COVID-19 updates: US cases down nearly 25%

Moderna announced Wednesday that the first participant has been dosed in the phase 2 study of its omicron-specific booster candidate, in case it becomes necessary. 

Moderna's trials will include people who received two doses of the original Moderna vaccine and people who received two doses of the original Moderna vaccine and a Moderna booster shot. 

Pfizer announced Tuesday that it's initiated clinical studies to evaluate an omicron-based vaccine for adults.

A study from the National Institutes of Health published in the New England Journal of Medicine found mixing and matching boosters are safe and create a similar immune response to sticking with your initial vaccine. 

An earlier version of this study, with more preliminary findings, helped guide the CDC's decision to allow mix-and-match. 

The study authors make no claims about specific combinations being more or less effective. The study did find that people who got an mRNA vaccine (Pfizer or Moderna) and then received the Johnson & Johnson booster had a significant increase in T-cell response, a part of immunity.  

The trial looked at 458 participants who received a vaccine with no prior COVID-19 infection. This data is only for the first 29 days after receiving the booster; researchers plan to follow the participants for one year, allowing for more data. 

-ABC News' Vanya Jain, Sony Salzman, Eric Strauss, Dr. Alexis Carrington

An unvaccinated child has died in Mississippi from COVID-19, according to the state's health department.

The department confirmed to ABC News that the child was between the ages of 11 and 17, an age bracket that is eligible to receive Pfizer’s COVID-19 vaccine. 

This marked the 10th child -- including an infant -- to die in Mississippi from COVID-19. None of the 10 children were vaccinated, according to the health department.

-ABC News' Josh Hoyos

Daily COVID-19-related deaths -- which are a lagging indicator -- are steadily increasing to their highest point in nearly one year, according to federal data.

The U.S. is reporting an average of more than 2,100 new fatalities each day, surpassing the average from last summer's delta surge. However, the nation's death toll remains significantly lower than last winter when the U.S. peaked at about 3,400 deaths per day.

Since Thanksgiving, there have been nearly 84,000 confirmed deaths.

Meanwhile, the U.S. case rate is steadily falling, down by 6% in the last week -- although case rates still remain higher than at any other point of the pandemic, according to federal data.

Out of the 3,220 counties in the U.S., just eight counties are not reporting high transmission, according to federal data.

Utah is leading the nation in new cases per capita, followed by Alaska and New Mexico.

-ABC News' Arielle Mitropoulos