Although Vice President Kamala Harris was infected by the coronavirus in April, it’s not impossible that she could become infected again — a prospect raised by her being with President Biden just two days before he tested positive.

According to her schedule for that day, Ms. Harris received the president’s daily briefing in the Oval Office. She also met with Olena Zelenska, the first lady of Ukraine, with Mr. Biden and others at the White House.

The prospect of both Ms. Harris, 57, and Mr. Biden, 79, contracting the virus could be unsettling, but both are fully vaccinated and have received two booster shots, which remain highly protective against severe disease. Ms. Harris received her second booster on April 1, and Mr. Biden on March 30.

Ms. Harris, who traveled to North Carolina on Thursday, said that she had spoken with Mr. Biden by phone and that he was “in good spirits.”

According to a White House official, Ms. Harris tested negative Thursday morning. On the advice of the White House medical team, she will remain masked, but her schedule will continue as planned.

She may not be in the clear. To account for the virus’s incubation period, many experts recommend taking a rapid test two to four days after a potential exposure, and taking at least two rapid tests about a day apart.

Ms. Harris previously tested positive for the coronavirus on April 26. In announcing her positive test, her office said she was not experiencing symptoms and would isolate at home. At that time, Ms. Harris was already fully vaccinated and had received two booster shots. She was prescribed the antiviral treatment Paxlovid.

At the time of her infection, the vice president had not been in close contact with Mr. Biden. She had spent several days in California and had not seen the president in person in eight days.

A month before that, Doug Emhoff, Ms. Harris’s husband, tested positive for the virus. Ms. Harris was not infected then, but while she isolated and continued to test negative, Mr. Emhoff’s positive test forced her to cancel an appearance at an event with Mr. Biden.

It is possible the vice president could contract the virus again. The antibodies that help protect against infection wane over time, and Omicron is more adept at dodging those antibodies than previous variants were.

Even a previous Omicron infection may not protect against a subsequent one. Although it is not clear what version of the virus Ms. Harris had in April, at the time, the BA.2 subvariant of Omicron was the dominant version in the United States.

Now, the BA.5 subvariant, which has spread even more quickly than previous versions, is causing a new surge of cases, including a spate of reinfections. The Centers for Disease Control and Prevention estimated last week that BA.5 was accounting for nearly 80 percent of new infections across the U.S.

“You can be previously infected — even as recent as the last couple of months — and have a very high rate of reinfection,” Dr. Ashish K. Jha, Mr. Biden’s coronavirus response coordinator, said in an interview last week.

A recent study from Qatar, which has not yet been reviewed by outside experts, suggests that BA.4 and BA.5 are better at evading antibodies from previous coronavirus infections than earlier versions of Omicron were.

However, people who were infected with an earlier version of Omicron should be better protected than those infected with other variants of the virus. According to the Qatar study, an infection with a pre-Omicron variant was 28 percent effective at preventing a subsequent infection with BA.4 or BA.5. A previous Omicron infection, however, was 80 percent effective at preventing a BA.4 or BA.5 infection.

Russia’s decision to restart the flow of natural gas through a vital pipeline on Thursday brought a moment of relief to Germany, which uses the fuel to power its most important industries and heat half its homes. But it is unlikely to be much more than that.

President Vladimir V. Putin of Russia has made clear that he intends to use his country’s energy exports as a cudgel, and even a weapon, to punish and divide European leaders — loosening or tightening the taps as it suits him and his war aims in Ukraine.

He is counting on that uncertainty to impose heavy economic and political costs on European leaders. Those elected officials are under growing pressure to bring down energy prices and avoid gas rationing that might force factories and government buildings to close and require people to lower thermostats in winter. Leaders in some nations, like Spain and Greece, are already chafing at a European Union plan to have every member country cut its gas use, arguing that they are already much less reliant on Russia than Germany.

Many questions remain about the stability of the gas supplies that began flowing again on the pipeline, the Nord Stream 1, which directly connects Russia and Germany. But, analysts said, it is clear that Europe, and Germany in particular, could remain on edge for months about whether there will be enough energy.

In the weeks leading up to the 10-day shutdown for planned maintenance that just ended, Gazprom, Russia’s state-owned energy monopoly, had already reduced flows through the pipeline to 40 percent of its capacity. Analysts have warned that such levels will not be enough to prevent an energy crisis next winter.

“The resumed gas supplies from Russia via Nord Stream 1 are no reason to give the all clear,” said Siegfried Russwurm, president of the Federation of German Industries. “It remains to be seen whether gas will actually flow in the long term and in the amount contractually agreed.”

He added, “Germany and Europe must not become the plaything of blackmailing Russian politics.”

