Coronavirus cases are falling in the US but experts say it's not from the COVID vaccine, yet

New coronavirus cases are on the decline in the United States following staggering post-holiday peaks last month, but experts say it's too early for new COVID-19 vaccines to be having an impact.

The positive trend also is not assured to continue, as new and more transmissible variants threaten to reverse it, according to Centers for Disease Control and Prevention Director Dr. Rochelle Walensky.

"Although we have seen declines in cases and admissions and a recent slowing of deaths, cases remain extraordinarily high, still twice as high as the peak number of cases over the summer," she said this week.

The decline in cases is likely due to a natural depression after record travel followed by indoor holiday gatherings triggered a surge in infections, said Dr. Sarita Shah, associate professor at Emory University’s Rollins School of Public Health.

According to the Transportation Security Administration, the agency screened 1.9 million travelers the day before Christmas Eve setting a pandemic record.

“We’ve seen these rises and falls in the COVID case counts now a few times and they seem to really track along holidays or people’s movements,” Shah said.

COVID-19 symptoms take between two to 14 days to appear after exposure, and cases peaked exactly two weeks after the Christmas holidays, noted Brittany Baker, undergraduate program coordinator and clinical assistant professor at North Carolina Central University.

Related: 400,000 Americans now dead from COVID-19

During that peak, the U.S. began reporting more than 200,000 new cases per day. Baker said Americans may have been scared into taking more precautionary measures against COVID-19, which could have contributed to the current decline.

“When we start to read as a general public that numbers are going up, we have a sense of retreat,” she said.

Dr. Wafaa El-Sadr, professor of epidemiology and medicine at Columbia University’s Mailman School of Public Health, said the falling case numbers can’t be attributed to the COVID-19 vaccine as not even a tenth of the U.S. population has been vaccinated, according to the CDC.

And it's not clear when the vaccine rollout, which began in December, will start to appear in falling case numbers.

Experts say the forecast keeps changing, as more drug makers, such as Johnson & Johnson, seek emergency authorization from the Food and Drug Administration, the Biden administration attempts to secure more doses from Pfizer-BioNTech and Moderna, and states expand vaccine eligibility to speed up rollout.

“We’re vaccinating our most vulnerable populations right now but once we start to move into the broad population, the population that’s driving the numbers … that’s when we’ll start to see an impact on the overall numbers,” Shah said.

Another vaccine on the horizon?: Johnson & Johnson requests FDA authorization for its one-dose COVID-19 vaccine

No cuts, please: Some people get COVID-19 vaccines before it's their turn

She said Americans may start to see the vaccine’s impact on case numbers as early as the summer, but it will be more evident in the fall.

Health experts also need to learn more about how the vaccine prevents COVID-19 spread before knowing how it will affect future case numbers, El-Sadr said.

Dr. Anthony Fauci, the nation’s leading infectious diseases expert, said in a CNN town hall last week evolving evidence shows it could prevent asymptomatic transmission but there’s no definitive proof, yet. It may be possible for someone to be exposed, present no symptoms and still have enough virus in the nasal pharynx to infect someone else.

Protecting against people who don’t show symptoms is vital as a CDC study in early January found that asymptomatic cases account for more than half of all transmission, El-Sadr added.

“We want to make a dent in the prevention of asymptomatic infection,” she said. “We want to protect people from disease in order to control the epidemic.”

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

This article originally appeared on USA TODAY: US COVID cases are falling but it's not from vaccine, yet, experts say