For example, if a person is infected with Covid-19 and has a test a day or two later, there might not be enough viral RNA to detect the virus.

So, testing as a litmus for traveling won't work, unless it reveals a positive result and the person stays home as a result, Reiner said.

"What I think I want people to understand, more importantly, is that a test that's negative today doesn't mean you're going to be negative tomorrow or the next day or the following day," Giroir told CNN.

"That negative test is not a free pass to do risky behavior."

It's not wrong to get a test, but a negative test "doesn't give you a free pass," he said.

The US is "at a critical and very dangerous point in this pandemic where cases are rising in nearly every jurisdiction and our hospital capacity is getting ...

it's really getting challenged by this," Giroir added.

"It is a dangerous situation but it's reversible," he said.

Public health officials, including Giroir, have stressed continued mitigation efforts, such as mask usage, social distancing, avoiding crowded places and frequent hand washing as a means of controlling the spread of Covid-19 and eventually bringing down the curve.