On Wednesday, Ursula von der Leyen, president of the European Commission, who previously held senior positions in the German government, introduced a proposal for European Union members to reduce their gas consumption 15 percent to prepare for uncertain and possibly unsteady supply before the winter.

Before Russian forces invaded Ukraine in late February, Germany got 55 percent of its natural gas from Russia. Few E.U. countries come close to that level of dependence — a fact that is starting to fracture European unity on Russia and energy policy.

Many Europeans already think Germany, the bloc’s largest economy, is a wealthy neighbor that is not always eager to help weaker countries. That characteristic was most recently highlighted by the country’s attitude toward helping Greece, Spain and other countries that use the euro when they were struggling financially about a decade ago.

Now, some of those very same countries are signaling that they are unwilling to make their businesses and people endure more suffering when energy prices are soaring to help bail Germany out of its dependence on Russia.

The Spanish energy minister, Teresa Ribera, said on Thursday that her country would encourage but not require its citizens to cut gas use. “Unlike other countries, we Spaniards have not lived beyond our means from an energy point of view,” she told El País newspaper, echoing the description some German ministers used during the eurozone crisis.

The Greek government has also pushed back against the European Union’s call for a 15 percent cut in gas use. Although Greece relies on Russia to meet 40 percent of its gas needs, its supplies have not been cut.

Stoking such divisions is at the heart of Mr. Putin’s strategy of cutting off gas deliveries through pipelines that cross Ukraine and Poland while limiting the volume of natural gas flowing under the Baltic Sea through the 760-mile Nord Stream 1 pipeline.

“The entire European energy system is going through a crisis, and even with today’s restart of Nord Stream 1, the region is in a tight position,” Rystad Energy, a research firm, wrote in a market note on Thursday.

Mr. Putin appears to be drawing out the uncertainty about whether and for how long the gas will keep flowing to Germany to try to maximize his leverage as long as he can.

Hours before the flow of gas resumed on Thursday, Gazprom said in a statement that it still had not received documents from Siemens for a turbine that was sent to Canada for repairs. The papers are necessary for the part to be returned, the company said, adding that the engine, and others like it, had “a direct influence on the operational safety of the Nord Stream pipeline.”

Robert Habeck, Germany’s economy minister and vice chancellor, rejected a statement from Gazprom earlier in the day that the resumption of gas through the pipeline was proof that the Russian company was a “guarantor” of energy security in Europe.

“The opposite is the case,” Mr. Habeck said. “It is proving to be a factor of uncertainty.”

The German government has already activated the second of three steps of its gas emergency plan. Included was the swapping of gas-fired power plants with ones that burn coal, which releases many more greenhouse gases into the atmosphere than burning gas. The third and final step would allow the government to ration supplies.

On Thursday, Mr. Habeck announced additional measures aimed at increasing the country’s gas reserves, like conservation incentives that include more ambitious targets for the storage facilities and reactivating power plants that burn lignite — the dirtiest fossil fuel.

He said the government was also weighing strict limits on how people could use gas. For example, the government might forbid people to heat private swimming pools with gas. When pressed on how such measures would be enforced, Mr. Habeck drew a parallel to bans on holding private gatherings during the initial lockdowns of the coronavirus pandemic, which were rarely enforced — unless neighbors alerted the authorities.

“I do not think that the police will visit every homeowner. That is not our intention and not the country I want to live in,” Mr. Habeck said. “But if it was pointed out that someone is not going along with it, then we would certainly look into that.”

Whether Germans, who were among the Europeans most willing to follow public health rules in 2020 when the pandemic started only to rebel months later, will be willing to sacrifice their comfort in solidarity with Ukraine has yet to be fully put to the test.

The German government is facing what Janis Kluge, an analyst on Russia with the German Institute for International and Security Affairs in Berlin, called “a very delicate balance” in how it communicates with the public.

“On the one hand, they have to mobilize everybody to save energy, to save gas and tell everybody that there could be an energy emergency in the winter, while at the same time avoiding that this turns into criticism about the sanctions policy and support for Ukraine,” he said.

“This is exactly what Putin wants to achieve,” Mr. Kluge added. “That when we make the next decision about arms deliveries to Ukraine, that somewhere in the back of our heads, there is this thought, well, what is this going to do to our gas deliveries?”

Berlin has been scrambling to buy more gas from the Netherlands, Norway and the United States. The government has set aside 2.94 billion euros, about $3 billion, to lease four floating terminals in the hopes that they will be operating by midwinter to help ward off a crisis that threatens a recession.

For years, Germany ignored warnings from its neighbors and allies that it was making itself vulnerable by becoming increasingly dependent on Russia for its energy needs.

“Germany will become totally dependent on Russian energy if it does not immediately change course,” President Donald J. Trump said at the United Nations in 2018.

In response, the German delegation, which included the country’s foreign minister, Heiko Maas, laughed.