HARI SREENIVASAN: The U.S. Food and Drug Administration today asked blood centers in two in Florida counties to suspend blood donations until each unit can be screened for the Zika virus.

This comes as state health officials investigate four non-travel-related cases of Zika, which may mean the first cases of local Zika virus transmission by mosquitoes in the United States.

For an update on all this, I spoke earlier this evening with Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

I'm joined now by Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the NIH.

Thanks for joining us.

DR. ANTHONY FAUCI, National Institutes of Health: Good to be with you.

HARI SREENIVASAN: All right, first of all, this change in status almost, what does this warning in Florida mean?

DR. ANTHONY FAUCI: Well, the warning from the FDA is saying that in the regions in Miami-Dade and Broward County where you have what looks like local transmission of cases, namely, cases that are not travel-related, don't appear to be sexually transmitted, and very likely mosquito-transmitted locally, mainly because those people never left the continental United States, so they didn't get infected elsewhere, given that, what the FDA is doing is, they're saying there may be more cases out there.

So, in order to protect the blood supply and to be very safe and to keep it safe, they are suspending the collection of blood from those two areas until they can implement either testing of the units as they are donated or decontaminating of any potential units that might be potentially contaminated.

So, they want to be quite safe about it and very prudent, so that we don't get contamination of the blood supply. That's what their particular order is regarding collection of bloods.

HARI SREENIVASAN: Now, halting the collection of blood, that is a pretty extreme step. That has got to affect local hospitals that use it, other places that use blood.

Is there some evidence that you would base a significant move like this on, an idea that there are local mosquitoes there that would be carrying Zika?

DR. ANTHONY FAUCI: Well, there's no doubt that the mosquitoes that are capable of transmitting Zika are in that area.

This is the Aedes aegypti mosquito that's extensively seen along the Gulf Coast, the Gulf Coast states, and up across in part of the East Coast and a little bit out towards the West, but certainly along the Gulf Coastal states.

We know that we have now more than 1,600 travel-related cases, which is the reason why one would have predicted that, sooner or later, you're going to see some local transmission. And the local transmission that we're seeing now is the kind of thing that makes one want to make sure we do what we can to prevent that local transmission from becoming sustained and from becoming disseminated.

And the answer to that is by very aggressive mosquito control.

HARI SREENIVASAN: If the FDA thinks that this is a good step in the Miami area, or Dade County, Broward county area, why not the entire southern half of the United States and where these mosquitoes could be traveling?

DR. ANTHONY FAUCI: Well, you don't want to do what is called overreact.

And at this point, with the very local in two areas cases now, now four cases that are under investigation, I think that that is prudent enough to do what they did, because experience with other viruses, such as chikungunya and dengue, is that when you do get local spread like this, it tends to stay well-contained, particularly if you do adequate and aggressive mosquito control.

So there is not a prediction that it is going to be widespread throughout the area. So what they're doing is, they're taking the prudent step of saying, for the time being, until we learn otherwise, we're going to hold off on the collection of blood that would supply this particular two areas in Florida.

HARI SREENIVASAN: Now, should there be a screen on blood donations throughout larger regions? Because the FDA came up for a test for that a couple of months ago, right?

DR. ANTHONY FAUCI: Right.

They came up with a test. And what's being done is essentially what's being done right now in Puerto Rico. So, Puerto Rico has a very extensive outbreak that's getting worse and worse as the weeks go by. There is so much transmission in Puerto Rico that, right now, they're actually doing the screening of the blood supply that we're talking about.

But when you have just a couple of cases here and there without the evidence of any massive spread, the prudent decision was to just let's just hold off on collection of blood, and if it gets any worse or it gets to the point where it's more disseminated, then we will do other things.

But, right now, they're going to ultimately — they're going to ultimately test the blood supply there.

HARI SREENIVASAN: All right, Anthony Fauci joining us from the National Institutes of Health tonight, thanks so much.

DR. ANTHONY FAUCI: Thank you.