(CNN) As an outbreak of a novel coronavirus has swept through Hubei province, China, the US Centers for Disease Control and Prevention has been preparing for its worst case scenario -- a widespread outbreak of illnesses in the United States.

"Right now we're in an aggressive containment mode," CDC Director Dr. Robert Redfield told CNN's Chief Medical Correspondent Dr. Sanjay Gupta in an interview on Thursday.

"We don't know a lot about this virus," he said. "This virus is probably with us beyond this season, beyond this year, and I think eventually the virus will find a foothold and we will get community-based transmission."

As of Thursday, there have been 15 cases of the novel coronavirus confirmed in seven states: eight in California; two in Illinois; and one in Arizona, Washington, Massachusetts, Wisconsin and Texas.

This 'will become a community virus'

While more research is needed to fully understand the virus, Redfield told Gupta that the CDC has focused on surveillance to track cases and containment strategies to slow possible progression of the virus in the United States. Slowing progression gives more time for researchers to work on developing and testing a vaccine and antiviral drugs for this novel coronavirus. Currently, there is no known cure for the virus.

"The containment phase is really to give us more time. This virus will become a community virus at some point in time, this year or next year," Redfield said. "We don't have any evidence that this coronavirus is really embedded in the community at this time, but with that said, we want to intensify our surveillance so that we're basing those conclusions based on data."

The containment strategy refers to efforts to prevent widespread transmission of the coronavirus in the United States, including having people with confirmed cases stay in isolation and placing restrictions on travel between affected areas in the world. Such containment measures were used widely during the SARS global outbreak of 2003 , during which 8,098 people worldwide became sick and of those, 774 died, according to the CDC.

Due to the current novel coronavirus outbreak, the Trump administration has put in place travel restrictions that block foreign nationals from entering the United States if they visited China in the 14 days prior to their arrival to the United States.

Restrictions also apply to US citizens who have been in China's Hubei province, the epicenter of the coronavirus outbreak, in the two weeks prior to their return to the United States. Upon their return, those citizens will be subject to a mandatory quarantine of up to 14 days.

World Health Organization officials and other experts have criticized travel restrictions on foreign nationals, even arguing that such restrictions could backfire.

"We reiterate our call to all countries not to impose restrictions that unnecessarily interfere with international travel and trade. Such restrictions can have the effect of increasing fear and stigma, with little public health benefit," WHO Director-General Tedros Ahanom Ghebreyesus said last week during a media briefing in Geneva.

US officials have defended the government's response, saying they're taking important steps to prepare for the virus and slow its spread -- and that the timing of their efforts is key.

"Frankly, some people criticized when we decided that we wanted to temporarily suspend travel into the United States from individuals who were not Americans or permanent residents who had been in the hot zone in the last 14 days. Some people didn't think that that was what they would do," Redfield told Gupta on Thursday.

"Well, we felt very strongly that our obligation was to do all we can to protect the American public," Redfield said. "I would rather be criticized for over-protecting America than under-protecting America at this stage."

Regarding the travel restrictions on foreign nationals, Gupta asked, "Obviously, Dr. Redfield, the virus doesn't discriminate based on race. Why do we?"

In response, Redfield spoke more to the restrictions on US citizens, saying, "The issue here was first a strong commitment to take care of the Americans that are going to come back, whether they happen to potentially have this virus or not -- and you saw that with the over 800 individuals that the State Department has repatriated and we've assisted in that."

Photos: Inside the CDC's coronavirus war room Employees with the Centers for Disease Control and Prevention work at the Emergency Operations Center in Atlanta on Thursday, February 13. Hide Caption 1 of 9 Photos: Inside the CDC's coronavirus war room Jay Butler, the CDC's deputy director for infectious diseases, speaks to reporters. Hide Caption 2 of 9 Photos: Inside the CDC's coronavirus war room A clock shows different time zones in the Emergency Operations Center. Hide Caption 3 of 9 Photos: Inside the CDC's coronavirus war room A CDC employee works on his computer. Hide Caption 4 of 9 Photos: Inside the CDC's coronavirus war room The center is staffed 24 hours a day, 365 days a year. Hide Caption 5 of 9 Photos: Inside the CDC's coronavirus war room Ed Rouse, the senior advisor for the CDC's Division of Emergency Operations, speaks to reporters in Atlanta. Hide Caption 6 of 9 Photos: Inside the CDC's coronavirus war room Many of the employees are monitoring the coronavirus outbreak, but that's not all they are monitoring. Hide Caption 7 of 9 Photos: Inside the CDC's coronavirus war room CDC employees work at the Emergency Operations Center. Hide Caption 8 of 9 Photos: Inside the CDC's coronavirus war room A look inside the main conference room at the Emergency Operations Center. Hide Caption 9 of 9

Concerns about the virus' spread

When it comes to preventing spread of the virus and learning more about the pathogen, Redfield told Gupta that he thinks asymptomatic transmission of the novel coronavirus is possible and concerning.

In other words, Redfield said that an infected person not showing symptoms could still transmit the virus to someone else, based on what he has learned from the Chinese Center for Disease Control and Prevention.

"There's been good communication with our colleagues to confirm asymptomatic infection, to confirm asymptomatic transmission, to be able to get a better handle on the clinical spectrum of illness in China. What we don't know though is how much of the asymptomatic cases are driving transmission," Redfield said.

"What I've learned in the last two weeks is that the spectrum of this illness is much broader than was originally presented. There's much more asymptomatic illness," Redfield said. "A number of the confirmed cases that we confirmed actually just presented with a little sore throat."

Weeks into the outbreak, CDC still not invited to China

Meanwhile, Redfield said that the CDC has been eager to help China in its efforts to fight this outbreak. Nearly six weeks after the CDC first offered to help China with the coronavirus outbreak, the offer still has not been accepted.

The outbreak of the novel coronavirus was first identified in early January.

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"There's a lot of information we don't know -- that's why I offered to provide assistance, direct assistance, and send our CDC folks over there back on January 6 to really help them gather that information and also to help us see the information first hand that we need to help make the right public health recommendations for our nation," Redfield said.

"That letter has not been responded to yet by the official Chinese government," he said. "We do believe that we're the best in the world in this space and we're ready to help and assist them, but they're an independent nation that has to make that decision that they're going to invite us in."