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

Spain has reported more COVID-19 reinfections in a recent span of two weeks than it has during the rest of the pandemic, according to the latest data from a Spanish public health research institute.

Data from the Carlos III Health Institute shows there were 20,890 repeat infections reported in Spain from Dec. 22, 2021, to Jan. 5, 2022. That figure tops the 17,140 reinfections reported in the European country from the start of the pandemic in March 2020 to Dec. 22, 2021. The data includes confirmed, probable and possible cases.

Alfredo Corell, an immunology professor at Spain's University of Valladolid, told Spanish news site NIUS that the rising number of reinfections were due to the new and highly transmissible omicron variant.

"Prior to this variant, reinfections were anecdotal at the global level," Corell told NIUS. "Omicron has changed this paradigm."

In southern Africa, where the variant was first identified in November, preliminary research suggests that omicron is three times more likely to cause reinfections compared to other known variants of the virus, including the highly contagious delta. However, symptoms of reinfected individuals appear to be mild, according to Anne von Gottberg, a microbiologist at South Africa's National Institute for Communicable Diseases who is studying omicron.

"Previous infection used to protect against Delta," von Gottberg said during a press briefing on Dec. 2. "But now, with Omicron, that doesn't seem to be the case."