Dr. Anthony Fauci says an additional Covid-19 booster shot will be recommended for previously vaccinated people with weakened immune systems.

He told NBC's Today show on Thursday that he expects the booster recommendation to come imminently.

People have compromised immune systems for a variety of reasons, including organ transplants, cancer or other conditions. Any authorisation for an additional booster shot would come from the Federal Drug Administration.

The chief medical advisor to US President Joe Biden said researchers are collecting data to determine when the protection for other vaccinated groups, like the elderly, will go below a “critical level”.

Fauci says for other vaccinated groups, such as the elderly, data is being collected to determine if or when their protection goes below a critical level and that's when you're going to be hearing about the implementation of boosters for others.

The nation's top infectious disease expert says the United States is not going to administer the dose in coming days to people other than those who are immuno-compromised.

Fauci says inevitably there will be a time when we'll have to get boosts because no vaccine, at least not within this category, is going to have an indefinite amount of protection.