The World Health Organization’s chief scientist warned that even as countries start rolling out vaccination programs to stop COVID-19, there will be no chance of herd immunity this year.

Dr. Soumya Swaminathan said on Monday it was critical countries and their populations maintain strict social distancing and other virus restrictions for the foreseeable future. Countries including Britain, the US, France, Canada, Germany, Israel and the Netherlands have started inoculating millions of their citizens against the virus in recent weeks.

Dr. Bruce Aylward, an adviser to WHO’s director-general, said the U.N. health agency was hoping vaccinations in poorer countries might begin in late January or February.

He said the WHO needed the co-operation of vaccine manufacturers alongside money and logistical help to start immunizing poorer populations and aims to have a “a rollout plan” next month.