• The BA.2 subvariant, which spreads 80% faster than the earlier omicron, has more than doubled in the U.S. over two weeks and will become the dominant variant.
• However, White House chief medical advisor Dr. Anthony Fauci said he doesn't expect another surge right now, though cases might rise.
• Ali Mokdad, a leading epidemiologist, said he expects cases to decline over the spring and summer, with a surge possible this winter.
• "The pandemic phase of the virus is over in our opinion," Mokdad said. "We are moving into an endemic phase."

Omicron's more contagious subvariant, BA.2, has more than doubled in prevalence over the past two weeks in the U.S. and now represents more than 34% of Covid-19 infections that have undergone genetic sequencing, according to data published by the Centers for Disease Control and Prevention this week.

BA.2 has been steadily growing as a proportion of the Covid variants circulating in the U.S. since Feb. 5, when it represented about 1% of genetically sequenced virus samples, according to the CDC. BA.2 probably already accounts for 50% of new infections in the U.S. because many people are taking tests at home that aren't picked up in the official data, according to Ali Mokdad, an epidemiologist at the Institute for Health Metrics and Evaluation at the University of Washington.

Data from Walgreens, which conducts testing at its pharmacies nationwide, shows BA.2 as the dominant variant, at 51% of all positive Covid cases for the week ending March 19.

Though BA.2 is rising in the U.S., leading public health officials are not expecting another dramatic surge in new cases, largely due to the level of immunity the population has from vaccination and the fierce outbreak during the winter omicron wave.

"The bottom line is we'll likely see an uptick in cases, as we've seen in the European countries, particularly the U.K.," White House chief medical advisor Dr. Anthony Fauci told ABC's "This Week." "Hopefully we won't see a surge — I don't think we will."

In the U.K., the number of people testing positive for Covid has jumped 16% over the past week, according to government data. The number of patients admitted to hospitals with the virus is also up about 20%. BA.2 now represents about 44% of all positive cases in London as of March 10, according to the U.K. Health Security Agency.

However, Mokdad said the situation in the U.S. is different from that in European countries, because there was much more infection from omicron here over the winter. European nations have also dramatically changed their behavior in recent weeks by lifting restrictive public health measures, which has led to the spike. In many parts of the U.S., on the other hand, restrictive measures were not implemented during omicron, so there's not as dramatic a change in behavior to drive new infections, Mokdad said.

In the U.S., new infections are down 96% from the pandemic record of more than 800,000 on Jan. 15, according to a CNBC analysis of data from Johns Hopkins University. However, the speed of the decline has slowed and new cases appear to have plateaued at a seven-day average of around 31,000 new infections daily. The number of hospital admissions of patients with Covid has dropped 90% from the peak of the omicron wave in January, according to the CDC.

Mokdad said that though he expects BA.2 to represent more than 80% of new cases in the coming months, the variant's doubling time has slowed recently. IHME is projecting that cases will continue to decline through the spring and summer, with another surge possible this winter, when immunity has started to wane substantially.

"The pandemic phase of the virus is over in our opinion," Mokdad said. "We are moving into an endemic phase."

Public health officials in England have found that the BA.2 subvariant is growing 80% faster than the earlier version of omicron, BA.1, according to a briefing paper published earlier this month. World Health Organization epidemiologist Maria Van Kerkhove has described BA.2 as the most transmissible Covid variant so far and said it's sweeping the world. The subvariant now represents more than 80% of sequenced Covid samples worldwide, according to an international database.

Due to vaccination and infection, an estimated 95% of the U.S. population ages 16 and older had developed antibodies against the virus as of late December 2021 before the omicron wave peaked, according to a CDC survey of blood donor samples. Mokdad said this level of immunity puts the U.S. in a good place until winter, when protection will start to wear off.

The antibodies induced from the vaccine decline after about three months, which can lead to breakthrough infections, though the shots still protect against severe illness. Younger healthy people who have recovered from Covid have immunity for at least six months, according to peer-reviewed studies in Denmark, the U.K. and the U.S. Though these studies were published before omicron, scientists in Qatar recently found that infection 10 months earlier provided about 46% protection against illness from BA.2 in people who weren't vaccinated. However, the elderly and people with compromised immune systems are much more vulnerable to reinfection.

BA.2 does not make people more sick than BA.1, which was less severe than the delta variant, according to a large real-world study from South Africa's National Institute of Communicable Diseases. Reinfection with BA.2 — though possible — appears rare, according to a February study from Denmark's Statens Serum Institut in Copenhagen. Public health authorities in the U.K. have come to the same conclusions on hospitalization and reinfection. Neither study has been peer reviewed yet.

"The fact that there are similar clinical manifestations of BA.1 versus BA.2 gives me a little bit of hope that it's not going to completely change the game on us in the same way that omicron changed the game from delta," said Jennifer Nuzzo, an epidemiologist at Johns Hopkins University.

There's also no significant difference in the duration of protection that Pfizer's and Moderna's shots provide against mild illness from BA.2 compared with BA.1, according to a study published this month by scientists based in Qatar that is also not peer reviewed. The vaccines are 50% effective at preventing mild illness from both omicron variants three months after the second dose, but protection is negligible after that time. However, the two-dose vaccines still provide more than 70% protection against hospitalization and death, and booster doses increase this protection to more than 90%.

Fauci said this week that there's no need to reimplement Covid restrictions at this time. The CDC said earlier this month 98% of people in the U.S. live in areas where they no longer need to wear masks in public places indoors under its new Covid guidance. Public health authorities in the U.S. have shifted their focus to hospitalizations, rather than just new infections, when assessing the threat the virus poses to communities.

The Biden administration is relying on a strategy of vaccination, testing, and treatment with antiviral pills to prevent the virus from disrupting daily life. About 75% of adults in the U.S. are fully vaccinated, according to CDC data.

Dr. Paul Offit, an infectious disease expert at Children's Hospital of Philadelphia, said the public should focus on hospitalizations, a measure of more severe illness, rather than just new infections. Offit said between vaccination and infection from omicron, there's likely enough immunity in the population to protect against a major spike in hospitalizations from BA.2.

"For right now, I choose to be optimistic that we're just going to see a lot of mild illness and not see a dramatic increase in hospitalizations," Offit said.