Utah health officials on Friday announced a change to testing guidelines due to a shortage of test availability in the state.

Symptomatic individuals in the general public are urged not to test and just isolate for five days and the "Test To Stay" program at schools is being suspended.

Since Christmas Day, daily tests in Utah have jumped from about 19,000 to nearly 48,000.

State epidemiologist Dr. Leisha Nolen said Utah must use its test supply where it has the biggest impact, and right now that's not in the general community or schools.

Utah officials say you should still get tested if: you have underlying conditions; you're visiting a vulnerable individual; you work in a health care setting and are symptomatic or have been exposed; you’re seeking confirmation that the infection has passed so you can end isolation.

-ABC News' Matt Fuhrman

Omicron became the dominant variant in New York City within five weeks after it was first detected, according to a new report released Thursday by the city's health department.

By comparison, it took 20 weeks for the delta variant to become dominant.

The report, which details preliminary findings on the city's omicron wave, found that there have been lower hospitalization rates but more total hospitalizations compared to the delta wave due to "significantly greater case numbers."

Unvaccinated New Yorkers were more than eight times more likely to be hospitalized than those who were fully vaccinated early in the omicron wave, the report found. Black New Yorkers and people ages 75 and older also were more likely to be hospitalized.

Unvaccinated pregnant women with COVID-19 and their newborn babies have a higher risk of complications from the disease compared with those who are vaccinated, a new study found.

In the study, published Thursday in Nature Medicine, researchers from Public Health Scotland looked at vaccination rates and COVID-19 outcomes in 131,875 pregnant women in Scotland between Dec. 8, 2020, and Oct. 31, 2021, when the delta variant was dominant.

They found that 90.9% of COVID-19 hospital admissions, 98% of intensive care admissions and all 450 newborn deaths were in unvaccinated pregnant women.

The study reiterates the importance of pregnant women getting vaccinated against the virus due to a greater risk of dangerous health complications from COVID-19. A growing body of research has shown the vaccines to be safe and effective for pregnant women.

-ABC News' Dr. Siobhan Deshauer, Sony Salzman and Dr. Alexis Carrington

Following heated exchanges between Dr. Anthony Fauci and several Republican senators at a Congressional hearing Tuesday, more than 200 prominent science and public health leaders have now penned an open letter voicing their support of the White House chief medical adviser's service and leadership -- and condemning attacks against him. 

"We deplore the personal attacks on Dr. Fauci," the letter says. "The criticism is inaccurate, unscientific, ill-founded in the facts and, increasingly, motivated by partisan politics. It is a distraction from what should be the national focus -- working together to finally overcome a pandemic that is killing about 500,000 people a year."

Signatories include former Senate majority leader Bill Frist, a Republican; Rich Besser, former acting director for the Centers for Disease Control and Prevention; members of President Joe Biden's transition COVID-19 task force; and several Nobel laureates.

The letter comes two days after a Health, Education, Labor and Pensions committee hearing, during which Fauci publicly accused Republican Sen. Rand Paul of fomenting the violent threats and harassment that he and his family have had to contend with during the pandemic's politicized climate.

-ABC News' Sasha Pezenik

The United Kingdom has launched a new advertising campaign that urges pregnant women who have not yet received a COVID-19 vaccine or booster shot to do so as soon as possible.

Testimonies of pregnant women who have had the shots will be played out in ads across U.K. radio stations and on social media starting Monday. The new campaign urges pregnant women "don't wait to take the vaccine" and highlights the risks of COVID-19 to both mother and baby as well as the benefits of getting vaccinated, according to a press release from the U.K. Department of Health and Social Care.

The press release cited the latest data from the U.K. Health Security Agency that suggests COVID-19 vaccination is safe for pregnant women and provides strong protection against the virus for both mother and baby. The press release also cited data from the U.K. Obstetric Surveillance System that shows more than 96% of pregnant women hospitalized with COVID-19 symptoms between May and October 2021 were unvaccinated, and a third of them required respiratory support. Around one in five women who are hospitalized with COVID-19 need to be delivered preterm to help them recover, and one in five of their babies need care in the neonatal unit.

"Getting a COVID-19 vaccine is one of the most important things a pregnant woman can do this year to keep herself and her baby as safe from this virus as possible," Lucy Chappell, chief scientific adviser to the U.K. Department of Health and Social Care, said in a statement Monday. "We have extensive evidence now to show that the vaccines are safe and that the risks posed by COVID-19 are far greater."