The COVID-19 pandemic has carried a key lesson for public health officials, Dr. Anthony Fauci said Monday: When talking to Americans, be clear that science is often a moving target.

"What we knew [about COVID-19] in January was very different from what we knew at the end of January, the beginning of February, and then very different from March," Fauci, who is stepping down next month after 38 years at the National Institute of Allergy and Infectious Disease, told CNN's Jake Tapper.

"Hopefully, we could have been more on top of appreciating the dynamic nature of how things change, thinking that it wasn't aerosol spread in the beginning, and then you find out it is aerosol spread. Thinking that, well, symptomatic people spread it, and then you find out that 50% to 60% of the transmissions occur from someone who has no symptoms."

Many Americans see these changes as signs of scientists "flip-flopping," he said, when in reality "it's that the data are evolving in a very dynamic way."

Fauci announced in August that he would be stepping down from NIAID and as chief medical adviser to U.S. President Joe Biden "to pursue the next chapter of my career."

Asked about Republican lawmakers' intent to hold hearings on the origins of the coronavirus and any relation to US funding of viral research, Fauci told Tapper that it would be "essentially molecularly impossible" for the viruses involved in funding from the National Institutes of Health to turn into SARS-CoV-2, which causes COVID-19.

Experts agree that the virus almost certainly jumped into humans from an animal market in China but that they may never know for sure. "It's possible that there was a lab leak," Fauci said. "But if you look at the viruses that the NIH funded -- and it was a very small grant, $120,000, $130,000 a year of granting -- to study bat viruses in a surveillance way to see what's out there. ... If you look at those viruses and you look at what was done with the viruses, it would be essentially molecularly impossible for those viruses to turn into SARS-CoV-2 because they are so evolutionarily different."

Fauci criticized China's controversial zero-Covid policy, saying shutdowns "should always be a temporary phenomenon, not a long-range strategy," and should be done with an end point in mind, such as buying time to secure PPE or provide vaccinations. "When you want to shut down in order to interrupt immediately a process that's going on, like the spread of infection, there should be a purpose to it."

As for what's next for him, Fauci said he wouldn't enter any negotiations for future roles until next year -- but would probably be taking in a New York Yankees game in the spring.