WASHINGTON (REUTERS) - The US Centres for Disease Control and Prevention (CDC) on Monday (Nov 29) said everyone aged 18 years and older should get a booster shot either six months after their initial Pfizer-BioNTech or Moderna vaccine doses or two months after their Johnson & Johnson shot.

The update comes after US health regulators last week expanded the eligibility for booster shots of Covid-19 vaccines to all adults, but stopped short of saying all adults aged 18 to 49 should get the additional shots.

The World Health Organisation (WHO) on Monday said the Omicron coronavirus variant, which was first detected in Southern Africa, carried a very high risk of infection surges.

The variant has now been confirmed in Australia, Belgium, Botswana, Britain, Denmark, Germany, Hong Kong, Israel, Italy, the Netherlands, France, South Africa and Canada.

"The recent emergence of the Omicron variant (B.1.1.529) further emphasises the importance of vaccination, boosters and prevention efforts needed to protect against Covid-19," CDC director Rochelle Walensky said in a statement.

Scientists in the United States and around the world are urgently examining vaccine effectiveness related to this variant, the CDC said.

The agency also said the 47 million adults who have not yet been vaccinated are encouraged to do so as soon as possible, adding that children and teens should also be vaccinated to lower their chances of a serious illness.

The comments echo those of US President Joe Biden, who has urged Americans to get vaccinated, take boosters and wear masks.

Mr Biden said he believed that the existing vaccines would continue to protect against severe disease, but added that his administration was working with vaccine makers to develop contingency plans if new vaccines were needed.

The CDC said increased testing in the country would help identify the variant quickly.