Its regional branch for the Americas, PAHO, warned that vaccines being developed now will take months to arrive and supplies might not become sufficient until after 2021.
Countries should at first aim to vaccinate 20% of the population, giving priority to medical workers and people over 65 and those that have prior health conditions, PAHO said.
In the meantime, he said in a briefing from Washington, countries must not relax preventive social distancing measures and the use of masks and the practice of washing hands.
Health experts do not know exactly what level of vaccination is needed to obtain herd immunity and stop the virus spreading, though the WHO has estimated this at 70%, he said.
A number of vaccine candidates are being tested but no vaccine has been approved for distribution yet in the region.
“We are working hard to ensure that once vaccines are approved and available, countries are prepared to roll them out” to ensure logistics systems and cold storage supply chains are in place, said PAHO director Carissa Etienne.
Meanwhile, the jumps in weekly Covid-19 case counts in the United States and Canada are particularly worrisome as the Northern Hemisphere winter approaches, Etienne said.
Mexico is experiencing a resurgence in cases in the state of Baja California, near the U.S. border, she said.
Brazil is currently reporting the highest incidence of new cases in South America, and the mounting cases and deaths have put its health system under strain and hospitals are at capacity in some areas, Etienne said.