Dr. Anthony Fauci, the nation's leading expert on infectious diseases, says a vaccine for COVID-19 could be ready by the end of the year, but it's not clear how long immunity will last. 

'We could have a vaccine by the end of the this calendar year or in the first few months of 2021. So that's the thing that makes me confident that the process is really on track,' he said.

'Things are clearly going in the right direction,' he added in an optimistic interview with Good Morning America on Wednesday.

Fauci, the director of the National Institute of Allergy and Infectious Diseases, said he predicts there will be more than just one COVID-19 vaccine, but it's not clear how long it will be effective for or whether it'll need to be taken annually like a flu shot.

'We'll take it one step at a time. The first thing we want to do is determine if it's safe and if it protects. How long that protection lasts... we're going to have to obviously observe that and that's the reason why when you make a vaccine it isn't the end of the game,' he said. 

Fauci says he's hopeful to have the vaccines ready by end of the year because development has been promising so far.

'What I'm confident about is how well the process is proceeding in the development of more than one candidate that in early testing both in the animals and phase one testing in humans looks quite promising,' he said. 'It makes me cautiously optimistic as we get into the early summer.