The head of the World Health Organization said Monday the emergence of new COVID-19 variants has raised questions about whether or not existing vaccines will work, calling it “concerning news” that the vaccines developed so far may be less effective against the variant first detected in South Africa.

Evidence is mounting that having COVID-19 may not protect against getting infected again with some of the new variants. People also can get second infections with earlier versions of the coronavirus if they mounted a weak defense the first time, new research suggests.

In the final World Health Organization (WHO) COVID-19 press conference of the year, senior officials warned that the virus is "not necessarily the big one" and that there is a real chance of another, more serious pandemic spreading across the world.