The head of the World Health Organisation is calling for a moratorium on administering booster shots of Covid-19 vaccines to help ensure that doses are available in countries where few people have received their first shots, according to AP.

WHO Director-General Tedros Adhanom Ghebreyesus made the appeal today mostly to wealthier countries that have far outpaced the developing world in numbers of vaccinations.

WHO officials say the science is unproven about whether giving booster shots to people who have already received two vaccine doses is effective in preventing the spread of the coronavirus. WHO has repeatedly called for rich countries to do more to help improve access to vaccines in the developing world.

Israel, France, Germany and many Middle Eastern countries have already started administering booster shots. Other nations, including the United States and Britain, are considering plans to do so in the wake of the emergence of the highly transmissible delta variant.