The positive trend also is not assured to continue, because 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 a natural drop 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.
During that peak, the U.S. began reporting more than 200,000 new cases a day. Baker said Americans may have been scared into taking more precautionary measures against COVID-19, which could have contributed to the 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, because not even a tenth of the population has been vaccinated, according to the CDC.
And it's not clear when the vaccine rollout, which began in December, will start to make a difference in falling case numbers.
Experts say the forecast keeps changing as more drugmakers, such as Johnson & Johnson, seek emergency authorization from the Food and Drug Administration, the Biden administration seeks 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.
She said Americans may start to see the vaccine's influence 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 that evolving evidence shows the vaccine could prevent asymptomatic transmission, but there's no proof yet. It may be possible for someone to be exposed, present no symptoms and still have enough virus in the nasopharynx to infect someone else.
Protecting against people who don't show symptoms is vital. A CDC study in early January found that asymptomatic cases account for more than half of all transmission, El-Sadr said.
"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."
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