Rise of coronavirus variants will define the next phase of the pandemic in the U.S.

The bottom line on all three remains positive. In laboratory tests, vaccines are just as effective against the variant identified in the United Kingdom as they are against the original strain of the virus. And there is only a modest drop-off in their effectiveness against two others.

“These variants emerged because we continued to give the virus more chance to spread,” said David D. Ho, whose lab at Columbia University is leading the research on the P. 1 variant first discovered in Brazil. “The sooner we vaccinate everyone, the faster we will contain the viral spread and reduce the chance for new variants to emerge.”

But the overall picture hides problems in some places. One or more of the variants — which also cause more severe disease than the original version of the virus — are racing through the Northeast and the Midwest. That has prompted officials in some communities to ask for more vaccine than they would receive under the government’s population-based formula. Officials in the Northwest are watching a major outbreak of the P. 1 variant in British Columbia.

“We need more vaccine down here on the Cape, now,” said Bruce G. Murphy, health director for the town of Yarmouth, Mass., on Cape Cod, which has 82 active cases among its population of just more than 23,000.

Mass-vaccination centers in the Boston area are little help to Yarmouth residents, especially older people who cannot travel, Murphy said.

“If we see 13 [cases], that could be just the tip of the iceberg coming in,” he said.

Michigan also has asked for more vaccine to control an astonishing surge during which its average daily coronavirus caseload rose from 1,503 on March 7 to 7,020 Wednesday, according to CDC data analyzed by The Washington Post.

Asked whether the administration would ever change its strategy, Andy Slavitt, senior adviser to the White House coronavirus response team, said Wednesday that the government already is able to move vaccine supply from other parts of a state to harder-hit areas.

“We are getting the amount of vaccines we think are needed for the population because that’s fundamental,” Slavitt said. “And then we are working on very tactical areas . . . how to maximize that vaccine distribution so we get the things we want: efficiency, health equity and the other goals that we have.”

In addition to those three variants, the CDC considers two in California “variants of concern” and is watching them closely. It is also monitoring a variant found in New York City.

With most of the rest of the world far behind on immunizations, the virus will continue to spread and mutate, every copy with the potential to spark a variation that current vaccines will not be able to control. The odds of that remain low, experts think, but they are not zero.

“I fear there will be one terrible variant that will come out and take us back to square one,” Ho said.

But experts said vaccines can be boosted, and new ones developed, to address any variants that emerge.

As of April 6, there were 16,275 cases of B.1.1.7, 386 of B. 1.351, first discovered in South Africa, and 356 of P. 1 in the United States. Experts think the actual numbers are much larger, but because the U.S. effort to sequence the genomes of the virus lags, it is difficult to know how widespread the variants truly are.

“The landscape is this big explosion of U.K., the worrisome uptick in P. 1 and then other strains as well,” said Daniel Jones, vice chairman of the Division of Molecular Pathology at the Ohio State University Wexner Medical Center. Within a few weeks, he said, there will be enough infections in the United States to gather better data on immune response from people, rather than through tests in a laboratory.

Lab tests usually involve looking at one mutation at a time; the variants circulating can have many, said Stephen Kissler, an immunology expert at the Harvard T.H. Chan School of Public Health.

And “the human immune system is much more complex than we can emulate in a petri dish, so we can only get partial information,” he said.

Found in more than 24 countries and 25 U.S. states, P. 1 is thought to be up to 2.2 times as contagious and 61 percent more infectious than the original form of the virus. In other words, it produces a bigger viral load, which probably makes people sicker.

Some experts have wondered whether competing variants have slowed its spread. Northern California, for example, is dominated by B.1.427/B.1.429 cases, while Southern California has a higher share of B.1.1.7.

“We are seeing different variants in different parts of the country competing to take over populations,” said Benjamin Pinsky, medical director of the Stanford Clinical Virology Lab.

For now, B.1.1.7 appears to be dominant. According to the latest data from Helix, a company that is working with the CDC to track variants, B.1.1.7 comprises about 50 percent of all sequenced cases in the United States.

“My feeling is if you went in a boxing ring with all the variants, the U.K. variant is going to win in terms of transmissibility,” said Peter Chin-Hong, an infectious-disease expert at the University of California at San Francisco.

Immune response to the P. 1 type “is going to be better than the South African variant but probably not as good as the U.K. variant,” Ho said.

The variants discovered in California appear to be potentially more benign than the others. Several states with B.1.1.7 surges have reported unusual outbreaks among school-age children that spreads to their communities, but that hasn’t been the case for the California variants.

“We’ve dropped hospitalizations really low” in areas where B.1.427/B.1.429 is becoming dominant, Chin-Hong said. “You’d expect to see more cases among kids and spring breakers, but we haven’t seen that. It suggests it’s not marking its territories by making younger people sicker.”

In Canada, public health officials have been struggling with the largest P. 1 variant surge outside Brazil. There were more than 787 cases in British Columbia as of Tuesday, and a small but growing number in the adjacent province of Alberta. It has sickened young skiers at the Whistler Blackcomb resort and workers at an energy company. Twenty-one players and four staff members of the Vancouver Canucks hockey team have been infected by a variant of the virus, although the team has not determined which one.

Deena Hinshaw, chief medical officer for Alberta, tweeted Monday that her team was investigating a P. 1 outbreak linked to “a large employer” that began with a traveler who returned from a trip out of province. She said there was a separate workplace outbreak in a different part of the state.

Hinshaw said that the province had identified 887 new cases over the previous 24 hours and that 39 percent of active cases are now variants.

In Connecticut, the variant first discovered in the United Kingdom has helped give 20- to 29-year-olds the highest case rate of any age group in the state. Whether that reflects properties of the variant, relaxed precautions among young people or vaccinations that are protecting older people — or some combination of the three — is not clear.

Another variant, originally detected in India, has been making headlines in California this week as a “double mutant” because it contains characteristics from the variants first discovered in California and Brazil/South Africa.

Described by Indian researchers on March 24, it was first identified in the United States March 25 in a patient in the San Francisco Bay area.

Pinsky said “it’s too soon to tell how this variant will act. We know how these individual mutations change the virus but we haven’t been able to see how they behave in combination.”

Brazil, now fully in the grip of the P. 1 variant, shows how quickly it can seize control when not taken seriously. In the Amazon rainforest, it quickly devastated the city that spawned it, then stormed south. It was soon prevalent all over the country — and its assault on Brazil, now in the midst of a nationwide medical failure, has been merciless ever since.

Nearly 67,000 Brazilians died of covid-19 — the disease caused by the virus — in March, more than double any other month during the pandemic. The highest death toll — 4,195 — was recorded Tuesday. Scientists haven’t proved that the variant is more deadly. But physicians and health officials in Brazil say it has signaled a darker and deadlier phase of the pandemic.

The vaccination rate in Brazil is lagging far behind the rate in the United States.

Patients are arriving much sicker — and much younger.

“The P. 1 variant countrywide is clearly contributing to the increase,” Sylvain Aldighieri, a senior official with the Pan American Health Organization, told reporters last week. “We are also receiving signals and reports in different states that young adults are hospitalized in higher numbers.”