South Africa suspended its vaccination campaign against COVID-19 on Sunday after a new study revealed that the AstraZeneca vaccine is less effective against a variant of the virus found in the country.

The World Health Organization is holding a meeting Monday about the latest development in South Africa.

The study, conducted by the University of the Witwatersrand in Johannesburg and not yet peer reviewed, concluded that the British vaccine offered only “limited protection against moderate forms of the disease caused by the South African variant, in young adults.”

South Africa has seen more than 46,000 people die from the virus. It had planned to begin inoculating its population with a million doses of the AstraZeneca vaccine in the coming days. However, the study found that the vaccine was only 22% effective in moderate cases of the South African variant of the disease.

AstraZeneca said on Sunday that it was developing another vaccine that would be more effective against the South African variant, which could be expected by this autumn.