GENEVA - SARS-CoV-2, the virus causing the ongoing COVID-19 pandemic, is likely to keep evolving as transmission continues globally, but its severity will reduce due to immunity acquired by vaccination and infection, the World Health Organization (WHO) said on Wednesday.

Speaking at an online briefing, WHO Director-General Tedros Adhanom Ghebreyesus gave three possible scenarios for how the pandemic could evolve this year.

"Based on what we know now, the most likely scenario is that the virus continues to evolve, but the severity of the disease it causes reduces over time as immunity increases due to vaccination and infection," he said, warning that periodic spikes in cases and deaths may occur as immunity wanes, which may require periodic boosting for vulnerable populations.

"In the best-case scenario, we may see less severe variants emerge, and boosters or new formulations of vaccines won't be necessary," he added.

"In the worst-case scenario, a more virulent and highly transmissible variant emerges. Against this new threat, people's protection against severe disease and death, either from prior vaccination or infection, will wane rapidly."

The WHO chief put forward outright his recommendations for countries to end the acute phase of the pandemic in 2022.

"First, surveillance, laboratories, and public health intelligence; second, vaccination, public health and social measures, and engaged communities; third, clinical care for COVID-19, and resilient health systems; fourth, research and development, and equitable access to tools and supplies; and fifth, coordination, as the response transitions from an emergency mode to long-term respiratory disease management."

He reiterated that equitable vaccination remains the single most powerful tool to save lives. However, as high-income countries now roll out fourth doses of vaccination for their populations, one third of the world's population is yet to receive a single dose, including 83 percent of the population of Africa, according to WHO's data.

"This is not acceptable to me, and it should not be acceptable to anyone," Tedros said, vowing to save lives by ensuring everyone has access to tests, treatments and vaccines.