WASHINGTON (BLOOMBERG) - US regulators have cleared giving a third Covid-19 vaccine dose to people with weakened immune systems in an effort to improve protection for those with impaired responses to standard shots.

The authorisation applies to both Moderna and Pfizer vaccines, with organ transplant recipients or those whose immune systems are similarly compromised covered, the US Food and Drug Administration's (FDA) said in a statement late on Thursday (Aug 12).

Other fully vaccinated individuals do not need an additional dose right now, the FDA said.

Bolstering the agency's move was a study published in the New England Journal of Medicine on Wednesday showing that an extra dose of Moderna's vaccine significantly raised antibody levels against Covid-19 in transplant patients.

The findings showed that among patients who received a placebo shot, rather than the third dose, just 18 per cent showed the desired antibody levels.

"I feel very relieved that they are doing this, because it will save lives," said Ms Janet Handal, a kidney transplant recipient. "I wish that they had done it sooner."

While the US-authorised Covid-19 vaccines are highly effective at preventing severe disease, hospitalisation and death in most people, booster shots are especially important for transplant patients and others with weak immune systems, who often do not get adequate responses to their first vaccine course.

Immune-compromised people compose about 3 per cent of adults, Dr Rochelle Walensky, director of the Centres for Disease Control and Prevention (CDC), said on a press call.

Many patients sought out extra doses on their own as findings emerged on their responses to vaccines.

Surgery and epidemiology professor Dorry Segev of the Johns Hopkins University said his research team has observed more than 200 transplant recipients who have gotten third shots, and he suspects many more immunocompromised patients have done so as well.

Advisers for the CDC raised concerns at a meeting last month about patients getting additional doses in an unsupervised manner and the inequities that could arise.

"The people who have gotten it now are the people who navigate the healthcare system well, who are willing to go to great lengths," Prof Segev said.

FDA approval would expand access of these boosters in a more equitable way to people who might not be as good as navigating the healthcare system and "might have been too shy or too worried about pursuing something", he added.

Follows other countries

Only those with weakened immune systems will be offered boosters, White House chief medical adviser Anthony Fauci said on NBC's Today show.

For other vaccinated groups, such as the elderly, data is being collected to determine if or when their protection goes "below a critical level".

"Right now at this moment, other than the immune-compromised, we're not going to be giving boosters to people, but we will be following them very carefully, and if they do need it, we will be ready to give it to them," he said.

The United States now joins a number of other countries, such as Israel, Germany and France, that are offering boosters to vulnerable groups. Other countries, including Russia, Hungary and the United Arab Emirates, are offering a third shot more broadly.

About 60 per cent of eligible Americans, aged 12 and up, are fully vaccinated, according to the CDC. While the pace of shots is rising after falling sharply from a high set in mid-April, the Joe Biden administration is urging people to get vaccinated as infections surge in the US, fuelled by the Delta variant.

An advisory committee to the CDC is also set to discuss booster doses of Covid-19 vaccine for immunocompromised patients on Friday.

Ms Handal and other transplant recipients formed the Transplant Recipients and Immunocompromised Patient Advocacy Group during the pandemic. It has urged the FDA to streamline approval of additional doses for those with weakened immune systems.

"People who are immunocompromised are continuing to live with the fear that everyone felt in the early days of Covid-19," Ms Handal said in an interview.

They are carrying on with their lives as the world opens up, but "it's just an undercurrent of fear, and it's exhausting to live with".