The Omicron BA.2 variant is increasing its share of U.S. cases and putting particular pressure on parts of the Northeast, where wastewater readings and Covid-19 cases have ticked upward again from low levels.

Recorded Covid-19 cases remain low nationally and hospitals are treating the smallest number of Covid-19 patients after any surge. Places where cases are rising, including New York, are reporting relatively small increases from recent troughs. The mixed signals are prompting officials to warn that BA.2 will likely lead to more viral transmission, but also hope among public-health experts that warming weather and built-up immunity in the population are muting the variant’s impact.

“There are certainly some big factors that are in our favor right now compared to the situation back in November and December,” said Bruce Y. Lee, professor of health policy and management at the City University of New York School of Public Health.

The pressure point is BA.2, a version of the Omicron variant, which continues to take over in the U.S., according to new estimates from the Centers for Disease Control and Prevention this week. The subvariant represented an estimated 72% of national cases by the week ended April 2, and around 84% in both New England and a region including New York and New Jersey, the CDC estimates show.

An earlier version of Omicron hit the U.S. late last year and fueled a record number of cases and hospitalizations, as well as the second-highest peak in daily death reports since the pandemic began.

Public-health officials say they are concerned about BA.2 because it appears to spread even more easily than the Omicron variant detected earlier, though evidence thus far hasn’t suggested differences in the severity of cases. The subvariant has recently triggered surges in Europe, where pandemic trends have tended to foreshadow patterns in the U.S.

In the U.S., some wastewater-sampling sites used to detect how much virus is circulating in the population have recorded increases recently. The latest data from Biobot Analytics Inc., which tests wastewater samples for hundreds of treatment sites around the U.S., show readings from the Northeast in particular have been edging higher from low levels.

“The amount of virus in wastewater has been a pretty reliable indicator of what is likely to come,” said Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women’s Hospital in Boston.

Some parts of the Northeast are recording higher case counts, even as more at-home testing makes it harder for health departments to know when people test positive. Seven of 10 states with the highest rates of new cases per 100,000 are in the Northeast, including most New England states, according to the CDC.

A Biobot Analytics lab in Cambridge, Mass., where wastewater is tested to detect how much of the virus is circulating in the population.

Photo: Allison Dinner/Reuters

New York City’s daily case average over the past seven days is about 1,400, compared with about 1,000 over the past 28 days. City officials on Friday reiterated their recommendation that people wear masks indoors and said they would distribute more than six million home tests at schools, houses of worship and community centers.

Nationwide, the seven-day average for newly reported cases is about 26,000 a day, according to Johns Hopkins University, down from recent weeks where the average hovered closer to 30,000. National counts of newly admitted Covid-19 patients also recently fell to the lowest level after any surge.

CDC Director Rochelle Walensky on Tuesday said there are a handful of counties where data show rising cases and evidence of severe disease, such as hospitalizations.

“The high level of immunity in the population from vaccines, boosters and previous infection will provide some level of protection against BA.2,” Dr. Walensky said at a White House Covid-19 briefing. “However, we strongly encourage everyone to be up-to-date on their Covid-19 vaccines.”

Public-health and virus experts expect BA.2 will drive cases higher nationally, but they say they hope the U.S. will avoid a major surge. While Omicron hit just as winter weather and end-of-year holidays drew people together indoors, where the virus spreads most easily, BA.2 is spreading in the spring, when people tend to spend more time outdoors.

SHARE YOUR THOUGHTS

Do you think sewage monitoring will help communities respond more quickly to future Covid-19 outbreaks? Why, or why not? Join the conversation below.

The winter surge also likely left a large chunk of the U.S. with increased resistance, because BA.2 is similar to the coronavirus version that recently infected millions of people. “It does seem that there’s generally pretty good protection,” said Melissa Maginnis, an associate professor and virologist at the University of Maine.

Hackensack Meridian Health in New Jersey has contracted with temporary staffing agencies to have extra workers on standby in case hospitalizations rise rapidly with BA.2, said the system’s chief executive, Robert Garrett. Covid-19 hospitalizations among patients in the system climbed slightly in the past week to about 100 from a prior low of 90.

That is far below the Omicron peak of 1,400 Covid-19 patients reached in January across Hackensack Meridian’s hospitals. “I am feeling cautiously optimistic,” Mr. Garrett said.

—Melanie Evans and Jimmy Vielkind contributed to this article.

Write to Jon Kamp at jon.kamp@wsj.com and Brianna Abbott at brianna.abbott@wsj.com