Former President Trump withdrew membership from the WHO last July, which was to come into effect in July this year.
US remains the worst-hit country, with around a fifth of the two million global Covid-19 deaths.
Biden, 78, became the oldest US president in history.
Operators say the embittered leader could use his accounts to foment more unrest aimed at Biden's inauguration.
Democrat Joe Biden will be sworn in as president after four turbulent years under Donald Trump.