U.S regulators authorised a third dose of Covid-19 vaccines by Pfizer Inc-BioNTech and Moderna Inc on Friday for people with compromised immune systems who are likely to have weaker protection from the two-dose regimens.

The US Food and Drug Administration amended its emergency use authorisation for both vaccines on Thursday, paving the way for people who have had an organ transplant, or those with a similar level of weakened immune system, to get an extra dose of the same shot they have initially received.

An advisory panel to the US Centers for Disease Control and Prevention (CDC) voted to recommend the additional shots, and the agency's director signed off on that recommendation on Friday. Immunocompromised individuals can begin receiving the shots immediately, according to an agency spokesperson.

Mixing of mRNA vaccines is permitted for the third shot if their original vaccine is not available.

Patients will not need a prescription or the sign-off of a healthcare provider in order to prove they are immunocompromised and receive the additional dose, according to officials.