Dr. Rochelle Walensky, U.S. President-elect Joe Biden’s appointee to run the U.S. Centers for Disease Control and Prevention (CDC), listens as Biden announces nominees and appointees to serve on his health and coronavirus response teams during a news conference at his transition headquarters in Wilmington, Delaware, December 8, 2020.Kevin Lamarque | ReutersThe U.S. government is planning for the potential need for Covid-19 vaccine booster shots “just in case,” the head of the Centers for Disease Control and Prevention told CNBC.CDC Director Dr. Rochelle Walensky didn’t say what those plans were. However, should Americans require booster shots, the U.S. would likely need to make arrangements with drugmakers to supply additional doses and make plans for vaccine distribution.Walensky told CNBC that U.S. health officials are “thinking ahead” in the event that vaccine-induced immunity against the coronavirus begins to wane. Officials still don’t know if that will happen or if booster shots will be needed, “but we are planning for it just in case,” she said.”Right now, if you have two doses of the mRNA vaccines, you are protected,” Walensky said in comments that aired Tuesday during CNBC’s Healthy Returns Summit.”What we’re talking about is thinking ahead,” she said in the interview with CNBC’s Meg Tirrell. “What happens if in a year from now or 18 months from now your immunity wanes? That’s really our job is to hope for the best and plan for what might happen if we need further boosters in the future, the way we get flu vaccine boosters every year.”The CNBC Evolve Global Summit is coming up on June 16, 2021On deck: CEOs from Novartis, Coca-Cola, Adidas, McDonald’s, Hyatt and more international leaders join the CNBC Evolve Global Summit on June 16 for provocative conversations about adapting, innovating, and transforming in this new era of business. Learn more & register.Walensky said scientists are currently looking at the immunity of a small population of people who were vaccinated months ago. However, she said, she doesn’t expect scientists will do mass testing looking for reduced immunity against the virus.Drugmakers and some scientists now say people will likely need a booster shot of the Covid-19 vaccines and possibly additional shots each year, just like for the seasonal flu.Pfizer’s and Moderna’s Covid-19 vaccines currently require two doses given three to four weeks apart, while Johnson & Johnson’s shot requires just one jab. All three vaccines have been shown to be highly effective against Covid, though company executives now say they expect that strong protection to wane over time.All three companies have said they are developing booster shots or next-generation versions of their vaccines that will bolster protection against the original strain of the virus as well as against some of the new variants.Moderna announced last week that a third dose of its vaccine generated a promising immune response against the original virus as well as the B.1.351 and P.1 variants, first identified in South Africa and Brazil, respectively.Last month, the Biden administration’s Covid response chief science officer, David Kessler, said Americans should expect to receive booster shots to protect against coronavirus variants. He told U.S. lawmakers that currently authorized vaccines are highly protective but noted new variants could “challenge” the effectiveness of the shots.Covid-19 booster shots will be free to the public if they are needed to control the ongoing pandemic, Kessler told lawmakers on Tuesday. “We do have the funds to purchase the next round and to assure if there are boosters that they are free just as the last round,” Kessler said in testimony on Capitol Hill.Walensky told CNBC scientists still don’t know whether the coronavirus is seasonal with new infections slowing down during warm summer months and speeding up in the winter.Some scientists predict states will see repeated outbreaks when the U.S. enters its colder months.”We have other coronaviruses be seasonal viruses and yet this coronavirus has not proven to be seasonal,” she said, adding that the U.S. witnessed a surge in cases last summer.