed cases and 2,442 deaths, according to the Times database.

• Health officials in Toronto said that about 550 people may have been exposed to the coronavirus at a strip club bar after an employee tested positive for the virus. The occupation of the infected employee was not disclosed.

• Vietnam’s health ministry announced that it had registered to buy Russia’s coronavirus vaccine, despite experts’ concerns that the Kremlin is distributing it before the last phase of human trials have even begun. The ministry said it had also registered to buy a vaccine from the United Kingdom. It cautioned that using the vaccines would depend on the progress of clinical trials and compliance with Vietnam’s “strict regulations.

U.S. Roundup

Obesity alone, apart from accompanying health problems, adds to Covid-19 risks for men.

Various factors are known to increase the risk of severe Covid-19, including older age and chronic health conditions like high blood pressure and heart disease. The Centers for Disease Control and Prevention also lists extreme obesity as a high risk.

But is the excess weight to blame? Or is it the health problems that accompany obesity, like metabolic disorders and breathing problems?

Among female Covid-19 patients, body mass index — a measure of body fat based on height and weight — does not appear to be independently associated with an increased risk of dying at any age, the authors said, possibly because women carry weight differently than do men, who tend to have more visceral and abdominal fat. The study was published in Annals of Internal Medicine.

“Body mass index is a really important, strong independent risk factor for death among those who are diagnosed with Covid-19,” said Sara Tartof, the study’s first author, a research scientist at Kaiser Permanente of Southern California.

In other news from around the United States:

• Georgia Gov. Brian Kemp is expected to sign an executive order on Saturday allowing local governments to issue mask ordinances, according to The Associated Press, which cited the Atlanta Journal-Constitution. The governor has staunchly opposed issuing a statewide mask ordinance — even suing Mayor Keisha Lance Bottoms for signing a mask order in Atlanta. Mr. Kemp’s executive order is expected to give businesses the option of enforcing local mandates and comes after a “strong recommendation” from the White House coronavirus task force. Officials announced 3,319 new cases in Georgia today, according to a New York Times database.

• An Arizona school district that had planned to restart in-person classes next week in defiance of the state’s health benchmarks abruptly reversed course on Friday after staff members staged a “sick out” in protest. The state’s Republican governor, Doug Ducey, has recommended that in-person classes not resume until counties have met a number of targets, including going two weeks with Covid-19 positivity tests below 7 percent. The J.O. Combs Unified School District in greater Phoenix had planned to ignore the advice, but said on Friday evening that all instruction on Monday would be canceled because of “insufficient staffing.”

• The five metropolitan areas that have the highest rate of new coronavirus cases relative to their population are all in South Texas, according to data compiled by The Times.

• The Department of Homeland Security announced an extension of the U.S. agreement with Canada and Mexico to limit nonessential travel through Sept. 21. It was the fifth extension since the measure was put in place in March.

• A federal judge dismissed a lawsuit by an Arizona woman who claimed New York’s 14-day quarantine requirement for travelers from hot spot coronavirus states infringed on her “fundamental right to travel.” This was the second challenge to Gov. Andrew Cuomo’s quarantine order to be thrown out by a federal judge, The Associated Press reported.

• Gov. Gretchen Whitmer of Michigan said that the state would give four million free face masks to homeless shelters, tribal organizations, community health centers, schools and grocery stores. The effort, which targets people particularly vulnerable to the virus, includes one million masks provided by the Federal Emergency Management Agency and 1.5 million masks donated by Ford Motor.

• At least 22 workers at a remote mining camp in Alaska have been infected with the virus, forcing a temporary stoppage to underground work. The Kensington Mine, about 45 miles north of Juneau, has about 200 to 250 workers on site at a time, said a spokeswoman for its owner, Coeur Alaska. The state has embraced a broad reopening but implemented protocols to keep infections in check, and though Alaska has had a spike in reported infections this summer, its numbers remain low compared with other states’.

The health plan said in an email sent to members on Wednesday that it would raise the floor for eligibility from those earning $18,040 a year to $25,950, effective Jan. 1. Premiums will also increase.

“The Trustees of the SAG-AFTRA Health Plan have taken a difficult but necessary action to address financial deficits facing the plan,” the Screen Actors Guild-American Federation of Television and Radio Artists (SAG-AFTRA) Health Plan said in a statement.

At a time when film jobs are scarce and live theater is almost completely shut down because of the pandemic, many actors have seen their income slow to a trickle or disappear entirely.

SAG-AFTRA represents approximately 160,000 actors, singers, journalists and other media professionals in film and television. Membership is considered a rite of passage that can lead to more prestigious work.

According to the email, the changes are in response to projected deficits of $141 million this year and $83 million in 2021. The health plan estimated that, without the changes, it would run out of reserves by 2024.

In other U.S. cultural news:

A virtual cottage industry of companies and consultants has emerged to help families organize these small-group, in-home instruction pods and pair them with instructors, many of whom are marketing themselves on Facebook pages and neighborhood email lists.

But the cost — often from $30 an hour per child to $100 or more — has put them out of reach for most families, generating concerns that the trend could make public education even more segregated and unequal.

In Washington, D.C., one parent started a GoFundMe page to raise money to subsidize learning pods for low-income students in the district.

Education experts say fund-raising efforts and “pod scholarships,” however well meaning, are no solution for millions of low-income parents juggling the educational, child care and economic challenges of the pandemic.

More useful, they say, would be if school districts or city governments created their own version of learning pods, especially for at-risk students or children of essential workers.

In other education news:

• Barnard College and Columbia University said that all undergraduate classes would be held remotely for the fall semester and that student housing would be mostly closed. The announcement came days before students were to move into dormitories.

• The president of Villanova University in Pennsylvania has warned students that they will be sent home if they are caught disregarding the school’s coronavirus protocols, which include wearing a mask “at all times” and social distancing. Videos that appear to show a gathering of dozens of new Villanova students recently drew backlash on social media.

Retail sales rose 1.2 percent from June, the Commerce Department reported. Though smaller than increases in the previous two months, the jump in sales showed that the bounceback in spending to pre-pandemic levels was not a fluke. Sales have returned to their February level, a sign that consumerism, buoyed by government support, remains resilient even as many other facets of American life are increasingly bleak.

“It shows there is a willingness and a desire to spend,” said Michelle Meyer, chief U.S. economist at Bank of America. “There is no doubt the recovery in consumer spending has been robust.”

Retail sales in June rose 8.4 percent. That followed a May jump, 18.2 percent, which was the largest monthly surge on record. But that had followed two months of record declines.

But some of the recovery was helped by the $600 a week in unemployment assistance that expired at the end of July, and Congress’ failure so far to extend the emergency benefit could derail the retail rebound in coming months. And certain sectors of the industry may not truly bounce back until a vaccine is approved and widely distributed, allowing people to shop and dine indoors again without fear.