The World Health Organization's (WHO) vaccine advisory panel recommends that people who are immunocompromised or received an inactivated vaccine should receive a booster dose of a Covid-19 shot, it said on Thursday.

Many countries have been rolling out booster shots, targeting the elderly and people with underlying health issues, but worries about the new Omicron variant have prompted some to expand their use to larger portions of their populations.

With vaccination rates worryingly low in much of the developing world, the WHO has said in recent months that administering primary doses - rather than boosters - should be a priority.

The recommendation comes after the Strategic Advisory Group of Experts (SAGE) on immunisation held a meeting on Tuesday to evaluate the need for Covid-19 boosters.

Speaking in a briefing, SAGE chair Alejandro Cravioto said emerging data showed that vaccines' efficacy against COVID-19 wanes, with a significant decline seen in older people in particular.

Covid-19 vaccines protect "very well" through six months after the last dose with some "minor, modest reduction" in protection, Kate O'Brien, director of the WHO's immunisation department, said.

Inactivated vaccines which take the SARS-CoV-2 virus and inactivate or kill it using chemicals, heat or radiation, are made by Chinese manufacturers Sinovac Biotech, state-owned Sinopharm and India's Bharat Biotech.

A single dose of the Johnson & Johnson vaccine is still effective, but data from the company's clinical trials using two doses clearly show the benefit of having further vaccination, Cravioto said.