LONDON: Britain will give severely immunosuppressed people a third dose of COVID-19 vaccine to increase their chances of generating a better immune response, although officials stressed the offer was separate from any broader booster vaccine programme.

With the move, Britain follows the United States, which last month authorised a third dose of COVID-19 vaccines made by Pfizer Inc-BioNTech and Moderna Inc for people with compromised immune systems who are likely to have weaker protection from two-dose regimens.

Public Health England on Wednesday said the decision to offer third doses to the immunosuppressed followed data that showed 40% of immunosuppressed people generated only low levels of antibodies from two vaccine shots.

However, officials stressed the third dose was not a "booster" dose as it would be administered as part of the primary vaccination schedule and aimed at helping the initial process of generating an immune response.

"We want people with severely suppressed immune systems to have the best chance of gaining protection from COVID-19 via vaccination," said Wei Shen Lim, Chair of COVID-19 Immunisation for the Joint Committee on Vaccination and Immunisation (JCVI).

The JCVI's final advice on the broader booster programme for the elderly and vulnerable is expected soon. The committee previously said it could start this month.

Third doses will be given to people with immunosuppressive conditions such as HIV/AIDS, and those who have received immunosuppressive treatments like chemotherapy. The move will impact less than 1% of the population, around 400-500,000 people.

Officials said mRNA vaccines made by Pfizer and Moderna should be used as the third dose, as there was more data on these shots, but AstraZeneca's vaccine could be added later if the data supported the move.

The government said it accepted the JCVI's advice.

"The NHS (National Health Service) will contact people as soon as possible to discuss their needs and arrange an appointment for a third dose where clinically appropriate," health minister Sajid Javid said.

"This is not the start of the booster programme – we are continuing to plan for this to begin in September."